Substances, Changes, and Fibers

A change in which one or more than one new substances are formed is known as a chemical change. 

All of the following changes are irreversible because we can not convert the product into reactant again.
1) Milk to paneer.
2) Cow dung to biogas.
3) Grain to its flour.

Burnt sparklers and burnt books pages are examples of irreversible changes, as they are permanent changes which cannot be undone. Once these materials are burnt in to ashes, the original materials cannot be recovered back.

A reversible change can be undone to return to the original state. Cooking an egg involves chemical changes that permanently alter the proteins, making it irreversible.

Irreversible changes $-$ because when they are burnt formation of gas and ashes takes place i.e. chemical reaction is taking place and new substance are formed which can't be reversed back.

Sheep are specifically known for their soft under hair, which is the primary source of wool. While other animals have hair, the term fine soft under hair is most commonly associated with sheep in textile contexts.

Silk production, known as sericulture, originated in ancient China, where it was a closely guarded secret for centuries.

The most fibre are chemically composed of non-starch polysaccharides like cellulose, inulin, lignin, chitins, pectins, oligosaccharides. They are either obtained from plants or animals. They are either soluble or insoluble. 

Coal is a fossil fuel. Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure. 

Petroleum products such as petrol, diesel, kerosene and petroleum gas are used as fuel as they have high calorific value. 

B, LPG is clean fuel, it has high calorific value and low ignition temperature.