Chapter 6 Physical and Chemical Changes

Class 7 | Science | Chapter 6 | Physical and Chemical Changes | NCERT Solutions

Question 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

Answer 1 (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

Question 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) Keywords Chemical change Chemical reaction Crystallisation Galvanisation Physical change Rusting 2018-19 66 SCIENCE
(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)

Answer 2 (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)

Question 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 _________.

(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.

Answer 3 (a) When carbon dioxide is passed through lime water, it turns milky due to the formation of Calcium carbonate (CaCO3).

(b) The chemical name of baking soda is Sodium bicarbonate (NaHCO3).

(c) Two methods by which rusting of iron can be prevented are greasing 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.

Question 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.

Answer 4 Baking soda reacts with lemon juice to form Carbon dioxide , water and salt.The bubbles are due to the formation of carbon dioxide.This is a chemical change as new products are formed.

Question 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.

Answer 5 When a candle burns, both physical and chemical change occur:

1) Physical change: On heating, candle wax gets melted , is a physical change. Since it again turns into solid wax on cooling.

2) Chemical change: The wax burns to gives new substances like Carbon Dioxide, Carbon soot, water vapour, heat and light.

Cooking of food, LPG comes out of cylinder are the examples where both the chemical and physical changes take place.

Question 6. How would you show that setting of curd is a chemical change?

Answer 6 Setting of curd is a chemical change because

1) Curd cannot be converted into milk again.

2) A new substance is formed which differs in taste and chemical properties from the original substance

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

Answer 7 Burning of wood is a chemical change because wood on burning gives new substances like ash (carbon), carbon dioxide gas, water vapour, heat and light. The change is irreversible.

Cutting of wood in smaller pieces is a physical change because the original composition of wood does not change. No new substances are formed.In this only the size of the wood is reduced.

Question 8. Describe how crystals of copper sulphate are prepared.

Answer 8 Take a clean beaker (250 ml) and put the powdered sample of copper sulphate in it. Add distilled water and stir well using glass rod.
2. Make the solution more clearly by adding 2 – 3 drops of concentrated sulphuric acid in it and heat up 70° C then add more impure copper sulphate until no more of it dissolves.
3. Filter the solution and collect the filtrate in a china dish heat it over wire gauze kept on tripod with proper stirring to get constant temperature till it becomes half of the taken, cool the other end of glass rod by blowing air.
4. Formation of thin crust indicates that crystallization points is reached, then turn off the burner and cover it with a watch glass without disturbing it. As the solution cools down, crystals separate out. Slow cooling ensures for better crystallization.
5. Decant the mother liquor and wash the crystals with a thin stream of cold water with the help of a wash bottle press these between filter paper sheets. Then blue coloured copper sulphate crystals will be formed.

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

Answer 9 Both oxygen and water is required for rusting of iron.The painting prevents direct contact of iron with air and oxygen and thus prevents it from rusting.

Question 10. Explain why rusting of iron objects is faster in coastal areas than in deserts.

Answer 10 Rusting take place in the presence of oxygen and water.As moisture content is higher in coastal area than in deserts therefore the process of rusting is faster in coastal areas than in deserts.

Question 11. 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.

Answer 11 (ii) Process – B is a chemical change.

Question 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.

Answer 12 (iii) Both processes A and B are chemical changes.

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