Question

The fibre that burns completely leaving no residue is:
(A) Cotton
(B) Acrylic
(C) Polyester
(D) Polythene

Answer 1

Hint: Natural fibres are fibres that are produced or derived from plants, animals and other geological processes. Synthetic fibres are artificial or man-made fibres. Acrylic, polyester and polythene are synthetic fibres. The fibre that leaves no residue is a natural fibre.

Complete step by step solution:
Cotton is a soft, fluffy fibre that grows in a boll or a protective case. The seeds of the cotton are covered by this fibre. The fibre is pure cellulose. Cotton plants are a shrub and native to tropical and subtropical regions around the world. Cotton fibre burns rapidly with a yellow flame. Cotton burns rapidly and quickly leaving no residue.
Acrylic fibres are synthetic, artificial fibres made from a polymer. The average molecular weight of polyacrylonitrile is 100000 which is about 1900 monomer units. It is used for making sweaters and track suits. It is also used for linings of boots and gloves. Acrylic being a synthetic fibre burns, melts and shrinks at the same time. Acrylic flames and burns rapidly with hot, sputtering flame and a black smoke and it leaves residue.
Polyester: Coal, petroleum, water and air are used to obtain this type of synthetic fibre. Repeating units of a chemical known as esters is used to make polyester. Polyester remains crisp and retains its shape. Polyester being a synthetic fibre burns, melts and shrinks at the same time. It also leaves residue when burnt.
Polythene is the common plastic we use nowadays. It is man-made, linear, homo-polymer and it is primarily used for packaging. Polythene being synthetic fibre burns, melts and shrinks at the same time. Polythene releases toxic and poisonous gases like dioxin, furans while burning. Polythene does not burn quickly, hence leaving a residue.
Therefore, the correct answer is option A, that is, cotton.

Note:
Natural fibres can be used as a material for composite. Natural fibres can be matter into sheets to make paper felt. Synthetic fibres are prepared mostly from petroleum which are also known as petrochemicals. All fabrics are derived from fibres while fibres are made from artificial man-made resources.