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


Air and atmosphere



1) What is pressure ?

A gas pushes upon all surfaces with which it is in contact: it is said it exerts a pressure.

2) What is atmospheric pressure ?

The atmospheric pressure corresponds to pressure exerted by air from the atmosphere.

A simple experiment demonstrates the existence of atmospheric pressure:
A glass is completely filled with water. A sheet of paper is pressed against the glass and then the glass is returned.
Water remains in the glass and doesn't flow.
Despite its weight the water is kept inside the glass because the outside air pressure is greater.



3) Pressure units

The legal unit of pressure is Pascal (Pa)
Is also used:
- The hectopascal (hPa) = 100 Pa 1 hPa
- The bar (bar) 1 bar = 100000 Pa

4) How to measure pressure ?

The atmospheric pressure is measured with a barometer.
At sea level atmospheric pressure is about 101300 Pa or 1013 hPa or about 1 bar but it may vary and leads to high pressures (anticyclone) corresponding to areas of good weather or it can leads to low pressures (depression) corresponding to areas of bad weather.

When a gas is enclosed its pressure is measured with a manometer.