The NCERT Solutions for Class 7 Science, Chapter 5: Physical and Chemical Changes, are available here to aid students in thoroughly understanding the topic. Each solution is expertly crafted with detailed explanations, covering all textbook exercise questions and answers. These resources are designed to help students complete their assignments and homework efficiently.
NCERT Solution: Physical and Chemical Changes Class VII Science Chapter 5
Exercises -
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
Answer:
| Physical Changes | Chemical Changes |
|---|---|
| (b) Dissolving sugar in water | (a) Photosynthesis |
| (d) Melting of wax | (c) Burning of coal |
| (e) Beating aluminium to make aluminium foil | (f) Digestion of food |
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)
Answers:
(a) False
(b)False
(c) True
(d) False
(e) 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 .............
(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.
(e) Changes in which new substances are formed are called ............... changes.
Answers:
(a) calcium carbonate
(b) sodium hydrogen carbonate
(c) painting, galvanization
(d) physical
(e) chemical
4. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of gas. What type of change is it? Explain.
Answer:
This is a chemical change as new products are formed.
Lemon juice + Baking soda → Carbon dioxide + Other substances
When acid reacts with carbonates, salt is formed releasing carbon dioxide and water.
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 takes place.
Answer:
When a candle burns, both physical and chemical changes take place.
- Physical change → Melting of wax
- Chemical change → Burning of wax
Eating of food is another example where both physical and chemical changes occur simultaneously.
- Physical change → Breaking down of larger food particles into smaller particles.
- Chemical change → Digestion of food
6. How would you show that setting of curd is a chemical change?
Answer:
The process of curd formation is a chemical change because it results in the creation of a new substance, lactic acid, which has a distinct taste compared to the original components in milk, such as fats and proteins. Additionally, this change is irreversible, as curd cannot be reverted back into milk.
7. Explaining why burning of wood and cutting it into small pieces are considered as two different types of changes.
Answer:
Burning wood is a chemical change because it produces a new substance, charcoal, along with other byproducts. In contrast, cutting wood is a physical change, as it only alters the shape and size without creating any new substances.
8. Describe how crystals of copper sulphate are prepared.
Answer:
Copper sulfate crystals can be prepared as follows:
1. Pour water into a beaker and add a few drops of dilute sulfuric acid.
2. Heat the water, and once it starts boiling, slowly add copper sulfate powder while stirring continuously.
3. Continue adding copper sulfate powder until the solution reaches saturation.
4. Filter the solution and set it aside to cool.
5. Leave the solution undisturbed as it cools down.
6. After a few hours, copper sulfate crystals will form and can be observed.
9. Explain how painting of an iron gate prevents it from rusting.
Answer:
Rusting requires both oxygen and water (or water vapor) to occur. A coat of paint acts as a barrier, preventing direct contact between iron and air, which includes oxygen, thereby inhibiting the rusting process.
10. Explain why rusting of an iron objects is faster in coastal areas than in deserts.
Answer:
Rusting of iron occurs when it comes into contact with both water (moisture) and oxygen. In coastal areas, the humidity is higher compared to deserts, which leads to a faster rate of rusting for iron objects in these regions than in arid desert environments.
11. The gas we use in 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 process 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 process A and B are chemical changes.
(iv) None of these processes is a chemical change.
Answer:
(iii) Both process A and B are chemical changes.