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Chemistry and physics lessons
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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
Volumetric mass density
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


Sates of matter

Molecules in different states of matter

1) Different states of matter

Matter can exist in three different states: solid, liquid or gaseous.
It may also change of state:
From solid to liquid state (fusion or melting)
From liquid to gaseous state (vaporization)
From the gaseous to the liquid state (liquefaction or condensation)
From liquid to solid state (solidification or freezing)
From solid to gaseous state (sublimation)
From the gaseous to the solid state (deposition)

Although matter has different properties depending on its state:
Regardless of the state of a pure substance its molecules are identical.

Example: there is the same molecules of water in ice, in liquid water and water vapor.



2) Molecules in a solid

Schematic representation of molecules in a solid
(Molecules are symbolized by triangles)

Molecules in solid state

In the solid state, molecules are very close to each other (almost stacked) and the space they occupy is small: they are said to have a compact arrangement.
Each molecule has a fixed position: it is an orderly arrangement.

The solid state is compact and ordered

3) Molecules in a liquid

Schematic representation of molecules in a liquid
(Molecules are symbolized by triangles)

Molecules in liquid state

L'état liquide est compact et désordonné.

Molecules are still very close to each other and form a compact arrangement. Nevertheless, the molecules are not stationary, they can move by sliding over each other and are slightly agitated: they are organized in a disorderly manner.

The liquid state is compact and disordered.

4)  Molécules in a gas

Schematic representation of molecules in a gas
(Molecules are symbolized by triangles)

Molecules in gaseous state

The molecules are relatively far from each other and form a dispersed arrangement.
They are highly agitated and move very quickly in a disorderly manner.

The gaseous state is dispersed and disordered.