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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



Electric current in metals

1) The electric charges

There are two kinds of electric charges:
- Positive electrical charges (+)
- Negative electrical charges (-)

These electrical charges are comparable to north and south poles of magnets: identical poles repel while different poles attract.
We can see a similar phenomenon with electric charges:

- Electrical charges of same sign repel (two negative charges repel each other or two positive charges repel each other).
- Electrical charges of opposite signs attract (positive charge and negative charge attract).



2) The electric charge of electrons

Electrons are electrically charged particles:
Each electron carries a negative elementary charge denoted - e.
The term " elementary " means that there is no electric charge smaller.

3) Electric current in metals

If a metal is a part of an electrical circuit including a generator, the free electrons are influenced by positive and negative terminals of the latter:
Because of their negative charge, electrons are repelled by the negative terminal of the generator and are attracted by its positive terminal.

The free electrons are set in motion and this movement constitutes the electric current.
The free electrons move from the negative to the positive terminal: that is the opposite of conventional direction of electric current