6. Physical and Chemical Changes Class 7 Science Solutions English Medium-NCERT Exercise

Exercise 

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Q1. 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:

(a) chemical change 

(b) physical change

(c) chemical change

(d) physical change

(e) physical change

(f) 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)

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

(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:

(a) False  (b) False  (c) True  (d) False  (e) 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 _________.

(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:

(a) Calcium carbonate

(b) Sodium hydrogen carbonate

(c) galvinisation and painting

(d) Physical

(e) 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.

Answer:

It is chemical change. Here, a new substance, carbon dioxide gas is formed. 

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.

Answer: 

Process1: burning of coal

Physical change: Melting of candle form physical change. 

Chemical change: when we burn candle , smoke and carbon-di-oxide are formed which are new subtance .Thus,it is chemical change.

Process2: Digestion of food

Physical change: Breaking of food in large particles.

Chemical change: Breakdown of food into small particles by the action of digestive exzymes is chemical change.  

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

Answer:

Setting of curd is a chemical change beacouse this reaction is not reversible and thus milk can not be obtaned from curd.

Q7. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

Answer:

Burning of wood produses ash and smoke. hence the properties of wood are changed and new substance are formed. so, it is a chemical change.

when a log of wood is cut into small pieces, no new substance is formed there. each small piece bears the properties of wood. so, it is a physical change.

Q8. Describe how crystals of copper sulphate are prepared.

Answer: 

The process of getting pure and big crystalls form its solution is called crystallisation in copper sulfate - few drops of water and boiled. when it starts boiling then copper sulfate powder is mixed into until no more copper sulfate powder dissolve into it. when this solution cools downs, then we get copper sulfate crystals.

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

Answer:

Painting prevents the iron gate from rusting as the painted iron gate do not come in contact of water and air (oxygen) which are necessary for rusting.

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

Answer:

Rusting of iron object is faster in coastal area is very humid where as air in desert area is dry.

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 

Answer:

(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.

Answer:

(iii) Both process A and B are chemical change.