What common water pollution caused mainly by power plants and factories resulting in the death of many aquatic organisms

This type of water pollution is known as thermal pollution. Thermal pollution is the release of heat into the environment. In the case of power plants they release heated water into waterways. Water is used in power plants to cool down machinery. As machines operate they release heat so the water is used as a coolant. Releasing heated water into an aquatic ecosystem reduces the oxygen carrying capacity of the water, so less oxygen is available for organisms to use for respiration. 

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The mass has escaped into a gas which you can not see so the law stays true.