NCERT Solutions for Class 7th Science Chapter 5 Physical and Chemical changes according to the Latest Syllabus Of JKBOSE and CBSE NCERT Books.
1. Classify the changes involved in the following processes as physical or chemical changes:
(a) Photosynthesis
- Answer: Chemical change
(b) Dissolving sugar in water
- Answer: Physical change
(c) Burning of coal
- Answer: Chemical change
(d) Melting of wax
- Answer: Physical change
(e) Beating aluminum to make aluminum foil
- Answer: Physical change
(f) Digestion of food
- Answer: Chemical change
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. Cutting a log of wood into pieces is a physical change.
(b) Formation of manure from leaves is a physical change. (True/False)
- Answer: False. Formation of manure from leaves is a chemical change.
(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. Iron and rust are different substances.
(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 __________.
- Answer: calcium carbonate
(b) The chemical name of baking soda is __________.
- Answer: sodium bicarbonate
(c) Two methods by which rusting of iron can be prevented are __________ and __________.
- Answer: painting, galvanization
(d) Changes in which only __________ properties of a substance change are called physical changes.
- Answer: physical
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: It is a chemical change. The reaction between baking soda (a base) and lemon juice (an acid) produces carbon dioxide gas, which forms bubbles.
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, the melting of wax is a physical change, while the burning of wax to produce carbon dioxide and water is a chemical change. Another example is the digestion of food, where the chewing of food is a physical change, and the breakdown of food by enzymes is a chemical change.
6. How would you show that setting of curd is a chemical change?
- Answer: The setting of curd is a chemical change because it involves the conversion of lactose in milk into lactic acid by the action of bacteria. This process results in the formation of new substances and cannot be reversed.
7. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.
- Answer: Cutting wood into small pieces is a physical change because it only changes the size and shape of the wood without altering its chemical composition. Burning wood, however, is a chemical change because it results in the formation of new substances, such as ash, carbon dioxide, and water vapor.
8. Describe how crystals of copper sulphate are prepared.
- Answer: Dissolve copper sulphate in hot water to make a saturated solution. Allow the solution to cool slowly without disturbing it. Crystals of copper sulphate will form as the solution cools.
9. Explain how painting of an iron gate prevents it from rusting.
- Answer: Painting an iron gate prevents rusting by providing a barrier that prevents moisture and oxygen from coming into contact with the iron. This stops the oxidation process that leads to rust.
10. Explain why rusting of iron objects is faster in coastal areas than in deserts.
- Answer: Rusting of iron objects is faster in coastal areas due to the high humidity and presence of salt in the air, which accelerates the oxidation process. In deserts, the dry conditions slow down rusting.
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 of 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.
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.
13. Rust is
(a) Carbon dioxide (b) Iron (c) Oxygen (d) Iron oxide
- Answer: (d) Iron oxide
14. Which of the following is not a physical change?
(a) Rusting of iron (b) Melting of ice (c) Freezing of water (d) Dissolving sugar in water
- Answer: (a) Rusting of iron
Extended Learning – Activities and Projects of Physical and Chemical changes
1. Describe two changes that are harmful. Explain why you consider them harmful. How can you prevent them?
- Answer:
- Rusting of iron: Harmful because it weakens structures and causes economic loss. Prevention: Use of paints, coatings, and galvanization.
- Food spoilage: Harmful because it leads to waste and potential health hazards. Prevention: Proper storage, refrigeration, and use of preservatives.
