NCERT Exemplar Class 7 Science Chapter 6 Physical and Chemical Changes

NCERT Exemplar Class 7 Science Chapter 6 Physical and Chemical Changes are part of NCERT Exemplar Class 7 Science. Here we have given NCERT Exemplar Class 7 Science Solutions Chapter 6 Physical and Chemical Changes.

NCERT Exemplar Class 7 Science Solutions Chapter 6 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:
(d) Melting of wax is a physical change. All other changes are chemical changes.

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:
(b) Cooking of vegetables is a chemical change.

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:
(d) A chemical change may involve change in colour, temperature, smell or evolution of gas, heat, etc.

Question 4.
Which of the following are true when milk changes into curd?
(i) Its state is changed from liquid to semi solid.
(ii) It changes colour.
(iii) It changes taste.
(iv) 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:
(c) When milk changes to curd it becomes semi solid. The taste of milk changes and the change is irreversible but there is no change in colour. It remains white only.

Question 5.
A man painted his main gate made up of iron, to
(i)prevent it from rusting
(ii) protect it from 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:
(d) Painting can prevent rusting and the objects look more beautiful.

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:
(d) Iron pillar near Qutub Minar in Delhi is famous for its stability because it has not rusted over a long period.

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:
(a) During galvanisation iron is coated with zinc in order to prevent it from rusting.

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) physical change that can be reversed
(b) chemical change that can be reversed
(c) physical change that cannot be reversed
(d) chemical change that cannot be reversed.
Solution:
(a): Mixing of sugar in water is a physical change and crystals of sugar can be separated from a supersaturated solution hence it is a reversible change.

Question 9.
Which of the following statements is incorrect for a chemical reaction?
(a) Heat may be given out but never absorbed.
(b) Sound may be produced.
(c) A colour change may take place.
(d) A gas may be evolved.
Solution:
(a) Heat may be given out or absorbed during a chemical reaction.

Question 10.
Two drops of dilute sulphuric acid were added to 1 g of copper sulphate powder and then small amount of hot water was added to dissolve it (step 1).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:
(c) Dissolution of copper sulphate powder in water in presence of sulphuric acid is a physical change. On cooling this hot solution, the crystals of copper sulphate are separated, This is also a physical change.

Very Short Answer Type 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 rubber band is not a physical change.
Solution:
(a) True
(b) True
(c) True
(d) False – Stretching of rubber band is a physical change.

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

Question 13.
What kind of change is shown by tearing of paper?
Solution:
Tearing of paper is a physical change which cannot be reversed.

Short Answer Questions

Question 14.
Match the items of column I with the items of column ll.

Column 1		Column II
(a)	Large crystals	(i)	Turns lime water milky
(b)	Depositing a layer of zinc on iron	(ii)	Physical change
(c)	Souring of milk	(iii)	Rust
(d)	Carbon dioxide	(iv)	Sugar candy (Mishri)
(e)	Iron oxide	(v)	Chemical change
(f)	Dissolving common salt in water	(vi)	Galvanisation

Solution:
(a) (iv)
(b) (vi)
(c) (v)
(d) (i)
(e) (iii)
(f) (ii)

Question 15.
Fill in the blanks in the following statements using the words given in the box.
[ rusted, colourful, substance, chemical, physical, reversible, iron, oxide, object ]
(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:
(a) physical
(b) reversible
(c) physical
(d) rusted, chemical, substance

Question 16.
Classify the following processes into physical or chemical changes:
(i) 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:
(i) Beating of aluminium metal to make aluminium foil – physical change
(ii) Digestion of food – chemical change
(iii) Cutting of 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.
[ Air, copper sulphate, iron, vinegar, iron oxide, carbon dioxide, iron sulphate, copper, lime water, water ]
Solution:
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:
(a) When carbon dioxide gas is passed through lime water, it turns milky because a white precipitate of calcium carbonate is formed.
Calcium hydroxide + Carbon dioxide →Calcium carbonate + Water
(b) When acetic acid is added to a solution of sodium hydrogen carbonate, carbon dioxide gas is formed which comes out in the form of bubbles.
Sodium hydrogen carbonate + Acetic acid → Sodium carbonate + Water + Carbon dioxide

Long Answer Type Questions

Question 19.
Give two examples for each of the following cases:
(a) Physical changes which are reversible.
(b) Physical changes which are not reversible.
(c) Chemical changes.
Solution:
(a) (i) Folding of paper
(ii) Melting of ice
(b) (i) Cutting of paper
(ii) Breaking of glass
(c) (i) Burning of paper
(ii) Curdling of milk

Question 20.
Give an example of a chemical reaction for each of the following situations:
(a) A change in colour is observed.
(b) A gas is evolved.
(c) Sound is produced.
Solution:
(a) Reaction between copper sulphate and iron metal – blue solution of copper sulphate changes to green solution of ferrous sulphate.
(b) Reaction between Sodium hydrogen carbonate and acetic acid – carbon dioxide gas is evolved.
(c)Burning of crackers produces a loud sound.

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 formation of rust from iron is a chemical change?
(d) Give two other examples of a similar type of change.
Solution:
(a) Yes, rust is different from iron. It is a brown powder of iron oxide.
(b) No rusting is a chemical change and cannot be reversed.
(c) Yes, rusting is a chemical change since a new substance is formed.
(d) (i) Burning of magnesium ribbon to give magnesium oxide.
(ii) Burning of wood to give coal and ash.

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:
(a) Copper sulphate solution is blue in colour. When an iron nail is left in it for some time the blue colour of copper sulphate solution changes to green and the surface of iron nail gets a brown coloured deposit.
(b) Yes, the changes are chemical changes because new substances iron sulphate (green) and copper (brown) are formed in the reaction.

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