Water — CHEMISTRY STD 7 — Question

The release of substances into surface water or groundwater which makes the water unfit for use by humans or affects living organisms and their environment is called water pollution. Causes of water pollution –
1. Domestic sewage/wastes – Untreated sewage from cities and towns contains disease – causing microorganisms. When it is discharged into water bodies, the water gets contaminated.
2. Chemical pollution – Water gets contaminated by harmful . chemicals Industrial waste contains poisonous chemicals which affects the quality of water when discharged into it carelessly. Use of pesticides and fertilizers in agricultural fields pollute the groundwater. Accidental oil -spills in the oceans and seas damage marine life too.
3. Thermal pollution – Thermal power plants and oil refineries use large quantities of water for cooling purposes. They release this water at a very high temperature into nearly water bodies (rivers). This leads to thermal pollution of water. Heat decreases the capacity of water to hold dissolved oxygen. This reduced oxygen content badly affects fish and other aquatic animals.