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NCERT Solutions for Class 7: Chapter 6 – Physical and Chemical Changes

NCERT Solutions for Class 7, Chapter 6 (Physical & Chemical Changes) is available here. With this article, you can also download PDF of Class 7 Science NCERT textbook in PDF format.

Jun 24, 2019 18:39 IST
NCERT Solutions for Class 7 – Chapter 6: Physical and Chemical Changes

NCERT Solutions for Class 7 Science, Chapter 6 - Physical & Chemical Changes is available here. You can also download Chapter 6 of Class 7 Science NCERT textbook in PDF format.

NCERT Solutions for Class 7 Science, Chapter 6 - Physical & Chemical Changes:

Question1. 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

Solution1:

Processes

Types of Changes

Photosynthesis

Chemical change

Dissolving sugar in water

Physical change

Burning of coal

Chemical change

Melting of wax

Physical change

Beating aluminium to make aluminium foil

Physical change

Digestion of food

Chemical change

Question2. 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)

Solution2:

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

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

(c) Iron pipes coated with zinc do not get rusted easily. (True)

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

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

Question3. 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 _________.

(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.

Solution3:

(a) When carbon dioxide is passed through limewater, it turns milky due to the formation of ___calcium carbonate__ .

(b) The chemical name of baking soda is ___sodium hydrogen carbonate___.

(c) Two methods by which rusting of iron can be prevented are __ painting__ and __ galvanization__.

(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.

Question4. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.

Solution4:

The bubbles are basically due to the formation of carbon dioxide gas. On mixing baking soda with lemon juice, they react with each other to form carbon dioxide (and other products).

Question5. 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.

Solution5:

• Melting of wax: Physical Change

• Burning of wax: Chemical Change

Digestion of food is another example, in the process; food is physically broken down into smaller pieces by chewing & also chemically broken down by enzymes in the saliva & acid in the stomach.

Question6. How would you show that setting of curd is a chemical change?

Solution6:

We can get milk (original substance) back from curd; also the new substance (curd) has different chemical properties, taste, smell etc.

Question7. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

Solution7:

When we cut wood only its shape and size is changed and no new substance is formed. But, when we burn wood, a new substance is formed.

Question8. Describe how crystals of copper sulphate are prepared.

Solution8:

Crystallization is the process by which we can obtain crystals of copper sulphate. In this process, a few drops of dilute sulphuric acid is added to water.  The water is then boiled & as it starts boiling, copper sulphate powder is added with stirring. The powder must be added on till the solution becomes saturated. Afterwards, it is filtered into a china dish and allowed to cool, undisturbed. After some time the crystals of copper sulphate separate out.

Question9. Explain how painting of an iron gate prevents it from rusting.

Solution9:

Rusting of iron happens when it comes in contact with air and moisture. By painting the iron gate, we basically we are preventing the contact of water and air from iron.

Question10. Explain why rusting of iron objects is faster in coastal areas than in deserts.

Solution10:

The presence of moisture in the air promotes rusting of iron. The quantity of moisture in coastal areas is quite high, as compared to deserts, where moisture content in the air is very low.

Question11. The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder it exist 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.

Solution11:

Option (ii) is correct.

Question12. 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.

Solution12:

Option (iii) is correct.

Download Chapter 6 (Physical & Chemical Changes) of Class 7th Science NCERT book in PDF format