Maharashtra Board Class 7 Science Chapter 13 Changes Physical and Chemical Solutions

1. Distinguish between the following.

Question a. Physical change and Chemical change
Answer:

In simple words: Physical changes alter a substance's appearance without forming new materials and are usually reversible, while chemical changes result in new substances with different properties and are often irreversible.

🎯 Exam Tip: Clearly differentiating between physical and chemical changes with specific examples is crucial for scoring well. Focus on reversibility and new substance formation.

Question b. Periodic changes and Non-periodic change
Answer:

In simple words: Periodic changes happen repeatedly at regular intervals, like day and night, whereas non-periodic changes occur irregularly or at unpredictable times, such as earthquakes.

🎯 Exam Tip: Understanding the regularity and predictability of a change is key to distinguishing between periodic and non-periodic phenomena. Provide examples for both types.

Question c. Natural change and Man-made change
Answer:

In simple words: Natural changes occur without human intervention, like fruit ripening, while man-made changes are caused by human activities, such as burning fossil fuels.

🎯 Exam Tip: When distinguishing, focus on the origin of the change – whether it happens independently in nature or is a consequence of human actions. Provide clear contrasting examples.

2. Under which different types can the following changes be classified?

Question a. Conversion of milk into yoghurt.
Answer: Manmade (useful). In simple words: Converting milk into yoghurt is a useful change brought about by human action, making it a man-made change.

🎯 Exam Tip: Classify changes based on their origin (natural/man-made) and utility (useful/harmful) as well as other characteristics like speed or reversibility.

Question b. Bursting of cracker.
Answer: Manmade (fast change). In simple words: Bursting a cracker is a rapid change caused by human activity, thus categorized as a fast, man-made change.

🎯 Exam Tip: When describing changes, include multiple classification types (e.g., man-made, fast, irreversible) to provide a complete answer.

Question c. Occurrence of an earthquake.
Answer: Non-periodic. In simple words: An earthquake occurs unpredictably and irregularly, classifying it as a non-periodic change.

🎯 Exam Tip: Non-periodic changes are characterized by their irregular occurrence and lack of a fixed time interval. Use examples like natural disasters to illustrate this.

Question d. Revolution of earth around the sun.
Answer: Periodic. (Natural change) In simple words: The Earth's revolution around the sun is a regular, recurring event in nature, making it both a periodic and natural change.

🎯 Exam Tip: Remember that natural changes can also be periodic, demonstrating predictable patterns in nature.

Question e. Stretching of a spring.
Answer: Physical, reversible change. In simple words: Stretching a spring alters its shape but not its chemical composition, and it can return to its original state, making it a physical and reversible change.

🎯 Exam Tip: Physical changes primarily affect the form or state of matter and are often reversible, distinguishing them from chemical changes.

3. Give reasons.

Question a. While purchasing canned food, its expiry date should be checked.
Answer:
1. Canned food is processed food.
2. After certain period of time it may get spoilt due to some growth of micro-organism it may get intoxicated.
3. If we consume such food we may get food poisoning, so we should check the expiry date. In simple words: Canned food can spoil due to microbial growth after its expiry date, leading to intoxication and food poisoning, which is why checking the expiry date is essential for safety.

🎯 Exam Tip: When providing reasons, always link the action directly to its potential consequences, especially concerning health and safety.

Question b. An iron article should be given a coat of paint.
Answer:
1. When an iron article is kept for long time it gets corroded due to presence of oxygen in atmosphere.
2. Iron gets oxidized to iron oxide which forms a reddish brown layer on it and iron gets rusted.
3. It is called corrosion. Due to corrosion things become weak.
4. So an iron article should be given a coat of paint so that it is protected against corrosion.
5. Paint gives protection to iron. In simple words: Iron corrodes and rusts when exposed to atmospheric oxygen, forming a weak reddish-brown layer. Painting provides a protective layer, preventing corrosion and preserving the strength of the iron article.

🎯 Exam Tip: Emphasize the chemical process (oxidation, rusting, corrosion) and how the protective measure (painting) prevents this degradation, explaining both the "why" and "how."

Question c. A Wooden article should be polished.
Answer:
1. When a wooden article comes in contact with moisture there is possibility of fungal growth and also termites will make wooden article brittle and break.
2. So a wooden article should be polished which forms a protective layer.
3. It does not allow the wooden article to get decayed by moisture or attacked by termite. In simple words: Polishing wooden articles creates a protective layer that shields them from moisture, preventing fungal growth and termite attacks, which otherwise make the wood brittle and prone to decay.

🎯 Exam Tip: Focus on the specific threats to wooden articles (moisture, fungi, termites) and how polishing acts as a barrier to mitigate these issues, ensuring durability.

Question d. Copper and brass utensils should be tinned.
Answer:
1. Due to corrosion of copper or brass a greenish layer is seen to form on it.
2. So to protect copper and brass utensils from corrosion they are coated with tin.
3. This process is called thinning. In simple words: Copper and brass utensils corrode over time, forming a greenish layer. Tinning, which involves coating them with a thin layer of tin, protects them from this corrosion.

🎯 Exam Tip: Explain the corrosion process specific to copper/brass and then precisely define 'tinning' as the preventative measure, connecting the problem to its solution.

Question e. A dry handkerchief gets wet at once on dipping in water, but it takes long for a wet handkerchief to dry.
Answer:
1. The dry fibres of handkerchief absorbs water to its maximum capacity immediately that is why handkerchief gets wet at once.
2. When wet handkerchief dries, all the water absorbed has to become water vapour by evaporation.
3. This depends on surrounding air and heat. In simple words: A dry handkerchief quickly absorbs water due to its fibers' capacity. However, a wet handkerchief takes longer to dry because all the absorbed water must evaporate into vapor, a process influenced by surrounding air and heat.

🎯 Exam Tip: Distinguish between the rapid process of absorption (capillary action) and the slower process of evaporation, highlighting the factors that affect drying time.

4. What will you take into account while identifying the following:

Question a. A physical change in a substance.
Answer: I will take into account the following facts.
1. Whether the shape, size or state of the substance is changed.
2. Whether it is a temporary change.
3. Whether it is a reversible change. In simple words: To identify a physical change, I would check if only the substance's shape, size, or state has altered, if the change is temporary, and if it can be easily reversed to its original form.

🎯 Exam Tip: Key indicators for physical changes are changes in state, shape, or size, temporary nature, and the ability to revert to the original substance without forming new compounds.

Question b. A chemical change in a substance.
Answer: I will take into account the following facts.
1. Whether a new substance is formed.
2. Whether it is a permanent change.
3. Whether it is a non-reversible change. In simple words: To identify a chemical change, I would determine if a new substance with different properties has formed, if the change is permanent, and if it cannot be easily reversed.

🎯 Exam Tip: When identifying chemical changes, always look for the formation of a new substance, a permanent alteration, and the difficulty or impossibility of reversing the change.

5. Read the paragraph and note down the various types of changes it mentions.

Question a. Read the paragraph and note down the various types of changes it mentions. It was nearing six o'clock in the evening. The sun was setting. A breeze was blowing. Leaves on the tree were shaking. Sahil was sitting in the courtyard, rolling balls of wet soil and shaping them into various toys. Then he felt hungry and went into the house. Mother made a dough from wheat flour and fried purees. While eating hot purees, his attention was drawn outside the window. It had started raining. There was lighting too. Sahil was enjoying his dinner in the dim light.
Answer:
1. Clock showing six o'clock time – It is a periodic change.
2. Sun was setting – A periodic change.
3. Breeze was blowing – A natural change.
4. Leaves shaking – A natural change.
5. Making toys from wet soil – Physical change.
6. Making dough from wheat flour – Physical change
7. Frying purees – Chemical change.
8. Raining – Natural change.
9. Lightning – Natural change. In simple words: The paragraph illustrates various changes: periodic ones like clock time and sunset, natural changes such as a breeze, shaking leaves, rain, and lightning, and man-made changes like making soil toys (physical), dough (physical), and frying purees (chemical).

🎯 Exam Tip: For narrative-based questions, carefully read each event, categorize it based on scientific principles (periodic, natural, physical, chemical), and explain the classification concisely.

Project:

Question a. Visit a place where work like powder coating. spray painting is done. Make a note of the information you obtain.
Answer: [Text] In simple words: (This question requires an activity-based answer that varies based on individual observation.)

🎯 Exam Tip: For project-based questions, demonstrate understanding by clearly documenting observations, processes, and safety measures related to the industrial activities visited.

Fill in the blanks and rewrite the sentences:

Question 1. Burning of candle is a .................... change.
Answer: chemical In simple words: Burning a candle is a chemical change because the wax undergoes a reaction to produce new substances like ash, carbon dioxide, and water vapor.

🎯 Exam Tip: Remember that burning almost always signifies a chemical change, as new substances are formed and the process is generally irreversible.

Question 2. Cooking of vegetable is ......... change.
Answer: useful In simple words: Cooking vegetables is a useful process that changes their texture, taste, and nutritional availability, making it a useful change.

🎯 Exam Tip: Classify changes not just by physical/chemical but also by their utility to humans; cooking is a prime example of a useful, man-made chemical change.

Question 3. A man painted his main gate made up of iron to prevent it from ....................
Answer: rusting In simple words: Painting an iron gate prevents it from rusting, a chemical process where iron reacts with oxygen and moisture to form iron oxide.

🎯 Exam Tip: Connect the act of painting directly to the prevention of corrosion or rusting, highlighting its practical application in preserving metal.

Question 4. Galvanization is a process used to prevent the rusting of ....................
Answer: iron In simple words: Galvanization is a method used to prevent the rusting of iron by coating it with a protective layer of zinc.

🎯 Exam Tip: Understand galvanization as a specific anti-corrosion technique, and remember that it's primarily used for iron objects.

Question 5. Making sugar solution is a .................... change.
Answer: physical In simple words: Making a sugar solution is a physical change because sugar merely dissolves in water, retaining its chemical identity, and can be recovered by evaporating the water.

🎯 Exam Tip: Dissolving is generally a physical change unless there is a chemical reaction between the solute and solvent. The key is no new substance formation.

Question 6. A physical change is generally ....................
Answer: temporary In simple words: Physical changes are typically temporary because the substance involved retains its original chemical composition and can often revert to its initial state.

🎯 Exam Tip: The temporary nature and reversibility are defining characteristics of physical changes, contrasting them with permanent chemical changes.

Question 7. Grinding of wheat grain changes its size, it is a ....................
Answer: physical change In simple words: Grinding wheat grain alters its physical form (size) but not its chemical composition, thus it is a physical change.

🎯 Exam Tip: Changes in size, shape, or state without altering the fundamental composition are indicators of a physical change.

Question 8. Iron benches kept in lawns and gardens get rusted. It is a change because a new .................... is formed.
Answer: chemical, product. In simple words: Iron benches rust in gardens because it's a chemical change where iron reacts with oxygen and moisture, forming a new product called iron oxide (rust).

🎯 Exam Tip: Rusting is a classic example of a chemical change because a new substance (iron oxide) with different properties is formed, distinguishing it from physical alterations.

State whether the following statements are True or False. Correct and rewrite the false statement:

Question 1. Iron and rust are the same substance.
Answer: False, Rust is oxide of iron. In simple words: Iron and rust are not the same substance; rust is iron oxide, a new compound formed when iron reacts with oxygen and water, demonstrating a chemical change.

🎯 Exam Tip: Emphasize that rust is chemically different from iron, being an oxide, which solidifies its classification as a product of a chemical change.

Question 2. Iron pipes coated with zinc do not get rusted easily.
Answer: True In simple words: Iron pipes coated with zinc resist rusting because zinc provides a protective barrier and also acts as a sacrificial metal, corroding before the iron.

🎯 Exam Tip: Understand galvanization (coating with zinc) as an effective method for preventing rust, highlighting zinc's protective properties.

Question 3. Formation of manure from leaves is a physical change.
Answer: False, it is a chemical change. In simple words: The formation of manure from leaves is a chemical change, as leaves decompose through microbial action, transforming into new substances with different properties.

🎯 Exam Tip: Decomposition processes, like manure formation, involve chemical reactions and the creation of new substances, hence they are chemical changes.

Question 4. Cutting a log of wood into pieces is a chemical change.
Answer: False, it is a physical change. In simple words: Cutting wood into pieces is a physical change because only its size and shape are altered; the chemical composition of the wood remains the same.

🎯 Exam Tip: Simple mechanical actions that only change the form, size, or shape of a substance without altering its chemical identity are physical changes.

Question 5. Digestion of food is a chemical change.
Answer: True In simple words: Digestion is a chemical change because complex food molecules are broken down by enzymes into simpler substances that the body can absorb, fundamentally altering their chemical structure.

🎯 Exam Tip: Biological processes like digestion involve complex chemical reactions and transformations, making them classic examples of chemical changes.

Question 6. Melting of wax is a chemical change.
Answer: False, it is a physical change. In simple words: Melting wax is a physical change because it merely changes from a solid to a liquid state; its chemical composition remains the same and it can solidify again.

🎯 Exam Tip: Phase changes (solid to liquid, liquid to gas) are typically physical changes, as the substance itself does not transform into a new chemical entity.

Question 7. Dissolving sugar in water is physical change.
Answer: True In simple words: Dissolving sugar in water is a physical change because sugar molecules disperse in water without undergoing chemical alteration, and sugar can be recovered by evaporation.

🎯 Exam Tip: When a substance dissolves without forming new chemical bonds or substances, it is a physical change, as the original components can often be separated again.

Question 8. Changes in which new substances are formed are called physical changes.
Answer: False, they are called chemical change. In simple words: Changes that result in the formation of new substances with different properties are defined as chemical changes, not physical changes.

🎯 Exam Tip: The formation of a new substance is the definitive characteristic that distinguishes a chemical change from a physical change.

Question 9. Breaking down of larger food particles into smaller particles is a chemical change.
Answer: False, it is a chemical change. In simple words: While initial mechanical breaking of food into smaller particles (e.g., chewing) is physical, the subsequent breakdown by enzymes into simpler absorbable forms is a chemical change.

🎯 Exam Tip: Differentiate between mechanical breakdown (physical) and enzymatic breakdown (chemical) during digestion; both occur, but only the latter forms new substances.

Question 10. Setting of curd is a physical change.
Answer: False, it is a chemical change. In simple words: Setting of curd is a chemical change because lactic acid bacteria convert lactose in milk into lactic acid, leading to the coagulation of milk proteins and forming a new substance (curd).

🎯 Exam Tip: Processes involving bacterial action, like curdling, often lead to irreversible chemical changes where the original substance is transformed into a new one.

Question 11. Heating of iron is a physical change.
Answer: True In simple words: Heating iron is a physical change as it alters the iron's temperature or state (if melted) but does not change its chemical composition into a new substance.

🎯 Exam Tip: Simple heating or cooling, unless it causes a chemical reaction (like oxidation at high temperatures), typically results in a physical change, such as a change in state or expansion.

Question 12. Cooking a vegetable is chemical change.
Answer: True In simple words: Cooking a vegetable is a chemical change because heat alters its chemical structure, leading to changes in texture, color, and flavor, and this process is irreversible.

🎯 Exam Tip: Cooking often involves chemical changes, as heat breaks down complex molecules and forms new ones, fundamentally changing the food's properties.

Question 13. Freezing water into ice is a chemical change.
Answer: False, it is a physical change In simple words: Freezing water into ice is a physical change because water merely changes its state from liquid to solid, but its chemical composition (H₂O) remains the same.

🎯 Exam Tip: All changes of state (melting, freezing, boiling, condensation) are physical changes, as the substance's chemical identity remains constant.

Define the following.

Question 1. Physical change:
Answer: In a physical change the material may change shapes or forms but no new compounds are formed and change can generally be reversed by reversing the conditions. In simple words: A physical change alters a substance's appearance or state, like its shape or size, but does not create new chemical compounds and is often reversible.

🎯 Exam Tip: When defining physical change, highlight the absence of new substance formation and its general reversibility as core characteristics.

Question 2. Chemical change
Answer: A chemical change produces a new substance having new and different properties and which cannot be reversed by reversing the conditions. In simple words: A chemical change forms entirely new substances with distinct properties, and unlike physical changes, it cannot typically be reversed to restore the original material.

🎯 Exam Tip: Emphasize the formation of new substances and the irreversible nature of the transformation as the key elements in defining a chemical change.

Question 3. Periodic change
Answer: Some changes occur again and again after . a definite interval of time, such changes are called periodic changes. In simple words: Periodic changes are those that repeat consistently and predictably after a fixed duration, like the phases of the moon.

🎯 Exam Tip: The crucial aspects of a periodic change are its repetition and occurrence at regular, definite intervals of time.

Question 4. Non-periodic change
Answer: Changes that do not repeat themselves after a fixed interval of time are called non-periodic changes. In simple words: Non-periodic changes occur irregularly and unpredictably, without a consistent pattern or fixed time interval between occurrences.

🎯 Exam Tip: Highlight the irregular and unpredictable nature of non-periodic changes, contrasting them with the fixed intervals of periodic changes.

Question 5. Reversible change
Answer: The changes that can occur in a forward and reverse direction again and again are called reversible changes. In simple words: A reversible change is one that can be undone or reversed, allowing the substance to return to its original state by altering the conditions.

🎯 Exam Tip: Focus on the ability of the substance to revert to its initial form as the main characteristic of a reversible change.

Question 6. Irreversible change.
Answer: The changes which we cannot reverse back to same thing are called Irreversible changes e.g. Burning of wood, paper. In simple words: An irreversible change is a permanent alteration where the original substance cannot be restored after the change has occurred, such as burning wood.

🎯 Exam Tip: The inability to reverse the change and the permanent alteration of the substance are the hallmarks of an irreversible change.

Question 7. Condensation
Answer: The process by which matter changes from its vapour or gaseous state into its liquid state by cooling is called condensation. In simple words: Condensation is the process where a substance in its gaseous state transforms into its liquid state, typically due to cooling.

🎯 Exam Tip: Define condensation as a phase change from gas to liquid, explicitly mentioning the role of cooling in this process.

Question 8. Corrosion of metals
Answer: When an iron article rusts, a reddish brown layer (of iron oxide) forms on it. A greenish layer is seen to form on a copper article. This is called corrosion of metals. In simple words: Corrosion of metals is the gradual degradation of a metal due to chemical reactions with its environment, like iron forming reddish-brown rust or copper developing a greenish layer.

🎯 Exam Tip: When defining corrosion, provide examples specific to different metals (e.g., iron rusting, copper forming a green patina) to illustrate the concept fully.

Question 9. Galvanization
Answer: Iron articles are given a thin coat of zinc to prevent corrosion this is called galvanization. In simple words: Galvanization is the process of coating iron articles with a thin layer of zinc to protect them from corrosion and rusting.

🎯 Exam Tip: Clearly state that galvanization is a protective coating process specifically using zinc on iron to prevent rust.

Question 10. Tinning
Answer: Copper and brass articles are coated with tin. This is called tinning. In simple words: Tinning is the process of applying a protective layer of tin to copper and brass articles to prevent their corrosion.

🎯 Exam Tip: Identify tinning as a protective coating method, specifically for copper and brass, to prevent the formation of unwanted layers.

Question 11. Natural change
Answer: Changes that occur naturally like ripening of a fruit, spoiling of milk are called natural changes. In simple words: Natural changes are processes that happen spontaneously in nature without any direct human involvement, such as a fruit ripening or milk spoiling.

🎯 Exam Tip: Define natural changes by their spontaneity and independence from human activity, providing clear examples from nature.

Question 12. Man-made change
Answer: Man-made changes are result of human- activity. E.g. Burning of fossil fuel. In simple words: Man-made changes are those resulting directly from human actions and interventions, such as the burning of fossil fuels.

🎯 Exam Tip: When defining man-made changes, focus on the agency of human activity as the cause, providing relevant examples like industrial processes or daily activities.

Question 13. Useful change
Answer: Many changes such as baking bread, cooking food are useful to us, they are called useful changes. In simple words: Useful changes are transformations that benefit humans, directly contributing to our needs or improving our lives, like baking bread or cooking food.

🎯 Exam Tip: Define useful changes by their positive impact or benefit to humans, citing everyday examples that clearly demonstrate this utility.

Question 14. Harmful change
Answer: The changes that are not useful or changes that harms living things or the environment are called harmful changes. In simple words: Harmful changes are those that have negative consequences, causing damage or disadvantage to living beings or the environment.

🎯 Exam Tip: Clearly state that harmful changes are detrimental, impacting health, property, or ecological balance.

Name or Give two example of:

Question 1. Two physical changes.
Answer:
1. Melting of wax
2. Melting of ice In simple words: Melting wax and melting ice are both physical changes because they only alter the state of matter without forming new chemical substances.

🎯 Exam Tip: Always remember that changes of state (melting, freezing, boiling) are quintessential examples of physical changes.

Question 2. Two chemical changes.
Answer:
1. Burning of wax candle
2. Burning of wood or paper. In simple words: Burning a wax candle and burning wood are chemical changes because they produce new substances like ash, carbon dioxide, and smoke, which cannot be reversed.

🎯 Exam Tip: Combustion (burning) is a common example of a chemical change, characterized by the formation of entirely new substances and irreversibility.

Question 3. Two periodic changes.
Answer:
1. Occurrence of day and night
2. change of seasons In simple words: The occurrence of day and night and the changing of seasons are periodic changes because they repeat at regular, predictable intervals.

🎯 Exam Tip: Periodic changes demonstrate regularity and predictability over time, often driven by natural cycles like planetary rotation or revolution.

Question 4. Two non-periodic changes.
Answer:
1. A meteor moving across the sky.
2. A bird sitting on a tree flies away. In simple words: A meteor streaking across the sky and a bird flying away are non-periodic changes as they occur randomly and do not follow a fixed time interval or pattern.

🎯 Exam Tip: Non-periodic changes lack a discernible pattern or fixed interval, making them unpredictable in their occurrence.

Question 5. Two reversible changes.
Answer:
1. Melting of wax.
2. Freezing water into ice. In simple words: Melting of wax and freezing water into ice are reversible changes because the substances can be converted back to their original states by altering temperature.

🎯 Exam Tip: Reversible changes allow the original substance to be recovered by simply reversing the conditions that caused the change.

Question 6. Two irreversible changes.
Answer:
1. Ripening of a mango.
2. Burning of wood In simple words: Ripening of a mango and burning of wood are irreversible changes because new substances are formed and the original state cannot be restored.

🎯 Exam Tip: Irreversible changes often involve chemical transformations that fundamentally alter the substance, preventing its return to the initial state.

Question 7. Two natural changes.
Answer:
1. Lighting in the sky.
2. Earthquake. In simple words: Lightning in the sky and an earthquake are natural changes as they occur due to forces of nature without human intervention.

🎯 Exam Tip: Natural changes originate from processes within the environment or Earth's systems, independent of human influence.

Question 8. Two manmade changes.
Answer:
1. Baking bread
2. Cooking food. In simple words: Baking bread and cooking food are man-made changes as both processes are initiated and controlled by human activity to prepare edibles.

🎯 Exam Tip: Any change deliberately performed by humans, often to modify materials for specific purposes, falls under man-made changes.

Question 9. Two useful changes.
Answer:
1. Ripening of mango
2. Curdling of milk. In simple words: Ripening of mango makes it edible and curdling of milk produces a useful food product, making both beneficial for humans.

🎯 Exam Tip: Useful changes are those that provide a benefit or serve a specific purpose for human needs or well-being.

Match The Following

Question 1. Column 'A'
1. Photosynthesis
2. Dissolving sugar in water
3. Burning of wood
4. Melting of wax
5. Digestion of food
6. Heartbeat
7. Occurrence of earthquake.
8. Deforestation

Column 'B'
a. Physical change.
b. Chemical change
c. Man-made change.
d. Periodic changes.
e. Non-periodic change
f. Chemical change.
g. Physical change.
h. Chemical change.
Answer:

In simple words: This question tests your knowledge of different types of changes, matching specific examples to their correct classification as physical, chemical, man-made, natural, periodic, or non-periodic changes. Each change has distinct characteristics that define its category.

🎯 Exam Tip: Pay close attention to whether a new substance is formed or if the change is reversible to correctly distinguish between physical and chemical changes. Also, consider the regularity and human involvement for periodic/non-periodic and natural/man-made changes.

Question 2. Column 'A'
1. Depositing a layer of zinc on Iron
2. Souring of milk
3. Curdling of milk
4. Iron oxide
5. Depositing layer of tin on copper and brass article.

Column 'B'
a. Tinning
b. Rust
c. Galvanization
d. Chemical changes
e. Man-made changes.
Answer:

In simple words: This matching exercise helps you associate specific processes and substances with their correct scientific terms and classifications, such as galvanization for zinc coating, rust for iron oxide, and tinning for tin coating on copper/brass.

🎯 Exam Tip: Understanding the terminology related to preventing corrosion (galvanization, tinning) and the products of chemical changes (rust) is key for scoring here. Differentiate between natural chemical changes (souring/curdling of milk) and applied human-made processes.

Distinguish Between The Following

Question 1. Reversible change and Irreversible change
Answer:

In simple words: Reversible changes can be undone to get the original substance back, like melting wax. Irreversible changes cannot be undone, meaning new substances are formed permanently, such as burning wood into ash.

🎯 Exam Tip: Clearly define each type of change and provide contrasting examples to show a complete understanding. Focus on the ability to return to the original state as the primary distinguishing factor.

Under Which Different Types Can The Following Changes Be Classified:

Question 1. Formation of curd from milk.
Answer: Manmade change (useful), chemical
In simple words: Making curd from milk is a useful process initiated by humans (manmade), which results in a new substance with different properties, making it a chemical change.

🎯 Exam Tip: When classifying, consider if human intervention is involved, if new substances are formed, and if the change has a beneficial outcome.

Question 2. Burning of wood or paper.
Answer: Manmade, chemical change.
In simple words: Burning wood or paper is a chemical change because new substances (ash, smoke) are formed, and it's often initiated by humans, making it a man-made change.

🎯 Exam Tip: Recognize burning as a quintessential chemical change due to the formation of entirely new products and the irreversibility of the process. If humans light it, it's manmade.

Question 3. Glowing of bulb.
Answer: Manmade (fast).
In simple words: A bulb glowing is a man-made change because it's an artificial process involving electricity, and it happens very quickly, classifying it as a fast change.

🎯 Exam Tip: Classify based on the origin (natural vs. man-made) and speed of the change. The glowing of a bulb is a rapid, human-engineered effect.

Question 4. Rusting of iron.
Answer: Chemical change (harmful)
In simple words: Rusting of iron is a chemical change where iron reacts with oxygen and moisture to form iron oxide (rust), which is harmful as it deteriorates the metal.

🎯 Exam Tip: Identify rusting as a chemical change because a new substance (rust) is formed, and note its harmful nature due to material degradation.

Question 5. Burning of Camphor.
Answer: Chemical change
In simple words: Burning camphor is a chemical change because it transforms into new substances (ash, gases) that cannot be converted back into camphor.

🎯 Exam Tip: Any process involving combustion is generally a chemical change, as it leads to the formation of new compounds.

Question 6. Souring of milk.
Answer: chemical change (harmful)
In simple words: Souring of milk is a chemical change caused by bacteria, forming lactic acid and altering its composition, which is considered harmful as the milk becomes unsuitable for consumption.

🎯 Exam Tip: Changes involving decomposition or bacterial action that alter the chemical composition and make food inedible are chemical and often harmful.

Question 7. Digestion of food.
Answer: Chemical change (useful), irreversible
In simple words: Digestion of food is a useful and irreversible chemical change where complex food molecules are broken down into simpler nutrients by enzymes for absorption and energy.

🎯 Exam Tip: Digestion is a biological chemical change, crucial for life and irreversible in its process of breaking down food into usable components.

Question 8. Change of season.
Answer: Periodic change, natural change
In simple words: The change of seasons is a natural phenomenon that occurs regularly at fixed intervals, making it a periodic change driven by Earth's orbit.

🎯 Exam Tip: Recognize natural cycles that repeat over consistent timeframes as periodic changes.

Question 9. Day and night formation.
Answer: Periodic change
In simple words: Day and night formation is a periodic change because it occurs naturally and repeats itself regularly due to the Earth's rotation.

🎯 Exam Tip: Daily cycles are classic examples of periodic changes due to their predictable and repetitive nature.

Question 10. Heartbeat.
Answer: Periodic change
In simple words: A heartbeat is a periodic change as it is a natural, rhythmic contraction that occurs repeatedly at regular intervals throughout life.

🎯 Exam Tip: Biological processes that occur with regular rhythm are considered periodic changes.

Question 11. Water pollution.
Answer: Manmade, harmful change
In simple words: Water pollution is a man-made and harmful change, as human activities introduce contaminants into water bodies, degrading water quality and impacting ecosystems and health.

🎯 Exam Tip: Environmental degradations caused by human actions are generally classified as man-made and harmful changes.

Question 12. Air pollution.
Answer: Manmade, harmful change
In simple words: Air pollution is a man-made and harmful change resulting from the release of pollutants into the atmosphere, which degrades air quality and poses risks to health and the environment.

🎯 Exam Tip: Similar to water pollution, air pollution is a direct consequence of human activities and has detrimental effects, categorizing it as man-made and harmful.

Question 13. Burning of fossil fuel
Answer: Manmade (irreversible change)
In simple words: Burning fossil fuels is a man-made, irreversible chemical change as it converts fuel into new substances like carbon dioxide and water, which cannot be easily reverted to their original state.

🎯 Exam Tip: Combustion of any fuel is typically a chemical and irreversible change, and when initiated by humans for energy, it's also man-made.

Question 14. Ripening of fruit.
Answer: Natural, chemical
In simple words: Ripening of fruit is a natural chemical change where complex compounds within the fruit break down into simpler sugars, changing its color, texture, and flavor.

🎯 Exam Tip: Biological maturation processes like fruit ripening involve complex chemical reactions and occur without human intervention, making them natural chemical changes.

Question 15. Change-in climate.
Answer: Natural, periodic
In simple words: Changes in climate, over long periods, can be seen as natural periodic shifts, though human activities also contribute to current rapid climate changes.

🎯 Exam Tip: Differentiate between long-term natural climate cycles (periodic) and recent, rapid changes influenced by human activity (man-made, often non-periodic).

Question 16. Melting of ice into water
Answer: Physical change
In simple words: Melting of ice into water is a physical change because only the state of matter changes (solid to liquid), while the chemical composition (H₂O) remains the same.

🎯 Exam Tip: Changes of state (melting, freezing, boiling, condensation) are classic examples of physical changes, as no new chemical substances are formed.

Question 17. Germination of seed.
Answer: Natural, slow, desirable.
In simple words: Seed germination is a natural biological process that is slow, involves chemical changes within the seed, and is desirable as it leads to the growth of new plants.

🎯 Exam Tip: Biological growth processes are typically natural, slow, and often desirable, involving complex chemical transformations.

Question 18. Bursting of balloon.
Answer: Irreversible change (fast).
In simple words: The bursting of a balloon is an irreversible change because the balloon cannot be restored to its original intact state, and it happens very quickly.

🎯 Exam Tip: Changes that cause permanent damage or fragmentation of an object are usually irreversible and can be fast or slow depending on the cause.

Give Reasons:

Question 1. The burning of wax is a chemical change
Answer: 1. Because when the cotton wick embedded in the candle is burnt, ash and smoke is formed.
2. It is not possible to make wick again from the smoke and ash.
In simple words: Burning wax is a chemical change because the wick and some wax transform into new substances like ash and smoke, which cannot be changed back into their original form.

🎯 Exam Tip: Focus on the formation of new substances and the irreversibility of the process as key indicators of a chemical change, especially in combustion reactions.

Can You Tell?

Answer The Following Questions:

Question 1. We see many man-made materials in our day-to-day life. For what purpose are they produced.
Answer: We see many man-made matrical in our day to day life. For example,
1. Air conditioners for getting cool air in houses offices and cars.
2. Wooden furniture made from wood obtained by cutting trees.
3. Refrigerators to protect food from spoiling.
In simple words: Man-made materials are produced to fulfill various human needs and improve quality of life, such as providing comfort, utility, and preservation.

🎯 Exam Tip: When asked about purposes, think broadly about how man-made items enhance human comfort, convenience, and functionality, providing specific examples.

Question 2. You made a link from the glass piece. How will you change the shape of the link to give it its original shape?
Answer: Heat the glass piece again at the link. When it becomes soft break it.
In simple words: To restore the original shape of a glass piece that has been linked, you would need to reheat the glass to make it pliable and then carefully break the unwanted link.

🎯 Exam Tip: This question tests your understanding of the physical properties of glass and how heat can be used to alter its form, allowing for reshaping or separation.

Question 3. How will you restore the original candle after melting one ?
Answer: Collect the melted wax and give the required shape when it is soft.
In simple words: You can restore a melted candle by collecting the molten wax and molding it back into the desired shape once it cools and solidifies.

🎯 Exam Tip: This illustrates a physical change (melting) which is reversible; the substance (wax) remains the same, only its state changes, allowing it to be reformed.

Question 4. What comes after daytime?
Answer: Night time (periodic change)
In simple words: After daytime comes night time, which is a periodic change because it occurs regularly in a predictable cycle due to Earth's rotation.

🎯 Exam Tip: Daily occurrences like day and night are prime examples of natural periodic changes.

Question 5. What is the next stage after sunrise?
Answer: Sunset (periodic change)
In simple words: The next significant stage after sunrise in a day's cycle is sunset, representing another periodic change.

🎯 Exam Tip: Celestial events such as sunrise and sunset are predictable and repetitive, defining them as periodic changes.

Question 6. What comes after a high tide in the sea?
Answer: Low tide (periodic)
In simple words: After a high tide, a low tide follows, completing a cycle that is a periodic change in sea level.

🎯 Exam Tip: Tidal patterns are governed by gravitational forces and are classic examples of periodic changes due to their regular, predictable recurrence.

Question 7. A bird sitting on a tree flies away.
Answer: non-periodic change.
In simple words: A bird flying away from a tree is a non-periodic change because it happens unpredictably and does not repeat at fixed intervals.

🎯 Exam Tip: Events that lack a regular, predictable pattern or timing are classified as non-periodic changes.

Question 8. A meteor streaking across the sky.
Answer: non-periodic change.
In simple words: A meteor streaking across the sky is a non-periodic change because it is an unpredictable event that does not occur at regular, fixed intervals.

🎯 Exam Tip: Spontaneous and irregular events in nature, like meteor sightings, are examples of non-periodic changes.

Use Your Brainpower!

Answer The Following Questions:

Question 1. What kind of change is the falling of a tree in a storm?
Answer: Natural change.
In simple words: The falling of a tree in a storm is a natural change, as it is an event caused by natural forces without human intervention.

🎯 Exam Tip: Events driven by natural phenomena like weather are classified as natural changes.

Question 2. What kind of a change is the conversion of milk into yoghurt?
Answer: Manmade' change, useful change and a chemical change
In simple words: Converting milk into yogurt is a man-made, useful chemical change because bacteria transform lactose into lactic acid, creating a new product with different properties.

🎯 Exam Tip: This example demonstrates how a single event can be classified across multiple categories: man-made (human-initiated), useful (beneficial outcome), and chemical (new substance formed).

Question 3. What is meant by irreversible change? Give some examples.
Answer: 1. An irreversible change is a permanent change that cannot be undone, cannot reverse it back to original substance.
2. In irreversible change new materials are always formed.
3. Irreversible changes are permanent changes.
4. Heating, burning, mixing, powdering are few processes which causes irreversible changes.
Examples:
- burning of paper or wood
- cooking of raw egg.
In simple words: Irreversible changes are permanent transformations where new substances are formed, and the original substance cannot be retrieved, such as burning or cooking.

🎯 Exam Tip: For irreversible changes, emphasize the formation of new substances and the impossibility of reversing the process to obtain the original materials. Providing diverse examples reinforces understanding.

Question 4. Which type of change is the change of seasons from summer to rains to winter?
Answer: Natural and periodic change
In simple words: The change of seasons is a natural phenomenon that occurs in a predictable, repeating cycle, making it both natural and periodic.

🎯 Exam Tip: Seasonal changes are excellent examples of natural periodic changes due to their consistent, recurring pattern dictated by Earth's orbital mechanics.

Question 5. Which hands of a clock show periodic changes from 6.00 am to 6.00 pm? How many times?
Answer: Small (hour) hand (12 times).
In simple words: The hour hand of a clock shows periodic changes; it completes 12 full cycles between 6:00 AM and 6:00 PM (12 hours later), passing each hour mark once.

🎯 Exam Tip: This question tests the understanding of periodic motion in a practical context. The hour hand's movement is periodic, completing one cycle every 12 hours.

Question 6. Which of the following are physical changes and which are chemical changes?
(making a table from wood, burning wood, breaking of a glass object, ripening of a tomato, rusting of iron.
Answer:

In simple words: Physical changes alter an object's form but not its chemical composition, like shaping wood or breaking glass. Chemical changes create new substances, such as burning wood, rusting iron, or ripening a tomato.

🎯 Exam Tip: Distinguish physical from chemical changes by asking if a new substance is formed. If the material's identity remains the same (even if its shape or size changes), it's physical. If a new material is created, it's chemical.

Question 7. What are periodic and non-periodic changes?
Answer: 1. We find that some changes occur again and again after a definite interval of time. Such changes are called Periodic changes, e.g. Sunrise and Sunset.
2. On the other hand, it cannot be 'Slid for sure when certain changes will recur after one occurrence. Even if they recur, the time interval is not fixed. Such changes are called non-periodic changes, e.g. A meteor streaking across the sky.
In simple words: Periodic changes repeat regularly at fixed intervals, like sunrise and sunset, while non-periodic changes occur unpredictably, with no fixed time interval between recurrences, such as a meteor streaking.

🎯 Exam Tip: The key differentiator is the predictability and regularity of occurrence. Periodic changes have a fixed cycle, while non-periodic changes are irregular or random.

Question 8.a. Which changes shown in the pictures below are temporary.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र विभिन्न प्रकार के परिवर्तनों को दर्शाता है। इसमें (a) कच्चे आम का पकना, (b) चुंबक द्वारा लोहे की कीलों को हटाना, (c) कली का फूल बनना और (d) बर्फ का पानी में पिघलना शामिल है। ये उदाहरण भौतिक और रासायनिक, साथ ही अस्थायी और स्थायी परिवर्तनों को स्पष्ट करते हैं।
Answer: (b) Removing iron nails with help of magnet
(d) Conversion of ice cubes into water.
In simple words: The temporary changes depicted are removing iron nails with a magnet and ice melting into water, as the original substances can be easily restored or retrieved.

🎯 Exam Tip: Temporary changes are typically physical and reversible, meaning the substance retains its original chemical identity and can revert to its initial state.

Question b. Which of the changes are permanent?
Answer: Bud blooming into flower.
In simple words: The blooming of a bud into a flower is a permanent change because the flower cannot revert to a bud, representing a biological transformation.

🎯 Exam Tip: Permanent changes, often chemical or biological, result in a new state or substance that cannot be reversed to the original form.

Question c. In which did the original matter undergo a change?
Answer: 1. In permanent change
2. Bud blooming into flower.
In simple words: In permanent changes, like a bud blooming into a flower, the original matter undergoes a significant transformation, resulting in a new structure or substance.

🎯 Exam Tip: For changes where the original matter's identity or form is fundamentally altered, it's typically a chemical or permanent change. The question asks *in which* did it change, referring to the categories of change.

Question d. In which the original matter remain unchanged?
Answer: In temporary changes
(b) iron nails
(d) ice cubes.
In simple words: In temporary changes, such as separating iron nails with a magnet or ice melting into water, the original matter's chemical composition remains unchanged; only its arrangement or state changes.

🎯 Exam Tip: The essence of temporary or physical changes is that the fundamental nature of the substance does not alter, even if its appearance or state does.

Question e. In which of the changes was a new substance with a new property formed?
Answer: 1. Bud blooming into flower.
2. Ice to water, Ice is in solid state, Water is in liquid state.
In simple words: A new substance with new properties is formed when a bud blooms into a flower due to complex biological chemical changes. However, ice to water is a physical change, where the substance (water) remains chemically the same, only its physical state (solid to liquid) changes, not forming a new *chemical* substance. (Correction from OCR needed here for clarity on "new substance" vs "new state").

🎯 Exam Tip: Carefully distinguish between forming a *new substance* (chemical change, e.g., bud to flower, which involves chemical transformation) and a *change in state* (physical change, e.g., ice to water, where it's still H₂O). The question specifically asks for "new substance with a new property."

Question 9. Give some examples of fast and slow changes that occur in your surroundings.
Answer:

In simple words: Fast changes happen quickly, like burning or bursting, while slow changes take a significant amount of time to complete, such as rusting, germination, curdling, or ripening.

🎯 Exam Tip: Provide clear and distinct examples for both fast and slow changes, focusing on the time scale over which the change occurs to demonstrate understanding of this classification.