NCERT PRACTICE QUESTIONS
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
Ans.- a) Chemical change b) Physical Change c) Chemical change d) Physical Change e) Physical Change f) 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. (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)
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.
Ans.- (a) Calcium carbonate. (b) Sodium hydrogen carbonate. (c) galvanization and painting (d) physical
(e) chemical
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.- This is a chemical change, reaction can be given as-
Lemon juice + Baking soda → Carbon dioxide + other substances
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.
Ans.- (a) Physical change is melting of wax and chemical change is the burning of gas with the evolution of CO2
(b) Physical change is the breakdown of larger food molecules to simpler ones. And chemical change is the digestion of food by the action of HCl and enzymes.
6. How would you show that setting of curd is a chemical change?
Ans.- The setting of curd is a chemical change as curd cannot be turned back to milk. The properties of milk and curd are different so we can say new product is formed.
7. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.
Ans.- 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 gas.
8. Describe how crystals of copper sulphate are prepared.
Ans.-
ü Take a cupful of water in a beaker.
ü Add a few drops of dilute sulfuric acid to this.
ü Heat the water and when it starts boiling, add copper sulphate powder while still stirring.
ü Add the copper sulphate powder till the solution becomes saturated. Filter into a china dish and allow it to cool.
ü The solution should be kept undisturbed. Slowly, the crystals of copper sulphate separate out.
9. Explain how painting of an iron gate prevents it from rusting.
Ans. - Rusting of iron requires contact with water (moisture) and oxygen. By painting an iron gate, we prevent the contact between iron, atmospheric Oxygen and water (moisture), this helps in preventing rusting of iron.
10. Explain why rusting of iron objects is faster in coastal areas than in deserts.
Ans. - Rusting of iron requires contact with water (moisture) and oxygen. In coastal areas, humidity is more in comparison to that in deserts; hence, rusting of iron objects is faster in coastal areas than in deserts.
11. The gas we use in the kitchen is called liquefied 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
Ans. - (ii) Process – B 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 chang
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change
Ans. - (iii) Both processes A and B are chemical changes.
Junior Science Topic - Physical And Chemical Changes. Chapter notes, video lectures, power point presentation and NCERT exercise solution. (Class 7, CBSE syllabus, chapter 06)
By - EduPorm India - The Educational Platform
.png)
){kind=link}