Science class online
Chemistry and physics lessons
atom physics lesson


Free online chemistry lesson for elementary school, middle school and high school.
chemistry high school

Science class




Mechanics lessons
Water on Earth
Changes of state in the nature: the water cycle
Water in human body
Test for water
Properties of water in different states
States of matter
States of matter and its changes
Boiling water
Water: freezing and melting
Changes of state: mass and volume
Molecules in different states of matter

Mixtures and solutions
Heterogeneous mixtures
Homogeneous mixtures
Vaporization of water
Dissolving a solid in water
Conservation of mass on dissolving
Miscibility in water

Mass and volume
Volume and its units
Measuring volume with a graduated cylinder
Mass and its units
Measuring the mass of a liquid
Mass of a liter of water

Air and atmosphere
Earth's atmosphere protect us
Threats to the Earth's atmosphere
Composition of air
Air and life
Atoms and molecules
Molecules in mixtures and pure substances
Molecules and states of matter
Composition of molecules
Constituents of the atoms
The electrical neutrality of the atom

Basics of combustion
The combustion of carbon
The combustion of butane
Atoms and chemical reactions
Chemical reactions
Chemical equations
Law of conservation of mass
Chemical synthesis

Most common metals

How to distinguish metals ?

Corrosion of metals

Electrical conductivity of solid materials

Electrons and free electrons

Electric current in metals
The conductivity of aqueous solutions
Aqueous solutions and ions
The direction of movement of ions
Formation of ions
Tests for ions
Acidic and basic solutions
pH of aqueous solutions
Ions in acidic and basic solutions
Dilution of acids and bases
Composition of hydrochloric acid
Chemical reaction between iron and hydrochloric acid

Electrochemical cells and chemical energy
Chemical reaction beteween a copper sulphate solution and zinc
Copper sulfate and zinc battery
Basics of electrochemical cell




Water on Earth


The essential about water on Earth

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 .
Water on Earth - view from space
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).


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.
- 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.


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.