CBSE – 7th Standard Science


Question Bank – Chapter 6 – Physical & Chemical Changes




Q1. Classify the changes involved in the following processes as physical or chemical changes:

Ans. Physical changes are changes in the physical properties of substances, no new substances are formed in these changes and changes may be reversible but in chemical changes new substances are produced.


Q2.What is galvanization, what is its use in homes?           

Ans. Process of depositing a layer of zinc on iron is called. The iron pipes used in our homes to carry water are galvanized to prevent rusting


Q3. Why the ships are made by stainless steel?

Ans. Salt water of sea makes rusting faster and damage to ships, stainless steel is made by mixing iron with carbon and metals like chromium, nickel and manganese it does not rust thus ships base are made up by stainless steel.


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 , the bubbles formed are of carbon dioxide.  This is a chemical change. The reaction goes as Citric acid (lemon juice)+ baking sodaà carbon dioxide+ other substances


Q5. When a candle burns, both physical and chemical changes take place?

Ans. Candle melts after burning it gives light it’s a chemical change and wax remains wax after cooling that is a physical change, thus it is an example of both reactions.


Q6. How would you show that setting of curd is a chemical change?   

Ans. Formation of Lactic acid after setting of  milk into curd a new substance is formed thus it 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. Cutting of wood into small pieces no new substance is formed so it’s a physical change but after burning wood gives out heat and coal is formed thus it’s a chemical change.


Q8. Explain the crystallization with example?

Ans. Common salt is obtained by the vapourisation of sea water, but crystals of common salt are very small. When a small crystal of common salt is left dipped in the saturated solution of common salt for some time, big crystal of common salt is obtained. Formation of big and pure crystal of a substance from the saturated solution is called CRYSTALLISATION. Some substances can be obtained in pure state from their solutions by crystallization, like making salt by sea water and preparing crystals of copper sulphate.


Q9. Explain how painting of an iron gate prevents it from rusting.

Ans.. Applying a coat of paint or grease, causes a film between iron gate and water these coats should be applied regularly to prevent rusting.


Q10. Explain why rusting of iron objects is faster in coastal areas than in deserts .

Ans. For rusting, the presence of both oxygen and water  vapor is essential, in coastal area humidity is high containing water vapors this causes rusting faster while deserts are dry area thus rusting is slower.


Q11. Is  breaking down of the ozone a chemical change? What will be the effect on Earth?

Ans. Ozone breaks down in oxygen, Ozone acts as a natural shield against this radiation.

if ultraviolet radiation were not absorbed by ozone, it would reach the earth’s surface and cause harm to us and other life forms.


Q12. Describe how crystals of copper sulphate are prepared.

Ans. In a cup of water a few drops of dilute sulphuric acid is added. Heat the water upto boiling add copper sulphate powder slowly while stirring continuously, add powder till no more powder can be dissolved. Filter the solution. and cool down after some time the crystals of copper sulphate are prepared.


Q13. Why the chemical reactions are important in are lives?

Ans. Chemical changes are very important in our lives. All new substances are formed as a result of chemical changes, metals, medicines, detergents and plastics are made by these reactions.


Q14.Waht are other changes that could we found after chemical reactions?

Ans. Heat, light or radiation ultraviolet, may be given off or absorbed after chemical reaction and  Sound may be produced, change in smell and color may take place, gas may be formed after chemical reactions


Q15. Give an example where sound, heat, light and smell with formation of gas is found after chemical reaction?

Ans. Crackers are the best example of all these changes found after chemical reaction.


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