Page 99 - Chemistry - XI

Result

The rate of evaporation of liquid follows the order __________________.

Precautions
1. The temperature and surface area should be the same for the evaporation of each liquid.
2. The size and shape of weighing bottles should be the same.

3. Keep all the bottles at the same place.

PROJECT - 7

Introduction

Fibres are long thin thread-like materials which are characterised by great tensile strength. These fi bres are
twisted into threads which can be woven into a cloth. There are two types of fi bres:

1. Natural fi bres. The natural fi bres are cotton, wool silk, etc. These fi bres consist of long molecules
(polymer) with regular repeating structural units like beta-glucose unit of cellulose (monomer) (e.g.
cotton). Silk and wool fi bres are made of proteins and they contain long peptide chains.
2. Man-made fi bres. These are synthetic fi bres, e.g. nylon, terylene, rayon, etc. These fi bres are obtained
by polymerisation of unsaturated organic compounds.
Nylon is a synthetic polyamide fi bre made by condensation of adipic acid and hexamethylene diamine.

Terylene is a polyester of a ophthalmic acid and ethylene glycol.
The strength of fi bres depends upon the strength of the chemical bond of the polymer chain. In nylon,
these molecular chains are held by hydrogen bonds.

Tensile Strength of Fibres
The tensile strength of a fi bre is its resistance to longitudinal stress and measured by the minimum amount
of longitudinal stress required to break the material. The tensile strength of a fi bre tells us the extent to
which any fi bre can be stretched without breaking, i.e. to say it is the maximum force which the fi bres can
withstand.

The synthetic polymers which are used for making nylon, terylene, etc. possess high tensile strength and
their fi bres have strong intermolecular force due to hydrogen bonding. Eff ect of acid and base will be
diff erent on tensile strength of diff erent fi bres.

Objective
To determine tensile strength of various fi bres and eff ect of acid and base on tensile strength.

Materials Required

Iron stand, pan, beaker, weight box, threads of cotton, silk, nylon and terylene, HCl, CH COOH, NaOH,
3
and NH OH, and for constructing the samples, four types of threads are chosen namely, cotton, silk, nylon,
4
and terylene. Four diff erent brands of detergents are chosen: Surf, Ariel, Fena, and Wheel.

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