Chapter 3 Science CBSE 7th

Cloth is one of our basic needs. Cloth protects us from heat, cold, rain, dust, insects, etc. Clothes also make one civilized and smart. Clothes are made of cloth. Cloth is also known as fabric.
Fabric is made of fiber.
Types of Fibre: There are two types of fibre, viz. natural and man-made.

1. Natural fibers: Natural fibers are obtained from plants and animals; such as jute, cotton, wool, silk, etc.
2. Man-made fibers: Fibers that are synthesized in laboratory are called man-made fiber, such as terrylene, terry-cotton, acrylic, etc.

Types of Natural fiber: Natural fibers can be classified into two types – Plant fiber and Animal fiber.

1. Plant Fiber: Fiber obtained from plants is called plant fiber. For example – cotton, jute, flex, etc.
2. Animal Fiber: Fiber obtained from animals is called animal fiber. For example: wool and silk.

Wool: There are many animals that bear a thick coat of hair on their body. Such animals generally live in cold climates. Thick coat of hair over the body of such animals traps lot of air and keeps them warm as air is a bad conductor of heat. It prevents the warmth of the body from escaping and also prevents the coldness of the surroundings from entering. Thus, thick layer of hair over their body protects them from harsh cold. For example: Sheep, Goat, Camel, Yak, etc.Fleece and Wool bearing animals; like sheep, goat, camel, yak, etc. bear two types of hair – coarse hair and fine-soft under hair. Fine soft hair is found close to the skin in such animals. The fine soft under hair is called fleece. Fiber for wool is obtained from the fleece (hair) of such animals and hence such animals are called wool bearing animals.
Sheep are reared in many parts of our country for wool. The below Table gives the names of some breeds of sheep reared in our country for producing wool. The quality and texture of the fibres obtained from them is also indicated in the table.

Angora wool is obtained from Angora Goats. Angora Goats are found in hilly regions, such as Jammu and Kashmir. Pashmina wool is obtained from Pashmina Goats. Yak wool is obtained commonly in Tibet and Laddakh. Alpaca and Llama are other animals that give wool.

Selective breeding and rearing of sheep: Some breeds of sheep bear only a coat of fine hair. Such animals are reared by selective breeding. Selective breeding is the process to obtain animals or plants having special characteristics. In India, sheep are reared generally in the sates of Jammu and Kashmir, Himachal Pradesh, Uttaranchal, Arunachal Pradesh and Sikkim, or the plains of Haryana, Punjab, Rajasthan and Gujarat.
Food of sheep: Sheep are herbivores and feed generally on grass and leaves. Apart from grass and leaves they also feed on corn and oil cakes. Oil cakes are materials left after obtaining of oil from oil seeds.

Process to Obtain Wool From Sheep: Steps given below are followed to obtain wool from sheep or other wool bearing animals:Step 1: Shearing: The fleece (hair) of sheep is shaved off along with a thin layer of skin. In olden days this was done using pair of metal blades. But now-a-days machine is used to cut off the fleece. This is similar to shaving of beards or hair. This process is called shearing.Shearing is generally done in summer so that sheep could get new hair by winter to get protection against cold.
Step 2: Scouring: Fleece, after shearing, is washed properly to remove dirt and grease. The washing of fleece; after shearing; is called scouring.Step 3: Sorting: After scouring, fleece is sorted according to texture. This process is called sorting.
Step 4: Removing burr: After sorting, fluffy fibers; called burr; are picked out from hair. Burr is the fiber that gives wool.
Step 5: Dyeing: After sorting and picking out of burrs, these are dyed in desired colors.
Step 6: Making yarn: The fibers are then straightened, combed and rolled into yarns.
Wool yarn is used in knitting sweaters and woolen cloths, i.e. fabric.

Silk: Silk is another important animal fiber. Silk worm spins silk. Silkworm is reared to obtain silk.
History of Silk: Silk was discovered in China; around 3500 BC. Silk became a prized possession because of its fine quality and luster. Originally, it was used by emperors only. It was through trade that silk spread to other parts of the world over a period of time. Silk was a staple item of trade during ancient times. Due to this, the ancient trade routes which linked China to other parts of the world are called ‘Silk Route’.
As per historians, silk was produced in India also. Proof of use of silk during the Indus Valley Civilization has also been found.
Types of Silk: Different types of silk worm produce different types of silk; in terms of luster and texture. For example; tassar silk, mooga silk, kosa silk, etc. are produced by different types of silk moth. Mulberry silk is the most common silk moth.
Rearing of silkworm: Rearing of silkworm is known as SERICULTURE. Silkworms are reared on mulberry leaves as they feed on mulberry leaves.

Life Cycle of Silkworm: There are four stages in the development of silk moth:

1. Egg
2. larva
3. pupa
4. adult

Stage 1:- In the beginning female silk moth lays eggs on mulberry leaves.
Stage 2:- The eggs are hatched into very small larvae within a week. These larvae then feed on mulberry leaves. The larvae looks like a worm and are also called caterpillars or silkworms.
Stage 3:- They then grow in size (over three inches) and then the caterpillar is ready to enter the next stage of his life called pupa. An adult silkworm first weaves a net o hold itself. Then it swings its head from side to side in the form of figure 8. During these movements the head of the caterpillar secrets fibre made of a protein which hardens on exposure to air and becomes silk fibre. Soon the caterpillar covers itself completely by silk fibre. This covering is known a cocoon. The cocoon is made by caterpillar to protect its development as pupa. The silkworm continues to develop inside the cocoon to form silk moth. Pupa is the stage in the life history of silk moth when tie caterpillar becomes encased in the hard shell of silk fibre (cocoon).
Stage 4:- In this stage pupa which is encased in the cocoon, develops fully to form an adult silk moth. After the complete development the cocoon splits up and beautiful silk moth comes out of it. This completes the life history of a silk moth.Production of Silk from Silk Moth:
To obtain silk, silk moths are reared and their cocoons are collected to get silk threads or fabric. We will now learn more about the process. For this mulberry trees are grown.
(1) Rearing of silk worms to obtain cocoon:- Female silk moth lays hundreds of eggs at a time. These eggs are stores carefully on paper or cloth strips and are sold to silk worm farmers. The farmers keep eggs under hygienic condition and under suitable conditions of temperature and humidity. The eggs are then warmed to suitable temperature for hatching. when eggs hatch silkworms or caterpillar comes out. Silkworms are then fed on mulberry leaves day and night. They then increase enormously in size. The larvae are kept in clean bamboo trays along with young and freshly chopped mulberry leaves. After 25 to 30 days , silkworms stop eating and start spinning the cocoon.(2) Processing cocoons to obtain silk fibre:- The cocoons are collected and boiled in water to kill the pupa inside them. Hot water makes the silk fibres of cocoon to separate out. This resulting fibre is called raw silk. The process of taking out silk fibre from cocoons for use as silk is called reeling. Reeling is done in special machines which unwind the fibres of silk form cocoons.
(3) Converting silk fibre into silk cloth:- Silk fibres obtained from cocoons are spun to form silk yarn which is then woven in looms by wavers to make silk cloth.

Different Varieties of Silk
There is a variety of silk moths which look very different from one another. The silk produced by the silkworms of different varieties of silk moths is different in texture (coarse, smooth, shiny, etc.). Some of the varieties of silk are: Mulberry silk; Tassar silk; Mooga silk; Kosa silk; and Eri silk. These silks are obtained from cocoons spun by the silkworms of different types of silk moths. The most common silk moth is the mulberry silk moth. The silk obtained from the cocoons of mulberry silk moth is called mulberry silk. Mulberry silk is soft, lustrous (shiny) and elastic, and can be dyed in beautiful colours. Thus, the most common variety of silk is mulberry silk.