Solution of Chapter 6. Physical and Chemical Changes (NCERT Science Book)

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 aluminum to make aluminum foil

f) Digestion of food

State whether the following statements are true or false. In case a statement is false, write the corrected statement in your notebook.

a) Cutting of log of wood into pieces is a chemical change. (True/False)

b) Formation of manures 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)

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) Change in which only ───── properties of a substance change are called physical changes.

e) Changes in which new substances are formed are called ────── changes.

When baking soda is mixed with lemon juice, bubbles are formed with the evolution of gas. What type of change is it? Explain.

When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of familiar process in which both the chemical and physical changes take place.

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

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

Describe how crystals of copper sulphate are prepared.

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

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

The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder it exists 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.

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.

Describe two changes that are harmful. Explain why you consider them harmful? How can you prevent them?

Take three glass bottles with wide mouths. Label them A,B and C. Fill about half of bottle A with ordinary tap water. Fill bottle B with water which has been boiled for several minutes, to the same level as in A. In bottle C, take the same boiled water and of the amount as in other bottles. In each bottle put a few similar iron nails so that they are completely under water. Add a teaspoonful of cooking oil to the water in bottle C so it forms a film on its surface. Put the bottles away for a few days. Take out nails from each bottle and observe them. Explain your observations.

Prepare crystals of alum.

Collect information about the types of fuels used for cooking in your area. Discuss with your teachers/parents/others which fuels are less polluting and why ?