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Physical and Chemical Changes (Chemistry) Class 7 - NCERT Questions
Q 1.

Classify the changes involved in the following processes as physical or chemical changes :
(A) Photosynthesis
(B) Dissolving sugar in water
(C) Burning of coal
(D) Melting of wax
(E) Beating aluminium to make aluminium foil
(F) Digestion of food

SOLUTION:

(A) Chemical (B) Physical (C) Chemical (D) Physical (E) Physical (f) Chemical

Q 2.

State whether the following statements are true or false. In case a statement is false, write the corrected statement in your notebook.
(A) Cutting a log of wood into pieces is a chemical change. (True/False)
(B) Formation of manure from leaves is a physical change. (True/False)
(C) Iron pipes coated with zinc do not get rusted easily. (True/False)
(D) Iron and rust are the same substances. (True/False)
(E) Condensation of steam is not a chemical change. (True/False)

SOLUTION:

(A) False – Cutting a log of wood into pieces is a physical change.
(B) False – Formation of manure from leaves is a chemical change.
(C) True
(D) False – Rust is iron oxide (hydrated)
(E) True

Q 3.

Fill in the blanks in the following statements :
(A) When carbon dioxide is passed through limewater, it turns milky due to the formation of ______.
(B) The chemical name of baking soda is ________.
(C) Two methods by which rusting of iron can be prevented are ________ and ________.
(D) Changes in which only ________ properties of a substance change are called physical changes.
(E) Changes in which new substances are formed are called __________ changes.

SOLUTION:

(A) calcium carbonate (B) sodium bicarbonate (C) painting, oiling (D) physical (E) chemical

Q 4.

When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.

SOLUTION:

It is a chemical change. When baking soda is mixed with lemon juice, a gas carbon dioxide comes out and the following reaction takes place.

Q 5.

When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both the chemical and physical changes take place.

SOLUTION:

When a candle burns, first it melts which is a physical change. When the liquid is converted into vapours and mixed with air during burning, a chemical change takes place. Another example of a change in which both physical and chemical changes occur is burning of LPG gas. Liquified petroleum gas is filled in the cylinder in the form of a liquid. When these liquids come out of the cylinder in the form of vapours, physical change takes place. When these vapours are mixed with air and burn, chemical change takes place.

Q 6.

How would you show that setting of curd is a chemical change?

SOLUTION:

When milk is set to curd, the properties of milk are completely changed and a new product is formed. Curd cannot be converted back to milk, hence it is a chemical change.

Q 7.

Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

SOLUTION:

Burning of wood is a chemical change while cutting it into small pieces is a physical change.

Q 8.

Describe how crystals of copper sulphate are prepared.

SOLUTION:

A saturated solution of copper sulphate is prepared by dissolving in water and heating it. A small amount of dilute sulphuric acid is added in it to prevent hydrolysis of the solution. Saturated solution is filtered and the filtrate is kept undisturbed for some time. On cooling crystals of copper sulphate are separated from the solution.

Q 9.

Explain how painting of an iron gate prevents it from rusting.

SOLUTION:

When an iron surface is painted, iron does not come in contact with air or moisture. Paint acts as a protective layer and prevents exposure of iron to the atmosphere. Since air and water are necessary for rusting, iron does not get rusted if painted.

Q 10.

Explain why rusting of iron objects is faster in coastal areas than in deserts.

SOLUTION:

Percentage of moisture is high in coastal areas hence iron objects get both water and air for rusting. In deserts the air is dry and there is no water or moisture in the air hence rusting does not take place as fast as in coastal areas.

Q 11.

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.
(i) Process - A is a chemical change.
(ii) Process - B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.

SOLUTION:

(ii) Process - A is a physical change while process - B is a chemical change.

Q 12.

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.
(i) Process - A is a chemical change.
(ii) Process - B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.

SOLUTION:

(iii) Making of biogas and burning of biogas both are chemical changes.