CBSE Class 7 Science Chapter 3: Fibre to Fabric - NCERT Solutions

Solution:

1. Wool is obtained from the fleece of sheep. On a black sheep, the wool comes from the fine and soft under-hair that grows close to its skin. This layer provides the fibres we use.
2. The phrase 'white fleece of the lamb' refers to the woolly coat of the lamb, indicating that its natural colour is white. The fleece is the soft, curly hair covering the body of sheep and lambs.

Solution:

The correct option is (iii) both a and b. The silkworm is the larval stage of the silk moth. In biological terms, the stage after the egg hatches is called a larva, and this specific larva, which spins silk, is commonly known as a caterpillar.

Solution:

The correct answer is (iv) Woolly dog. Yaks, camels, and goats are animals that provide fibres used for making wool. A 'woolly dog' is not a recognized source of wool in the textile industry.

Solution:

Rearing: Rearing refers to the practice of raising and caring for animals, such as sheep, goats, or cattle, for various purposes like obtaining wool, meat, milk, or other products. For instance, sheep are reared primarily for their wool. They are provided with food, shelter, and protection, and are often kept in specific environments suitable for their well-being and yield.

Shearing: Shearing is the process of removing the fleece, or woolly coat, from a sheep. This is typically done once a year, usually in warmer seasons, using sharp blades or machines that cut the wool close to the skin without harming the animal.

Sericulture: Sericulture is the cultivation of silkworms for the production of silk. This involves raising silkworms, usually on mulberry leaves, and harvesting their cocoons, from which silk threads are extracted.

Solution:

The complete sequence of steps in the processing of wool is:

Shearing, Scouring, sorting, Carding, Spinning, Weaving/Knitting.

Let's clarify the missing steps:

• Scouring: After shearing, the fleece is washed thoroughly to remove dirt, grease (lanolin), and impurities.
• Carding: The cleaned fibres are then passed through a process called carding, where they are disentangled and straightened into a continuous web or sliver.
• Spinning: The sliver is then drawn out and twisted to form yarn or thread, which is the process of spinning.
• Weaving/Knitting: Finally, the yarn is used to make fabric through weaving (interlacing threads) or knitting (interlooping threads).

Solution:

The two stages in the life history of the silk moth that are directly related to the production of silk are:

1. The Larva (Silkworm): This is the stage where the silkworm hatches from the egg and begins to feed voraciously, usually on mulberry leaves. During this stage, it grows rapidly and eventually spins a protective cocoon around itself. The cocoon is made of a continuous thread of silk secreted by the silkworm.

   

2. The Cocoon: After the silkworm has finished growing, it spins a cocoon made of raw silk. This cocoon serves as a protective casing for the pupa developing inside. It is from these cocoons that the silk fibre is obtained by carefully unwinding the silk thread.

   

These two stages are crucial because the silkworm produces the silk thread, and the cocoon is the structure that contains this thread, which is then processed to make silk fabric.