NCERT Solutions for Class 7 Science Chapter – 6 Physical and Chemical Changes

Physical and Chemical Changes

Question 1.
Classify the changes involved in the following processes as physical or chemical changes

  1.  Photosynthesis
  2.  Dissolving sugar in water
  3.  Burning of coal
  4.  Melting of wax
  5.  Beating aluminium to make aluminium foil
  6.  Digestion of food

Solution:
(1) , (3), (6)  → Chemical changes.
(2) , (4), (5)  → Physical changes.

Question 2.
State whether the following statements are true or false. In case a statement is false, write the corrected statement in your notebook.

  1.  Cutting a log of wood into pieces is a chemical change. (True/False)
  2.  Formation of manure from leaves is a physical change. (True /False)
  3.  Iron pipes coated with zinc do not get rusted easily. (True/False)
  4.  Iron and rust are the same substances. (True/ False)
  5.  Condensation of steam is not a chemical change. (True/ False)

Solution:

  1.  False. Cutting a log of wood into pieces is a physical change.
  2.  False. Formation of manure from leaves is a chemical change.
  3.  True.
  4.  False. Iron and rust are different substances.
  5.  True.

Question 3.
Fill in the blanks in the following statements :

  1. When carbon dioxide is passed through lime water, it turns milky due to the formation of ……….. .
  2. The chemical name of baking soda is ……….. .
  3. Two methods by which rusting of iron can be prevented are ……….. and ……….. .
  4. Change in which only ……….. properties of a substance change are called physical changes.
  5. Changes in which new substances are formed are called ……….. changes.

Solution:

  1. calcium carbonate.
  2. sodium hydrogen carbonate.
  3. painting and galvanisation.
  4. physical.
  5. 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:
It is a chemical change. Here, a new substance carbon dioxide gas is formed.

Question 5.
When a candle bums, 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:
The wax of the candle first melt, then vaporises and burns. Melting of wax is a physical change since molten wax can be solidified back to the wax and there is no new substance formed.

When wax vapours burn, smoke and carbon dioxide are formed which are new substances. So, it is a chemical process.Lightning torch bulb using dry cell is another example where both physical and chemical processes take place. This lightning of the bulb is physical change since no new substance is formed there.

The current from the dry cell is obtained by the chemical substances inside it. Here, the chemicals in the cell get converted into new substances and hence the cell ultimately becomes useless. So, it is a chemical change.

Question 6.
How would you show that setting of curd is a chemical change?

Solution:
When some sour substance is added to milk or kept overnight, it turns into curd. The curd in no way can be converted into milk i.e. irreversible change. Curd is a different substance than milk. So, formation of curd 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 produces ash and smoke. Hence, the properties of wood are changed and new substances are formed. So, it is a chemical change. When a log of wood is cut into small pieces, there is no new substance formed.

Each small piece bears the properties of wood. So, it is a physical change. Obviously, burning and cutting of wood are two different types of change.

Question 8.
Describe how crystals of copper sulphate are prepared.

Solution:
A cup of water is taken in a beaker and a few drops of dilute sulphuric acid are added into it. The water is heated. When it starts boiling, copper sulphate powder is added slowly while stirring continuously.

Copper sulphate powder is added continuously till no more powder can be dissolved. The solution is filtered and allowed to cool down. Crystals of copper sulphate slowly form at the bottom of the beaker.
NCERT Solutions for Class 7 Science Chapter 6 Physical and Chemical Changes image 1

Question 9.
Explain how painting of an iron gate prevents it from rusting.

Solution:
For rusting, iron must be in contact with both air and moisture. When iron gate is painted, the layer of paint cuts the contact between air, moisture and iron. Thus, it prevents rusting.

Question 10.
Explain why rusting of iron objects is faster in coastal areas than in deserts.

Solution:
In coastal areas, there is more moisture in air due to the presence of sea. But in desert, there is a scarcity of water and hence air is almost dry there. Both air and moisture are necessary conditions for rusting. So, rusting is faster in coastal areas than in desert.

Question 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 bums (Change-B). The following statements pertain to these changes. Choose the correct one.

  1.  Process – A is a chemical change.
  2.  Process – B is a chemical change.
  3.  Both processes A and B are chemical changes.
  4.  None of these processes is a chemical change.

Solution:
2. Process – B is a chemical change.

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.

  1.  Process – A is a chemical change.
  2.  Process – B is a chemical change.
  3.  Both processes A and B are chemical changes.
  4.  None of these processes is a chemical change.

Solution:
3. Both processes A and B are chemical changes.

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