Physical and Chemical Changes(NCERT Solutions,Class 7)

(1)Classify the changes involved in the following processes as physical or chemical changes:

(a) Photosynthesis

(b) Dissolving sugar in water

(d) Melting of wax

(e) Beating aluminium to make aluminium foil

(f) Digestion of food

Ans-

The changes involved in the following processes as physical or chemical changes are given below:

S.N.	Processes	Changes
(a)	Photosynthesis	Chemical Change
(b)	Dissolving sugar in water	Physical Change
(c)	Burning of coal	Chemical Change
(d)	Melting of wax	Physical Change
(e)	Beating aluminium to make aluminium foil	Physical change
(f)	Digestion of food	Chemical Change

(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 leaes is a physical change. (True/ False)

(d) Iron and rust are the same substances. (True/ False)

(e) Condensation of steam is not a chemical change. (True/ False)

Ans-

(a) Cutting a log of wood into pieces is a chemical change. (False)

(b) Formation of manure from leaves is a physical change. (False)

(d) Iron and rust are the same substances. (False)

(e) Condensation of steam is not a chemical change. (True)

(3) Fill in the blanks in the following statements:

(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of …..

(b) The chemical name of baking soda is….

(d) Changes in which only …. properties of a substance change are called physical changes.

(e) Changes I which new substances are formed are called ……. changes.

Ans-

(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of calcium carbonate.

(b) The chemical name of baking soda is sodium hydrogen carbonate.

(d) Changes in which only physical properties of a substance change are called physical changes.

(e) Changes in which new substances are formed are called Chemical changes.

(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?

Ans-

Baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. This gas is carbon dioxide. It is a chemical change because a new substance, ie, carbon dioxide produced during the chemical process.

Lemon juice (Citric acid) + Baking soda (Sodium hydrogen carbonate) → Carbon dioxide + other substances

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

Ans-

Physical and chemical changes of a burning candle is given in table below:

S.N.	Effect of candle burning	Physical/Chemical changes
1.	Melting of wax	Physical change
2.	Production of heat	Chemical change
3.	Production of light	Chemical change
4.	Production of gas(Co₂)	Chemical change

Physical and chemical changes of making paper are given in table below:

S.N.	Process	Physical/Chemical changes
1.	Pulping	Chemical change
2.	Bleaching	Chemical change
3.	Sheeting	Physical change
4.	Pressing	Physical change
5.	Drying	Physical change

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

Ans-

The setting of curd is a chemical change because milk can not be get back from the curd.

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

Ans-

Burning of wood is a chemical change. Reasons are giving below:

(i) New substance ash is formed which is irreversible to wood.

(ii) Heat is produced when it burns.

(iii) Light is produced when it burns.

(iv) Smoke or gas is produced when it burns.

(v) Sound is also produced when it burns.

Cutting it into small pieces is a physical change. Reasons are giving below:

(i) Small pieces are pieces of wood also.

(ii) Size of wood changed. It becomes small.

(iii) Shape of wood changed. Any shape can be get from the log.

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

(8)Describe how crystals of copper sulphate are prepared.

Ans-

Add a few drops of dilute sulphuric acid in a cupful of water. Heat the water. Add copper sulphate when it stars boiling. Stir it continuously. Add copper sulphate powder continuously in the water till it stops dissolving. Filter the solution. Allow it to cool. After some time you can see the crystal of copper sulphate.

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

Ans–

Iron gate should be painted regularly to prevent rusting. It will prevent iron from coming in contact with oxygen and water.

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

Ans-

Rusting of iron objects is faster in coastal areas than in deserts because coastal areas have more humidity than desert areas. Oxygen and water present in the moisture acted with iron and deposit rust on iron.

(11)The gas we use in the kitchen is called liquefied 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.

Ans-

(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.

(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.

Ans-

(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.

Helping Topics

Physical and chemical changes

Effect of Magnesium hydroxide on litmus paper