The quantity of water present on the heart is constant. It evaporated from the sea and form clouds. These clouds shed water in the form of rain or snow and the water flows to the sea again. This is called the water cycle.
The domestic water supply comes from rivers and it contains available quantity of minerals, the type of mineral dissolved in it depends in the rocks over which it passed, the sources are:
Rain water: it’s not very good as drinking water, and it contains impurities and it hasn’t got many minerals;
Surface water: our many supply, but it has to be heated with a small amount of chlorine to help kill bacteria, before it can be used as drinking water;
Underground water: are usually cooler, cleaner and purer than surface water. Pollution is a very big problem, most people in third world countries use impure water and bad sanitation.
