NCERT Exemplar Solutions Class 8 Science Combustion and Flame

Multiple Choice Questions...............................

Question 1:  A substance which reacts with oxygen giving heat is called a combustible substance. Which one of the following is a combustible substance?

(a) iron nail

(b) glass

(c) stone piece

(d) wood 

Solution 1:  (d) wood 

In the presence of oxygen, wood can catch fire. It is a combustible material that generates energy, heat, and light when ignited. Glass, iron nails, and stone are not combustible so they do not fire like wood.

Question 2:  Which one of the following has the highest calorific value?

(a) kerosene

(b) biogas

(c) LPG

(d) petrol 

Solution 2: (c) LPG

The quantum of heat generated when a fuel is fully burned is represented by the term Calorific. LPG has a calorific value of 55000 kilo joules per kilogramme, which is high when compared to kerosene, biogas, and gasoline.

Question 3:  Magnesium ribbon on burning in air produces:

(a) magnesium oxide, water and light.

(b) magnesium oxide and heat.

(c) magnesium oxide, heat and light.

(d) magnesium oxide, water and heat. 

Solution 3:  (c) magnesium oxide, heat and light

Magnesium reacts with oxygen in the air to produce magnesium oxide, which releases heat and light.

Magnesium reacts with atmospheric oxygen to get magnesium oxide by liberating heat and light.

Mg+O₂   → MgO₂ + Heat+ Light is the chemical reaction for burning of magnesium ribbon in air.

Question 4:  Which of the following is not a combustible substance?

(a) camphor

(b) glass

(c) straw

(d) alcohol 

Solution 4:  (b) glass

Camphor, glass, and straw are combustible materials that can burn in the air. Glass is a non-combustible material that would not burn in the sunlight.

Question 5:  The substance that does not burn with flame is

(a) LPG

(b) camphor

(c) dry grass

(d) charcoal 

Solution 5:  (d) charcoal

As heated, the charcoal will not vaporise. As a result, charcoal would not burn by generating a flame. When charcoal is burned, it emits a luminous light.

Question 6:  On placing an inverted tumbler over a burning candle, the flame extinguishes after some time. This is because of the non-availability of

(a) oxygen

(b) water vapours

(c) carbon dioxide

(d) wax 

Solution 6: (a) oxygen

Combustion requires the presence of oxygen. When we put an inverted tumbler over a burning candle, the oxygen supply is cut off, and the flame is extinguished.

Question 7:  If a person’s clothes catch fire, the best way to extinguish the fire is to:

(a) throw water on the clothes.

(b) use the fire extinguisher.

(c) cover the person with a woollen blanket.

(d) cover the person with a polythene sheet. 

Solution 7:  (c) cover the person with a woollen blanket. 

We cut off the oxygen needed for clothing combustion by covering a person's clothes with a woollen blanket. As a result, fire will be identified.

Question 8:  The substance expected to have the highest ignition temperature out of the following is

(a) kerosene

(b) petrol

(c) coal

(d) alcohol 

Solution 8:  (c) coal 

The lowest temperature at which a material can catch fire is called the ignition temperature. Coal has the highest ignition temperature of the options available.

Question 9:  Choose the correct statement about inflammable substances from the following. They have:

(a) low ignition temperature and cannot catch fire easily.

(b) high ignition temperature and can catch fire easily.

(c) low ignition temperature and can catch fire easily.

(d) high ignition temperature and cannot catch fire easily. 

Solution 9: (c) low ignition temperature and can catch fire easily.

The term "inflammable" refers to substances that can quickly catch fire. They are substances with a low ignition temperature that quickly catch fire when exposed to air.

Question 10:  Choose the incorrect statement from the following. Forest fires are usually due to:

(a) carelessness of humans

(b) heat of sun

(c) cutting of trees

(d) lightning strike 

Solution 10:  (c) Cutting of trees. 

The forest fire is not caused by deforestation or tree cutting. Forest fires are commonly caused by the sun's heat, lightning strikes, and human carelessness.

Question 11:  The calorific value of a fuel is expressed in a unit called

(a) kilojoule per litre

(b) kilogram per millilitre

(c) kilojoule per gram

(d) kilojoule per kilogram 

Solution 11:  (d) kilojoule per kilogram 

The calorific value of a fuel is expressed in a unit called kilojoule per kilogram.

Question 12:  In villages, people use wood as fuel because:

(a) it is considered to be an ideal fuel.

(b) of its easy availability and low cost.

(c) it is environment-friendly.

(d) it catches fire easily. 

Solution 12:  (b) of its easy availability and low cost.

People use wood as fuel because of its easy availability and low cost, in villages.

Question 13:  Which among the following is considered as the cleanest fuel?

(a) cow dung cake

(b) petrol

(c) kerosene

(d) hydrogen gas 

Solution 13:  (d) hydrogen gas 

Hydrogen gas is referred to as a "clean" fuel because it emits no harmful gases. As far as products go, it can only generate heat and water.

Question 14:  Choose the incorrect statement from the following. A good fuel is one which:

(a) is readily available.

(b) produces a large amount of heat.

(c) leaves behind many undesirable substances.

(d) burns easily in the air at a moderate rate. 

Solution 14:  (c) leaves behind many undesirable substances. 

It is easy to find good fuel that produces a lot of heat. A good fuel burns quickly in the air at a moderate rate and does not produce any harmful by-products.

Question 15:  Shyam was cooking potato curry on a chulha. To his surprise, he observed that the copper vessel was getting blackened from outside. It may be due to:

(a) proper combustion of fuel.

(b) improper cooking of potato curry.

(c) improper combustion of the fuel.

(d) burning of the copper vessel. 

Solution 15:  (c) improper combustion of the fuel. 

Because of the lack of oxygen, the copper vessel turned black from the outside.

Very Short Answer Questions.........................

Question 16:  Fill in the blanks in the following sentences.

(a) A ____________ process in which a substance reacts with __________ to give off heat is called combustion.

(b) When the clothes of a person catch __________, the person is covered with a __________ to extinguish fire.

(c) The __________ temperature at which a substance catches fire is called its __________ temperature.

(d) The substances which have very ________________ ignition temperature and can easily catch fire with a flame are called __________ substances.

(e) The substances which vaporize during __________, give flame. 

Solution 16:

(a) A Chemical process in which a substance reacts with oxygen to give off heat is called combustion.

(b) When the clothes of a person catch fire, the person is covered with a blanket to extinguish the fire.

(c) The lowest temperature at which a substance catches fire is called its ignition temperature.

(d) The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances.

(e) The substances which vaporize during burning give flame.

Question 17:  Some words (underlined) in the following sentences are jumbled up. Write them in their correct form.

(a) Seldie is a combustible substance.

(b) Slags is a non-combustible material.

(c) Chittsmack does not burn by itself.

(d) Some substances on combustion produce thea and mafel.

(e) The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its Aricolci uelav. 

Solution 17:

(a) Diesel is a combustible substance.

(b) Glass is a non-combustible material.

(c) Matchstick does not burn by itself.

(d) Some substances on combustion produce heat and flame.

(e) The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value.

Short Answer Questions.........................

Question 18:  Two glass jars A and B are filled with carbon dioxide and oxygen gases, respectively. In each jar, a lighted candle is placed simultaneously. In which jar will the candle remain lighted for a longer time and why? 

Solution 18:   Since jar B contains oxygen, which is needed for combustion and flame, the candle remains in it. Owing to a lack of oxygen, a candle in a jar will not ignite.

Question 19:  Anu wants to boil water quickly in a test tube. On observing the different zones of the flame, she is not able to decide which zone of the flame will be best for boiling water quickly. Help her in this activity. 

Solution 19:   Anu can use the hottest portion of the flame, which is the outermost region.

Question 20:  Why is the use of diesel and petrol as fuels in automobiles being replaced by Compressed Natural Gas (CNG) in big cities? 

Solution 20:   CNG is a cleaner fuel than gasoline and diesel. Since it emits a small amount of toxic gases, CNG is increasingly being used in large cities in place of petrol and diesel.

Question 21: Boojho wants to separate the following materials as combustible and non-combustible. Can you help him?

Charcoal, chalk, stone, iron rod, copper coin, straw, cardboard, glass, paper, wood. 

Solution 21:

Question 22:  Indicate whether the following statements are True or False. Also write the false statements in their correct form.

(a) Air is necessary for combustion.

(b) Magnesium is a non-combustible metal.

(c) Carbon dioxide is an excellent fire extinguisher.

(d) Calorific value of wood is higher than that of coal. 

Solution 22:

1. True
2. False- Magnesium is a combustible metal, after all.
3. True
4. False- Due to the fact that coal has a higher calorific value than wood.

Question 23:  Match the items of Column A with the items of Column B

Solution 23:

Question 24:  Match the following for the flame of a candle.

Solution 24:

Question 25:  If you hold a piece of iron wire with a pair of tongs inside a candle flame or a Bunsen burner flame, what will you observe? Will it produce a flame? 

Solution 25:   Iron does not contain flame; instead, it glows a fiery red.

Question 26:  Fill in the blanks using the words given in the box.

(a) A chemical process in which a substance reacts with oxygen to give off heat is called __________.

(b) Wood, paper, CNG are __________ substances.

(c) The lowest temperature at which a substance catches fire is called its __________ temperature.

(d) Ignition temperature of __________ is lower than that of wood.

(e) The substances which have very low __________ temperature and can easily catch fire with a flame are called __________ substances.

(f) The amount of heat energy produced on complete combustion of 1kg of a fuel is called its __________ 

Solution 26:

(a) A chemical process in which a substance reacts with oxygen to give off heat is called combustion.

(b) Wood, paper, CNG are combustible substances.

(c) The lowest temperature at which a substance catches fire is called its Ignition temperature.

(d) Ignition temperature of petrol is lower than that of wood.

(e) The substances which have very low Ignition temperature and can easily catch fire with a flame are called Inflammable substances.

(f) The amount of heat energy produced on complete combustion of 1kg of a fuel is called its Calorific value.

Question 27:  People usually keep Angethi/burning coal in their closed rooms during winter season. Why is it advised to keep the door open?

Solution 27:   Carbon monoxide is produced when coal is burned, and it is toxic to those who inhale it. When the door is shut, the oxygen level drops and the carbon monoxide level rises. The person sleeping in the room will be harmed as a result of this. As a result, the door should be shut.

Question 28:  Write True/False against the following statements and also correct the false statement.

1. A physical process in which a substance reacts with oxygen to give off heat is called combustion.
2. Water is the best extinguisher for fires involving electrical equipment.
3.  Alcohol, CNG and LPG are inflammable substances.
4. Increased concentration of nitrogen in the air is believed to cause global warming.
5.  Greater the calorific value, better is the fuel.
6. Middle zone is the hottest zone of a flame.
7. The substances, which vaporize during burning, give flame. 

Solution 28:

1. True
2. False- For fires involving electrical appliances, carbon dioxide is the safest extinguisher.
3. True
4. False- Global warming is thought to be caused by an increase in carbon dioxide levels in the atmosphere.
5. True
6. False- The hottest part of a flame is the outer zone.
7. True

Question 29:  Cracker on ignition produces sound. Why? 

Solution 29:   When a cracker is lit, gas is released, and the gas is heated by the amount of heat produced by the cracker. This resulted in an explosion and the development of sound.

Question 30:  What do you understand by fuel efficiency? 

Solution 30:   The calorific value of a fuel determines its performance, which is the amount of heat energy emitted when 1 kg of fuel is fully burned. A fuel's calorific value is measured in kJ/kg.

Long Answer Questions.........................

Question 31:  You are provided with three watch glasses containing milk, petrol and mustard oil, respectively. Suppose you bring a burning candle near these materials one by one, which material(s) will catch fire instantly and why? 

Solution 31:   Since petrol is a highly flammable liquid with a calorific value of 45.8 MJ/kg, the watchglass containing petrol would catch fire first. When compared to milk or mustard oil, this is a very small amount. 

Question 32:  Manu was heating oil to fry potato chips. The cooking oil all of a sudden caught fire; he poured water to extinguish the fire. Do you think this action was suitable? If yes, why? If not, why not? In such a condition what should Manu have done? 

Solution 32:   No, Manu's method of putting out a fire caused by oil is ineffective. Since, along with the water, oil spreads, increasing the flame.

Manu should have taken the following steps:

Manu should have turned off the burner and covered the frying pan with a lid. The flame will be gradually extinguished as the contact between the flame and the oxygen is severed.

Question 33:  What are the three essential requirements to produce fire? How fire extinguisher is useful for controlling the fire. 

Solution 33:  Three essential requirements to produce gas are:

1. Oxygen essential for burning fire.
2. Combustible substance or fuel.
3. To get the ignition temperature, heat it up. 

The following are some of the ways that fire extinguishers will help you put out a fire:

1. Fire extinguishers put out a fire by cutting off the supply of oxygen or lowering the temperature of the gasoline, or both.
2. Water is the most effective fire extinguisher. Waterworks, on the other hand, just happen when objects like wood and paper catch fire. Water can conduct electricity and damage those attempting to extinguish a fire if electrical equipment is on fire. Water is also ineffective in fires involving oil or gasoline. Do you remember how much heavier water is than oil? As a result, it sinks under the oil, while the oil burns on top.
3. Carbon dioxide (CO₂) is the ideal extinguisher for fires involving electrical appliances and flammable materials such as gasoline. CO₂, being heavier than oxygen, acts as a blanket over the fire. The fire is regulated because there is no interaction between the fuel and the oxygen. CO₂ has the additional benefit of not causing damage to electrical equipment in most situations.

Question 34:  Give two examples each for a solid, liquid and gaseous fuel along with some important uses. 

Solution 34:

Solid Fuels

1. Coal- Steam engines are used to generate electricity.
2. Wood- Wood is mostly used in homes.

Liquid Fuels

1. Petrol- Used to power small vehicles such as bicycles and cars.
2. Kerosene- Used as a fuel for jet engines and for domestic heating.

Gaseous fuel

1. CNG – Automobiles operate on compressed natural gas (CNG).
2. Natural Gas – It's a material that's used in industry.

Question 35:  The calorific values of petrol and CNG are 45000 kJ/kg and 50,000 kJ/kg, respectively. If you have a vehicle which can run on petrol as well as CNG, which fuel will you prefer and why? 

Solution 35:   I prefer CNG to gasoline because it is a cleaner fuel that emits less harmful gases. As a result, air emissions caused by fuel combustion would be reduced. Since CNG has a higher calorific value than gasoline, it can be more cost effective.

Question 36:  Although wood has a very high calorific value, we still discourage its use as a fuel. Explain. 

Solution 36:   This is because wood comes from the felling of trees and woods, which has a negative impact on the climate. Cutting down trees causes a rise in CO₂ levels in the environment, resulting in global warming. Additionally, burning wood emits a variety of toxic gases such as carbon dioxide, carbon monoxide, and sulphur dioxide.

Question 37:  Forest fire produces a lot of air pollution. Write in brief about the reasons for forest fires. 

Solution 37:   The causes of forest fires are mentioned below:

• A campfire may also be the cause.
• A forest fire is started by lightning.
• As the temperature rises, dry grass will catch fire and spread across the forest.
• One of the major causes of forest fires is human negligence.

Question 38:  Complete the crossword given in the figure with the help of the clues.

Across

1. Non-metal which catches fire if exposed to air (10)
2. The lowest temperature at which a substance catches fire is called its __________ temperature. (8)
3. The most common fire extinguisher. (5)   

Down

1. A chemical process in which a substance reacts with oxygen to give off heat. (10)
2. Petrol is used as a __________ in automobiles. (4)
3. It is as hard as stone and black in colour. (4) 

Solution 38: