Chemistry

How much water is there on Earth ?
On Earth there are about 1 400 000 000 000 000 000 000 liters of waters.
This amount may seem very high nevertheless water exists under various forms and states and only a small percentage of it can be useful for humans.

Percentage of water on the Earth

Nearly 70% of the surface of the Earth (almost three quarters) is covered by water (seas, oceans, lakes, rivers). This  Percentage makes of water the most important material on the surface of Earth and justifies the expression " Blue planet " used to called the Earth. Earth is the only known planet ( so far ) whose surface is partially covered with liquid water .

Photo: water on Earth, view from space. Photo courtesy of  US Fish & Wildlife Service Photo Library.

Different sources of water
All available water resources on Earth are also called " hydrosphere ".
Mains sources of water are:
- Seas and Oceans
- The waterways (rivers, streams)
- The underground reserves
- Ices (glaciers, sea ice, icebergs, snow)
- Various forms of water present in air (clouds, water vapor).
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Fresh water and salt water
There are two kinds of water:
- Salt water (97.2%) present in the seas and oceans.
- Fresh water (2.8%), that is to say water that does not contain salt, in lakes, streams, groundwater, ice and in the atmosphere.
To survive man needs fresh water but it is rare compared to salt water. The fresh water reserves are very precious and must be protected.
Freshwater:
- Can be drunk by humans and animals (as opposed to salt water).
- Can be used for irrigation in agriculture (most plants can't tolerate salt water).
- Can be used for washing (salt water partially neutralizes the effects of soap and leaves some salt on the skin as it dries)

Drinkable water on Earth
Fresh water must not be mistaken with drinkable water. Fresh water can be consumed without danger for health only if it doesn't contain bacteria, virus or pollution.
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Fairfax water: official website of Fairfax that provides water to roughly 2 millions people in Virginia.
This website offers educational resources about water for kids and teachers like the story of drinking water.

Volume of Earth's oceans: This page provides acurates data and percentage about sources of water.

Earth's precious water: National geographic kids networks gives free access to educational resources based on standard science curriculum. This page is about percentage of water on Earth, drinkable water and water cycle.