Class 7 Science
Chapter 6 Physical and Chemical Changes (NCERT Solutions)
1 Classify the changes involved in the following processes as physical or chemical changes:
(a) Photosynthesis
Answer-chemical changes
(b) Dissolving sugar in water
Answer Physical change
(c) Burning of coal
Answer chemical changes
(d) Melting of wax
Answer Physical change
(e) Beating aluminum to make aluminum foil
Answer Physical change
(f ) Digestion of food
Answer chemical changes
2. 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)
Answer false
(b) Formation of manure from leaves is a physical change. (True/False)
Answer :- false
(c) Iron pipes coated with zinc do not get rusted easily. (True/False)
Answer :- true
(d) Iron and rust are the same substances. (True/False)
Answer:- false
(e) Condensation of steam is not a chemical change. (True/False)
Answer :- true
3 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 ……calcium carbonate(CaCO3)….
. (b) The chemical name of baking soda is ...........Sodium hydrogen carbonat
(c) Two methods by which rusting of iron can be prevented are ………….painting ……. and ……….galvanisation…………
(d) Changes in which only __.....physical……_____ properties of a substance change are called physical changes.
(e) Changes in which new substances are formed are called _...chemical……__ changes.
4. 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 :- When baking soda is mixed with lemon juice, bubbles are formed with the evolution of carbon di oxide (CO2)gas. This is chemical change .
Lemon juice + baking soda → chemical substance + carbon di oxide
5. 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 : -When a candle burns, both physical and chemical changes occur:
(i) Physical change: melting and vapourisation of melted wax.
(ii) Chemical change: Burning of vapours of wax to give a carbon dioxide
LPG is another example in which physical change occurs
when LPG comes out of cylinderin the form of liquid at high pressure and low temperature and when it used its converted from liquid to gaseous state and a chemical change occurs when gas burns in air with formation of carbon di oxide and water
6. How would you show that setting of curd is a chemical change?
Answer :- We can say that setting of curd (sour in test) from milk (sweet in test) is a chemical change because we can not get the original substance, i.e., milk (sweet in test )back and a new substance is formed with different taste, smell and other chemical properties
7. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes
Answer :- Burning of wood in the presence of air is a chemical change because in burning new substances are formed as
Wood + Oxygen ———–> Carbon dioxide + energy
But cutting it into small pieces is physical change because no new substance is formed. We can only reduce the size of wood.
. 8. Describe how crystals of copper sulphate are prepared.
Take a water(H2O) in a beaker and add some drops of dilute sulphuric acid (dil.H2SO4) and Heat the Solution . When it starts boiling, add copper sulphate(CuSO4) powder slowly. Continue to add copper sulphate (CuSO4) powder till no more powder can be dissolved. .During this process continuously stir the solution. After filtration Leave it for cooling. Look it after some time, you can see the blue colour crystals of copper sulphate (image with courtesy )
9. Explain how painting of an iron gate prevents it from rusting.
Rusting of iron is a chemical process in the presence of air and moisture . Painting stop rusting process because the iron gate from coming in contact with oxygen and moisture.
10. Explain why rusting of iron objects is faster in coastal areas than in deserts.
In the coastal area moisture always in the air higher than in the air in deserts. So, the process of rusting is faster in coastal areas because air and moisture both responsible for rusting process .
11. 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.
(Because during burning process new substance formed)
12. 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 processes A and B are chemical changes.