NCERT Exemplar problems and solutions, Ch 3, class 7

1. Complete the paragraph related to the life history of silk moth by filling in the blanks. The ____(a)___ silk moth lays ___(b)___, from which hatch ____(c)___ called ___(d)____ or ___(e)____. They grow in size and when the caterpillar is ready to enter the next stage of its life history called ___(f)____, it first weaves a covering to hold itself, which is known as ___(g)____.

Soln:

The a) female silk moth lays b) eggs, from which hatch c) larvae called d) caterpillars or e) silkworms. They grow in size and when the caterpillar is ready to enter the next stage of its life history called f) pupa, it first weaves a covering to hold itself, which is known as g) cocoon.

2. Paheli went to the market to buy sarees for her mother. She took out a thread from the edge of the two sarees shown by the shopkeeper and burnt them. One thread burnt with a smell of burning hair and the other burnt with the smell of burning paper. Which thread is from a pure cotton saree and which one from a pure silk saree? Give reason for your answer.

Soln:

Thread which burns with the smell of burning paper is of cotton because both paper and cotton are cellulose which are carbohydrates hence they burn with similar odor. Similarly the thread which is burnt with smell of burning hair is silk, because silk as well as hair are proteins hence they burn with similar characteristic smell.

3. Explain the phrase – “Unity is Strength” on the basis of the making of fabric from fibre.

Soln:

Fabric is made of thin hair like strand called as fibres. It is easy to break a fibre thread as it is fragile easy to break as well. On the other hand fabric is the accumulation of many fibre thread which are hard to break and needs enormous amount of energy to break fabric. Hence based on making of fabric from fibre it is said that unity is strength.

4. How do the hair of certain animals help in keeping their bodies warm?

Hairs of bad conductor of heat hence they will not allow escape of heat from body which ensures their body kept warm.

5. State whether the following statements are True or False. If false, correct them.

(a) Silkworms are caterpillars of silk moth.

(b) In India, camels and goats are generally reared for obtaining wool.

(c) The rearing of silkworms for obtaining silk is called silviculture.

(d) In the process of obtaining wool from fleece, sorting is done after scouring.

(e) Yak hair are not used to make woollen fabric.

Soln:

1. True
2. False- In India, sheep are reared for obtaining wool.
3. False- The rearing of silkworms for obtaining silk is called sericulture
4. True
5. False- Yak hair are used to make woollen fabric.