Class 7 Science Notes Chapter 3 (Chapter 3) – Examplar Problems Book

Right then, let's get straight into Chapter 3, focusing on the important points you'll need for your exams. This chapter deals with animal fibres, specifically Wool and Silk. Pay close attention to the processes involved and the key terms.

Chapter 3: Fibre to Fabric

1. Introduction to Animal Fibres

• Fibres obtained from animals are called animal fibres.
• The most common animal fibres are Wool and Silk.
• Wool comes from the fleece (hair) of sheep, goats, yaks, camels, llamas, and alpacas.
• Silk comes from the cocoons of silkworms.
• Both wool and silk fibres are made up of proteins.

2. Wool

• Sources of Wool:

  • Sheep: The most common source. The quality of wool depends on the breed of sheep.
  • Goat:
    • Angora goats: Yield Mohair wool (found in hilly regions like Jammu & Kashmir).
    • Kashmiri goats: Yield fine, soft wool used for Pashmina shawls.
  • Yak: Common in Tibet and Ladakh.
  • Camel: Fur on the body is used as wool.
  • Llama and Alpaca: Found in South America, also yield wool.

• From Fleece to Wool:

  • Sheep have two types of fibres in their fleece:
    • The coarse beard hair.
    • The fine soft under-hair close to the skin (This provides the fibres for making wool).
  • Selective Breeding: Some breeds of sheep possess only fine under-hair. Their parents are specially chosen to give birth to such sheep. This process of selecting parents for obtaining special characteristics in their offspring is termed selective breeding.

• Rearing and Breeding of Sheep:

  • Sheep are herbivores, preferring grass and leaves. Rearing also involves feeding them a mixture of pulses, corn, jowar, oil cakes, and minerals.
  • They are reared in many parts of India, including Jammu & Kashmir, Himachal Pradesh, Uttarakhand, Arunachal Pradesh, Sikkim, Haryana, Punjab, Rajasthan, and Gujarat.

• Processing Fibres into Wool: This is a multi-step process:

  • Step I: Shearing:
    • Definition: The removal of the sheep's fleece along with a thin layer of skin.
    • Timing: Usually done during hot weather so sheep can survive without their protective coat.
    • Method: Done using machines similar to those used by barbers.
    • Effect on Sheep: Does not hurt the sheep, as the uppermost layer of the skin is dead. Hair grows back.
  • Step II: Scouring:
    • Definition: Thorough washing of sheared skin with hair in tanks to remove grease, dust, and dirt.
    • Method: Nowadays done by machines.
  • Step III: Sorting:
    • Definition: Separation of hair of different textures. Hairy skin is sent to a factory for this.
  • Step IV: Cleaning of Burrs:
    • Definition: Small fluffy fibres, called burrs, are picked out from the hair. (These are the same burrs that sometimes appear on sweaters).
    • Process: The fibres are scoured again and dried. This wool is ready to be drawn into fibres.
  • Step V: Dyeing:
    • Definition: Fibres are dyed in various colours, as the natural fleece is usually black, brown, or white.
  • Step VI: Rolling into Yarn:
    • Definition: The fibres are straightened, combed, and rolled into yarn.
    • Outcome: Longer fibres are made into wool for sweaters. Shorter fibres are spun and woven into woollen cloth.

• Occupational Hazard:

  • Wool industry workers, especially sorters, risk getting infected by a bacterium called Bacillus anthracis.
  • This causes a fatal blood disease called Sorter's disease (Anthrax). Such risks faced by workers in any industry are called occupational hazards.

3. Silk

• Source: Silk fibres are obtained from the cocoons of the silk moth.
• Sericulture: The rearing of silkworms for obtaining silk is called sericulture.
• Life History of Silk Moth:
  • Stage 1: Egg: The female silk moth lays hundreds of eggs at a time.
  • Stage 2: Larva (Caterpillar or Silkworm): Eggs hatch into larvae. They feed voraciously on mulberry leaves and grow significantly in size.
  • Stage 3: Pupa:
    • The caterpillar stops eating and weaves a net to hold itself.
    • It swings its head from side to side in the shape of the figure eight (8).
    • During these movements, it secretes a fibre made of protein, which hardens on exposure to air and becomes the silk fibre.
    • Soon, the caterpillar covers itself completely with silk fibres, forming a cocoon.
    • The larva develops into a pupa inside the cocoon.
  • Stage 4: Moth: The pupa develops fully into an adult moth inside the cocoon. The moth then emerges from the cocoon. The silk threads are obtained from the cocoon.
• Types of Silk:
  • Different types of silk moths yield different varieties of silk yarn (e.g., Tussar silk, Mooga silk, Kosa silk). These silks differ in texture (coarse, smooth, shiny, etc.).
  • The most common silk moth is the mulberry silk moth. The silk fibre from its cocoon is soft, lustrous, elastic, and can be dyed beautifully.
• From Cocoon to Silk:
  • Rearing Silkworms:
    • Female moths lay eggs, which are stored carefully on strips of cloth or paper and sold to silkworm farmers.
    • Farmers keep eggs under hygienic conditions and suitable temperature/humidity.
    • Eggs are warmed to hatch larvae. This is done when mulberry trees bear fresh leaves.
    • Larvae (silkworms) are kept in clean bamboo trays with freshly chopped mulberry leaves. They eat day and night for about 25-30 days and increase enormously in size.
    • Then, they are moved to small chambers/racks in the trays (like twigs) where they spin cocoons.
  • Processing Silk:
    • Reeling: A pile of cocoons is used for obtaining silk fibres.
      • Cocoons are kept under the sun, boiled, or exposed to steam. This kills the pupa inside and separates out the silk fibres.
      • The process of taking out threads from the cocoon for use as silk is called reeling the silk.
      • Reeling is done using special machines that unwind the delicate threads (fibres) from the cocoons.
    • Spinning & Weaving: Silk fibres obtained are then spun into silk threads. These threads are woven into silk cloth by weavers.

Key Terms Recap: Cocoon, Fleece, Reeling, Scouring, Sericulture, Shearing, Silkworm, Silk Moth, Sorting, Selective Breeding.

Multiple Choice Questions (MCQs)

1. Which of the following is NOT a source of wool?
   (a) Yak
   (b) Camel
   (c) Goat
   (d) Woolly dog

2. The process of washing sheared wool to remove grease and dirt is called:
   (a) Shearing
   (b) Sorting
   (c) Scouring
   (d) Reeling

3. Sorter's disease is an occupational hazard associated with the wool industry, caused by:
   (a) A virus
   (b) A fungus
   (c) A bacterium
   (d) Dust allergy

4. Pashmina shawls are made from the wool obtained from:
   (a) Angora goat
   (b) Kashmiri goat
   (c) Alpaca
   (d) Sheep

5. The rearing of silkworms for obtaining silk is known as:
   (a) Apiculture
   (b) Silviculture
   (c) Sericulture
   (d) Pisciculture

6. In the life cycle of a silk moth, the silk fibre is secreted during which stage?
   (a) Egg
   (b) Larva (while forming cocoon)
   (c) Pupa
   (d) Adult Moth

7. The process of taking out silk threads from the cocoon is called:
   (a) Shearing
   (b) Scouring
   (c) Spinning
   (d) Reeling

8. Which of the following statements is INCORRECT regarding the processing of wool?
   (a) Shearing is done in cold weather to protect sheep.
   (b) Scouring removes grease, dust, and dirt.
   (c) Sorting involves separating fibres of different textures.
   (d) Burrs are small fluffy fibres picked out from the hair.

9. Silk fibre is primarily made of:
   (a) Carbohydrate
   (b) Fat
   (c) Protein
   (d) Cellulose

10. The most common type of silk, known for its softness and lustre, comes from the cocoons of the:
    (a) Tussar silk moth
    (b) Mooga silk moth
    (c) Kosa silk moth
    (d) Mulberry silk moth

Answer Key for MCQs:

1. (d)
2. (c)
3. (c)
4. (b)
5. (c)
6. (b)
7. (d)
8. (a)
9. (c)
10. (d)

Make sure you understand the 'why' behind each step in wool and silk processing, and the sequence of the silk moth's life cycle. That's crucial for tackling application-based questions in your exams. Good luck!