Physical and Chemical Changes Class 7 Notes NCERT and MCQs

30-08-2023 18:39 PM IST Priyanka Chaudhary

The chapter discusses physical change, chemical change and rusting of iron.

What is a Physical Change?

Physical change is a change in which a substance undergoes a change in its physical properties. Properties such as shape, size, colour and state of a substance are called its physical properties. 

Examples of Physical Change

a) Making a Sugar solution

b) Cutting of paper

c) Melting of Wax

d) Melting of Ice

e) Boiling of water

f) Breaking of bottle

g) Crushing a can

What is a Chemical Change?

A Chemical change is a change in which one or more new substances are formed. A chemical change is also called a chemical reaction. In addition to formation of new products, the following may accompany a chemical change:

• Heat, light or any other radiation (ultraviolet, for example) may be given off or absorbed.
• Sound may be produced.
• A change in smell may take place or a new smell may be given off.
• A color change may take place.
• A gas may be formed.

Burning of any substance is a chemical change.

Examples of Chemical change

a) Explosion of firework

b) Food gets spoiled

c) Rancidification of butter

d) Burning of magnesium ribbon- It burns with a brilliant white light and leaves a powdery ash.

Magnesium (Mg) + Oxygen (O₂) → Magnesium oxide (MgO)

Magnesium oxide (MgO) + Water (H₂O) → Magnesium hydroxide [Mg(OH)₂] – It turns red litmus to blue as it is a base.

e) Burning of wax

f) Burning of coal

g) Burning of wood

h) Fermentation of grapes

i) Ripening of fruits

j) Digestion of food

k) Photosynthesis

l)  Rusting of Iron

m) Breaking down of ozone is also a chemical change. Ozone layer protects us from the harmful ultraviolet radiation which comes from the sun. Ozone absorbs this radiation and breaks down to oxygen. Ozone acts as a natural shield against UV radiation.

n) Copper sulphate solution (blue) + Iron → Iron sulphate solution (green) + Copper (brown deposit)

o) Vinegar (Acetic acid) + Baking soda (Sodium hydrogencarbonate) → Carbon dioxide + other substances

Carbon dioxide (CO₂) + Lime water [Ca(OH)₂] →  Calcium Carbonate (CaCO₃) + Water (H₂O)

When carbon dioxide is passed through lime water, calcium carbonate is formed, which makes lime water milky.

Rusting of Iron

The process of rusting can be represented by the following equation:

Iron (Fe) + Oxygen (O₂, from the air) + water (H₂O)  →  rust (iron oxide Fe₂O₃)

For rusting, the presence of both oxygen and water (or water vapour) is essential. 

If the content of moisture in the air is high i.e. if it is more humid, rusting becomes faster.

Ways to prevent rusting

We can prevent rusting of iron articles by preventing them from coming in contact with oxygen, or water, or both. Rusting can also be prevented by applying a coat of paint or grease.

Another way to prevent rusting is to deposit a layer of metal like chromium or zinc on iron. This process of depositing a layer of zinc on iron is called galvanization.

The iron pipes we use in our homes to carry water are galvanized to prevent rusting.

Why do Ships suffer a lot of damage from rusting in spite of being painted?

This is because ships are made of iron and part of them remains under water. On the part above water, water drops keep clinging to the ship’s outer surface. Apart from this, salts present in sea water make the rusting process faster.

Crystallization

It is the process of obtaining crystals of substances from their solutions. It is an example of a physical change.

Crystallization of Copper Sulphate

1. Take a cupful of water in a beaker and add a few drops of dilute sulphuric acid.

2. Heat the water. When it starts boiling add copper sulphate powder slowly while stirring continuously. Add the powder till no more powder can be dissolved. Filter the solution.

3. Allow it to cool down and do not disturb the solution. Finally, you will see the crystals of copper sulphate.

MCQs based on NCERT Class 7 Science Chapter 6: Physical and Chemical Changes

1. Crystallization or process of obtaining crystals of substances from their solutions is an example of a

a. Chemical change

b. Physical change

c. Both chemical and physical change

d. Neither chemical nor physical change

Ans. b

Explanation: Crystallization or process of obtaining crystals of substances from their solutions is an example of a physical change. Physical change is a change in which a substance undergoes a change in its physical properties such as shape, size, colour and state of a substance. 

2. Which of the following may accompany a chemical change?

a. Formation of new products

b. Production of sound

c. Change in color

d. All of the above

Ans. d

Explanation: In addition to formation of new products, the following may accompany a chemical change:

• Heat, light or any other radiation (ultraviolet, for example) may be given off or absorbed.
• Sound may be produced.
• A change in smell may take place or a new smell may be given off.
• A color change may take place.
• A gas may be formed.

3. Which of the following is not a physical change?

a. Photosynthesis

b. Melting of wax

c. Dissolving sugar in water

d. Cutting of wood

Ans. a

Explanation: Photosynthesis is a chemical change as the new products are formed in this process. 

4. The chemical name of baking soda is ________.

a. Sodium hydrogen carbonate

b. Sodium carbonate

c. Potassium carbonate

d. Calcium hydrogen carbonate

Ans. a

Explanation: The chemical name of baking soda is Sodium hydrogen carbonate or Sodium bicarbonate. The formula of baking soda is NaHCO₃.

5. The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder, it exists as a liquid. When it comes out from the cylinder it becomes a gas (Change – A) then it burns (Change – B). The following statements pertain to these changes. Choose the correct one.

a. Process – A is a chemical change.

b. Process – B is a chemical change.

c. Both processes A and B are chemical changes.

d. None of these processes is a chemical change.

Ans. b

Explanation: When LPG comes out in the form of gas, there is only change in the state, hence process A is a physical change. When LPG burns, gas is evolved in the process, hence process B is a chemical change. 

6. Anaerobic bacteria digest animal waste and produce biogas (Change – A). The biogas is then burnt as fuel (Change – B). The following statements pertain to these changes. Choose the correct one.

a. Process – A is a chemical change.

b. Process – B is a chemical change. 

c. Both processes A and B are chemical changes.

d. None of these processes is a chemical change.

Ans. c

Explanation: Digestion by anaerobic bacteria is a chemical change as a new product- biogas is produced. When biogas is burnt, gases like Carbon dioxide are evolved. Hence, process B is also a chemical change.