Maharashtra Board Class 9 Science Chapter 4 Measurement of Matter PDF Download

Measurement Of Matter

Laws Of Chemical Combination, Atom - Shape, Mass, Valency, Molecular Mass And The Concept Of Mole, Radicals

Can You Recall?

1. What is the Dalton's atomic theory?

2. How are the compounds formed?

3. What are the molecular formulae of salt, slaked lime, water, lime, limestone?

In the previous standard we have learnt that compounds are formed by chemical combination of elements. We have also learnt that an important principle of Dalton's atomic theory is that molecules of a compound are formed by joining atoms of different elements.

The composition of a substance changes during a chemical change. The fundamental experiments in this regard were performed by scientists in the 18th and 19th century. While doing this, they measured accurately, the substances used and formed. The scientists Dalton, Thomson and Rutherford studied the structure of substances and the atom and thus discovered the laws of chemical combination. Scientists could then write the molecular formulae of various compounds on the basis of Dalton's atomic theory and the laws of chemical combination. Here we shall verify the laws of chemical combination by means of known molecular formulae.

Activity 1

Take 56 g calcium oxide in a large conical flask and put 18 g water in it.

Observe what happens.

Measure the mass of the substance formed.

What similarity do you find? Write your inference.

Activity 2

Take a solution of calcium chloride in a conical flask and a solution of sodium sulphate in a test tube.

Tie a thread to the test tube and insert it in the conical flask.

Seal the conical flask with an airtight rubber cork.

Weigh the conical flask using a balance.

Now tilt the conical flask so that the solution in the test tube gets poured in the conical flask.

Now weigh the conical flask again.

Try This

Apparatus: Conical flask, test tubes, balance, etc.

Chemicals: Calcium chloride (CaCl\(_2\)), sodium sulphate (Na\(_2\)SO\(_4\)), calcium oxide (CaO), Water (H\(_2\)O)

Teacher's Note

When you mix calcium oxide and water, the temperature increases a lot. In India, people use this reaction to make hot water for bathing in winter.

Exam Trick

Remember: In Activity 1 and 2, the total mass before the reaction equals the total mass after the reaction. This is the Law of Conservation of Matter - mass is never lost or gained.

Points To Remember

Chemical change is when substances become new substances.
Dalton's atomic theory explains how atoms combine to form compounds.
Laws of chemical combination tell us how elements join together.
In chemical reactions, mass is always conserved.
Scientists use experiments to find the laws of nature.

Laws Of Chemical Combination

In the above activities, the mass of the original matter and the mass of the matter newly formed as a result of the chemical change are equal. In 1785, the French Scientist Antoine Lavoisier inferred from his research that 'there is no rise or drop in the weight of the matter during a chemical reaction.' In a chemical reaction the total weight of the reactants is same as the total weight of the products formed due to the chemical reactions and this is called the law of conservation of matter.

Law Of Conservation Of Matter

In the above activities, the mass of the original matter and the mass of the matter newly formed as a result of the chemical change are equal. In 1785, the French Scientist Antoine Lavoisier inferred from his research that 'there is no rise or drop in the weight of the matter during a chemical reaction.' In a chemical reaction the total weight of the reactants is same as the total weight of the products formed due to the chemical reactions and this is called the law of conservation of matter.

Law Of Constant Proportion

In 1794 the French scientist J. L. Proust stated the law of constant proportions as "The proportion by weight of the constituent elements in the various samples of a compound is fixed," e.g., the proportion by weight of hydrogen and oxygen in water is 1:8. This means that 9 g water is formed by chemical combination of 1 g hydrogen and 8 g oxygen. Similarly, the proportion by weight of carbon and oxygen in carbon dioxide is 3:8. For example, in 44 g of carbondioxide there is 12 g of carbon and 32 g oxygen so that the proportion by weight of carbon and oxygen is 3:8.

An Introduction To Scientists

Antoine Lavoisier (1743-94)

French scientist, father of modern chemistry, substantial contribution in the fields of chemistry, biology and economics.

Nomenclature of oxygen and hydrogen.

Showed that matter combines with oxygen during combustion.

Was the first to use accurate weighing techniques to weigh reactants and products in a chemical reaction.

Discovered that water is made up of hydrogen and oxygen.

Assigned systematic names to the compounds, e.g. sulphuric acid, sulphate, sulphite.

Author of the first book on modern chemistry.

Studied the elements such as oxygen, hydrogen, nitrogen, phosphorus, mercury, zinc and sulphur.

First writer of the law of conservation of mass, in a chemical reaction.

Teacher's Note

The Law of Conservation of Matter means that nothing disappears in a chemical reaction. When you burn wood, it becomes ash and smoke, but the total weight stays the same if you could catch all the smoke.

Exam Trick

Remember: Conservation = nothing is lost. Just like your pocket money - if you spend 10 rupees from 50 rupees, you have 40 rupees left. The total is always conserved.

Points To Remember

Mass of reactants = Mass of products in any chemical reaction.
Antoine Lavoisier discovered this law in 1785.
Water is always made of hydrogen and oxygen in a fixed ratio of 1:8 by weight.
Different samples of the same compound always have the same composition.
J. L. Proust discovered the Law of Constant Proportion in 1794.

Atom: Size, Mass And Valency

Can You Recall?

1. From which experiments was it discovered that atoms have an internal structure? When?

2. What are the two parts of an atom? What are they made up of?

We have learnt that at the centre of an atom is the nucleus and that there are moving electrons in the extra-nuclear part. The electrons are negatively charged elementary particles while the elementary particles that make up the nucleus are positively charged protons and electrically neutral neutrons. Look at the image of an atom obtained with a field ion microscope.

Approximate Size Of Atom

\(\frac{1}{10^9} \text{ m} = 1 \text{ nm}\)

\(1 \text{ m} = 10^9 \text{ nm}\)

The size of an atom is determined by its radius. The atomic radius of an isolated atom is the distance between the nucleus of an atom and its outermost orbit. Atomic radius is expressed in nanometres.

Atoms are very very tiny. Modern instruments like the electron microscope, field ion microscope, scanning tunneling microscope have the capacity to show enlarged images of the atom.

The atomic size depends on the number of electron orbits in the atom. The greater the number of orbits the larger the size. For example, an atom of K is bigger than an atom of Na. If two atoms have the same outermost orbit, then the atom having the larger number of electrons in the outermost orbit is smaller than the one having fewer electrons in the same outermost orbit. For example an atom of Mg is smaller than an atom of Na.

Teacher's Note

Atoms are so small that we cannot see them with our eyes. Scientists had to invent special microscopes to see atoms. This shows how tiny the building blocks of matter are.

Exam Trick

Remember: More electron orbits = larger atom. Think of a building - more floors mean it is taller. Potassium has more orbits than Sodium, so Potassium atom is bigger.

Points To Remember

Atoms are extremely small and measured in nanometres.
The nucleus is at the centre of the atom.
Electrons move around the nucleus in orbits.
Protons and neutrons are inside the nucleus.
Atoms with more orbits are larger than atoms with fewer orbits.

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