NCERT Class 7 Science Chapter 6 - Physical and Chemical Change

Chapter 6 - Physical and Chemical Change

Question and Answers
Question 1: Classify the changes involved in the following processes as physical or chemical 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
 
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. (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 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.
 
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: When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a carbon dioxide gas. This is a chemical change.
Lemon juice + Baking soda → Carbon dioxide + other substances
 
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:
Candle Burning
1. Physical change is melting of wax.
2. The chemical change is the burning of gas with the evolution of CO2.
Digestion of Food
1. Physical change is the breakdown of larger food molecules to simpler ones
2. The chemical change is the digestion of food by the action of HCl and enzymes.
 
Question 6: How would you show that setting of curd is a chemical change?
Answer: The setting of curd is a chemical change because curd cannot be turned back to milk. The properties of milk and curd are different.
 
Question 7: Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.
Answer: Cutting of wood is a physical change as it does not change the nature of the wood. On the other hand, burning of wood is a chemical change as wood is converted to charcoal with the liberation of CO2.
 
Question 8: Describe how crystals of copper sulphate are prepared.
Answer: Crystals of copper sulphate are prepared using the crystallization method, which is described as follows
1. Take a cupful of water in a beaker.
2. Add a few drops of dilute sulphuric acid to this.
3. Heat the water and when it starts boiling, add copper sulphate powder while still stirring.
4. Add the copper sulphate powder till the solution becomes saturated. Filter into a china dish and allow it to cool.
5. The solution should be kept undisturbed. Slowly, the crystals of copper sulphate separate out.
 
Question 9: Explain how painting of an iron gate prevents it from rusting
Answer: Rusting of iron requires contact with water (moisture) and oxygen. By painting an iron gate, we prevent the contact between iron, Oxygen and water (moisture), this helps in preventing rusting of iron.
 
Question 10: Explain why rusting of iron objects is faster in coastal areas than in deserts.
Answer: Rusting of iron requires contact with water (moisture) and oxygen. In coastal areas, humidity is more in comparision to that in deserts; hence, rusting of iron objects 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: (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: (iii) Both processes A and B are chemical changes.