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NCERT Solutions for Class 8 Science Chapter 5 - Physical and Chemical Changes

Exercise

Q1. Classify the changes involved in the following processes as physical or chemical changes:
(a) Photosynthesis
Ans: Chemical change

(b) Dissolving sugar in water
Ans: Physical change

(c) Burning of coal
Ans: Chemical change

(d) Melting of wax
Ans: Physical change

(e) Beating aluminium to make aluminium foil
Ans: Physical change

(f) Digestion of food
Ans: Chemical change


Q2. 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)
Ans: False

(b) Formation of manure from leaves is a physical change. (True/False)
Ans: False

(c) Iron pipes coated with zinc do not get rusted easily. (True/False)
Ans: True

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

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

 

Q3. 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 _________.
Ans: Calcium carbonate.

(b) The chemical name of baking soda is _________.
Ans: Sodium hydrogen carbonate.

(c) Two methods by which rusting of iron can be prevented are _________ and _________.
Ans: Polishing, Glavanisation.

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

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


Q4. 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: When baking soda is mixed with lemon juice, bubbles formed because of evolution of carbon dioxide gas. This is a chemical change, because in this reaction new substances are formed.
Lemon juice + Baking soda → Carbon dioxide + other substances

 

Q5. 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: In burning of candle some of the wax melts, which is physical change, while most of the wax burnt, which is a chemical change.
In the burning of wood, water present in wood changes into vapour, this is physical change, while burning of wood is a chemical change.

 

Q6. How would you show that setting of curd is a chemical change?
Ans: In the setting of curd, milk changes into new substance, curd. This is an irreversible process that means milk cannot get back from curd. Formation of new substance and irreversible process shows that setting of curd is a chemical change.

 

Q7. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.
Ans: In burning of wood, new substances are formed and hence is a chemical change. While cutting the wood into small pieces no new substance is formed, thus it is a physical change.
Hence, burning of wood and cutting it into small pieces are considered as two different types of changes.

 

Q8. Describe how crystals of copper sulphate are prepared.
Ans: Process:

  • Water is taken in a beaker.
  • Few drops of sulphuric acid is added to the water.
  • Water is let to boil.
  • When water starts boiling, copper sulphate powder is added to the water with continuous stirring the solution.
  • When copper sulphate starts deposited into bottom of the beaker, then solution is filtered and left for cooling.
  • After cooling, the crystals of copper sulphate is prepared.


Q9. Explain how painting of an iron gate prevents it from rusting.
Ans: Iron get rusted because of reaction with oxygen present in moist air. By painting iron gate, iron is prevented to come into contact with oxygen present in moist air, which prevent it be getting rusted.

 

Q10. Explain why rusting of iron objects is faster in coastal areas than in deserts.
Ans: Iron get rusted because of reaction with oxygen present in moist air. In coastal areas, air contains more moisture because of sea or ocean, while in deserts air is dry and hot. Thus because of getting more moist air iron objects getting rusted faster in coastal areas than in deserts.

 

Q11. 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.
Ans: (ii) Process – B is a chemical change.


Q12. 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: (iii) Both processes A and B are chemical changes.

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FAQs on NCERT Solutions for Class 8 Science Chapter 5 - Physical and Chemical Changes

1. What are physical changes?
Ans. Physical changes are those changes that alter the physical form of a substance but not its chemical composition. For example, melting, freezing, boiling, evaporation, condensation, sublimation, etc., are all physical changes.
2. What are chemical changes?
Ans. Chemical changes are those changes that alter the chemical composition of a substance, resulting in the formation of a new substance. For example, burning, rusting, digestion, fermentation, etc., are all chemical changes.
3. What is the difference between physical and chemical changes?
Ans. The main difference between physical and chemical changes is that physical changes only alter the physical form of a substance, while chemical changes alter the chemical composition of a substance, resulting in the formation of a new substance. Physical changes are usually reversible, while chemical changes are usually irreversible.
4. What are some examples of physical changes?
Ans. Some examples of physical changes include melting of ice, boiling of water, dissolving sugar in water, cutting a piece of paper, etc.
5. What are some examples of chemical changes?
Ans. Some examples of chemical changes include burning of wood, rusting of iron, digestion of food, fermentation of grapes to make wine, etc.
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