Lesson: Physical and Chemical Changes

Question: 1

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

Solution:

(a)               Photosynthesis $–$ Chemical change

(b)               Dissolving sugar in water $–$ Physical change

(c)               Burning of coal $–$ Chemical change

(d)               Melting of wax $–$ Physical change

(e)               Beating Aluminium to make Aluminium foil $–$ Physical change

(f)                Digestion of food $–$ Chemical change

Question: 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)

(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)

Solution:

(a)               False. Cutting a log of wood into pieces is a physical change.

(b)               False. Formation of manure from leaves is a chemical change.

(c)               True.

(d)               False. Iron and rust are different substances.

(e)               True.

Question: 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 _________.

(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.

(f)                Changes in which new substances are formed are called _________ changes.

Solution:

(a)               Calcium carbonate (CaCO3)

(b)               Sodium bicarbonate (NaHCO3)

(c)               Greasing and galvanisation

(d)               Physical

(e)               Chemical

Question: 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.

Solution:

Baking soda + Lemon juice $\to$ Salt + Carbon dioxide + Water

When baking soda, which is a base, reacts with lime juice, which is an acid, they form salt and water with the evolution of carbon dioxide gas which results in bubble formation.

This reaction is a type of neutralisation reaction, i.e., a chemical change,

since a new substance with completely different properties is formed.

Question: 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.

Solution:

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

Melting of wax $\underset{}{\overset{}{\to }}$ Physical change

Burning of wax $\underset{}{\overset{}{\to }}$ Chemical change, since it leads to formation of new substances, like carbon dioxide, soot, water vapour, heat and light.

Eating of food is another such example where both physical and chemical changes take place.

Breaking down of larger food particles into smaller particles $\underset{}{\overset{}{\to }}$ Physical change

Digestion of food $\underset{}{\overset{}{\to }}$ Chemical change

Question: 6

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

Solution:

Setting of curd from milk is a chemical change. By the action of lactobacillus bacteria lactic acid (a new substance) is formed which is sour in taste. Once the curd has formed, milk cannot be re-obtained from it. Moreover, both milk and curd differ in chemical properties. Hence it is a chemical change.

Question: 7

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

Solution:

Burning of wood is a chemical change and cutting it into small pieces is a physical change. Cutting of wood into smaller pieces is a physical change because the original composition of wood does not change. No new substances are formed. On the other hand, burning of wood is a chemical change as new substances, like ash (carbon), carbon dioxide gas, water vapour, heat and light are formed.

Both the changes are irreversible.

Question: 8

Describe how crystals of copper sulphate are prepared.

Solution:

1.                  Pour a cup of water in a beaker and add a few drops of dilute sulphuric acid to it.

2.                  Now heat the water.

3.                  When it starts to boil, add copper sulphate powder slowly to it and stir it continuously.

4.                  Continue adding copper sulphate powder till no more powder can dissolve in the solution.

5.                  Filter the solution and allow it to cool. (Do not disturb the solution while it is cooling.)

6.                  We can observe the formation of crystals of copper sulphate after a few hours.

Question: 9

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

Solution:

Rusting of iron is a process of formation of reddish- brown ferric oxide, which damages the iron. For rusting the presence of both oxygen and water (or water vapour) is essential. By applying a layer of paint, we can block the exposure of iron to oxygen and water, thereby preventing the rusting of the iron gate.

Question: 10

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

Solution:

If the moisture content in the air is high, which means, if it is more humid, rusting becomes faster. This is the reason why rusting is faster in the coastal areas where there is higher humidity than in the deserts which is a comparatively dry place.

Question: 11

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.

Solution:

(ii)

Question: 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.

Solution:

(iii)