NCERT Exemplar Solutions Class 7 Science Physical and Chemical Changes

Multiple Choice Questions

Question 1:  Which of the following is a physical change?

(a) Rusting of iron

(b) Combustion of magnesium ribbon

(c) Burning of candle

(d) Melting of wax 

Solution 1:  (d) Melting of wax 

Iron rusting, combustion and candle burning are chemical modifications that can not be transformed into reactants when made.

Question 2:  Which of the following is a chemical change?

(a) Twinkling of stars

(b) Cooking of vegetables

(c) Cutting of fruits

(d) Boiling of water 

Solution 2:  (b) Cooking of vegetables 

Cooking of vegetables is a chemical change as when vegetables get heated the raw ingredients cannot be regained again.

Question 3:  A chemical change may involve –

(a) change in colour only

(b) change in temperature only

(c) evolution of gas only

(d) any or all of the above 

Solution 3:  (d) any or all of the above

A chemical change can entail a change in gas, heat, etc. in colour, temperature, smell or evolution.

Question 4:  Which of the following is/are true when milk changes into curd?

1. Its state is changed from liquid to semi-solid.
2. It changes colour.
3. It changes the taste.
4. The change cannot be reversed. Choose the correct option from below :

(a) (i) and (ii) are correct

(b) (ii) and (iii) are correct

(c) (i), (iii) and (iv) are correct

(d) (i) to (iv) are correct 

Solution 4:  (c) (i), (iii) and (iv) are correct

When milk changes its form to semi-solid, its flavour turns sour, but its colour stays white, so the correct answer is (c)

Question 5:  A man painted his main gate made up of iron, to

i.  prevent it from rusting.

ii. protect it from the sun.

       iii. make it look beautiful.

       iv. make it dust-free.

Which of the above statement(s) is/are correct?

(a) (i) and (ii)

(b) (ii) and (iii)

(c) only (ii)

(d) (i) and (iii) 

Solution 5:  (d) (i) and (iii)

Painting of iron prevents Iron Gate from rusting and will enhance its beauty.

Question 6:  Iron pillar near the Qutub Minar in Delhi is famous for the following facts. Which of these facts is responsible for its long stability?

(a) It is more than 7 metres high.

(b) It weighs about 6000 kg.

(c) It was built more than 1600 years ago.

(d) It has not rusted after such a long period. 

Solution 6:  (d) It has not rusted after such a long period.

The Iron pillar of Qutub Minar is not rusted after such a long period is the fact behind popularity of Iron pillar.

Question 7:  Galvanisation is a process used to prevent the rusting of which of the following?

(a) Iron

(b) Zinc

(c) Aluminium

(d) Copper 

Solution 7:  (a) Iron

To resist rusting, iron is coated with zinc during galvanization.

Question 8:  Paheli’s mother made a concentrated sugar syrup by dissolving sugar in hot water. On cooling, crystals of sugar got separated. This indicates a –

(a) a physical change that can be reversed.

(b) the chemical change that can be reversed.

(c) a physical change that cannot be reversed.

(d) the chemical change that cannot be reversed. 

Solution 8:  (a) physical change that can be reversed.

Sugar can be extracted from a supersaturated solution here, because that is a move that is reversible.

Question 9:  Which of the following statement is incorrect for a chemical reaction?

(a) Heat may be given out but never absorbed.

(b) The sound may be produced.

(c) A colour change may take place.

(d) A gas may be evolved. 

Solution 9:  (a) Heat may be given out but never absorbed.

Heat can be consumed or gave out during chemical reaction, thus alternative (a) is a wrong assertion.

Question 10:  Two drops of dilute sulphuric acid were added to 1 g of copper sulphate powder and a then a small amount of hot water was added to dissolve it (step I). On cooling, beautiful blue coloured crystals got separated (step II). Step I and Step II are:

(a) physical and chemical changes, respectively.

(b) chemical and physical changes, respectively.

(c) both physical change.

(d) both chemical change. 

Solution 10:  (c) both physical change.  

A physical transition is the degradation of copper sulphate powder in water in the presence of sulphuric acid. The crystals of copper sulphate are isolated as this hot solution is cooled, which is also a physical transition.

Very Short Answer Questions................

Question 11:  State whether the following statements are true or false:

(a) When a candle burns, both physical and chemical changes take place.

(b) Anaerobic bacteria digest animal waste and produce biogas.

(c) Ships suffer a lot of damage though they are painted.

(d) Stretching of the rubber band is not a physical change. 

Solution 11:

1. True
2. True
3. True
4. False- Rubber band stretching is a physical transition.

Question 12:  Melting of wax is a change where a solid changes to a liquid state. Give one more such change which you observe in your surroundings. 

Solution 12:   Another example of which solid transforms to liquid is the melting of ice.

Question 13:  What kind of change is shown by tearing of paper? 

Solution 13:   There is a physical transition that is not easy to undo.

Short Answer Questions........................

Question 14:  Match the items of Column I with the items of Column II.

Solution 14:

Question 15:  Fill in the blanks in the following statements using the words given in the box.

(a) Making sugar solution is a ____________ change.

(b) A physical change is generally____________.

(c) Grinding of wheat grain changes its size. It is a ____________ change.

(d) Iron benches kept in lawns and gardens get____________. It is a _________ change because a new _________ is formed. 

Solution 15:

(a) Making sugar solution is a Physical change.

(b) A physical change is generally reversible.

(c) Grinding of wheat grain changes its size. It is a physical change.

(d) Iron benches kept in lawns and gardens get rusted. It is a chemical change because a new substance is formed.

Question 16:  Classify the following processes into physical or chemical changes:

(i) The beating of aluminium metal to make aluminium foil.

(ii) Digestion of food.

(iii) Cutting of a log of wood into pieces.

(iv) Burning of crackers. 

Solution 16:

(i) The beating of aluminium metal to make aluminium foil: Physical change

(ii) Digestion of food: Chemical change

(iii) Cutting of a log of wood into pieces: Physical change

(iv) Burning of crackers: Chemical change

Question 17:  Write word equations for two chemical reactions with the help of materials given in the box.

Solution 17:

(1) Iron + Air + Water _ Iron oxide

(2) Copper sulphate + Iron _ Iron sulphate + Copper

Question 18:  Explain the following:

(a) Lime water turns milky on passing carbon dioxide gas into it.

(b) Bubbles are produced when acetic acid is added to a solution of sodium hydrogen carbonate. 

Solution 18:

(a) Insoluble white coloured calcium carbonate is produced.

(b) Because of the chemical response between acetic acid and sodium hydrogen carbonate, carbon dioxide has formed.

Long Answer Questions...................

Question 19:  Give two examples for each of the following cases:

(a) Physical changes are reversible.

(b) Physical changes are not reversible.

(c) Chemical changes.  

Solution 19:

(a) Physical changes are reversible: Folding of paper, melting of ice

(b) Physical changes are not reversible: Tearing of paper, breaking of glass

(c) Chemical changes: Curdling of milk, cooking of food

Question 20:  Give an example of a chemical reaction for each of the following situations:

(a) A change in colour is observed.

(b) Gas is evolved.

(c) Sound is produced. 

Solution 20:                                                                                                             

(a)  The reaction between a solution of copper sulphate and an iron metal.

(b) The reaction between baking soda and vinegar (carbon dioxide is evolved).

(c) Burning of crackers.

Question 21:  If you leave a piece of iron in the open for a few days, it acquires a film of brownish substance, called rust.

(a) Do you think rust is different from iron?

(b) Can you change rust back into iron by some simple method?

(c) Do you think the formation of rust from iron is a chemical change?

(d) Give two other examples of a similar type of change 

Solution 21:

a) Rust is iron oxide which varies from iron oxide.

b) Rust formation is permanent, and may thus not be reversed by any process.

c) As iron reacts with water, iron oxide is formed from atmospheric oxygen, which is called rust. As a new compound is formed, this is a chemical transition.

(d) (i) The setting of curd from milk.

       (ii) In order to form magnesium oxide burning of magnesium ribbon.

Question 22:  A student took a solution of copper sulphate in a beaker and put a clean iron nail into it and left it for about an hour.

(a) What changes do you expect?

(b) Are these changes chemical in nature?

(c) Write a word equation for the chemical change, if any. 

Solution 22:

(a) (i) In the beaker, the colour of the solution changes from blue to green.

      (ii) There is a brown-coloured deposit on the top of the iron nail.

b) When new compounds are produced in the form of iron sulphate and copper, these changes are chemical changes.

(c)