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


Mixtures and solutions


1) What is decantation ?

It simply consists in waiting that, in a heterogeneous mixture, constituents separate one each other spontaneously.

2) Example: decantation of a mixture of water and soil

Decantation of water and soil
After shaking the mixture,  the soil particles are dispersed in water.
We then observe:
- A layer of soil forms slowly to the bottom: it consists of particles that fall as a result of their weight.
- The liquid gradually brightens. Less dense particles fall slower.
- After a sufficiently long time, the liquid becomes clear since all particles have fallen to the bottom of the beaker.



3) Advantages and drawbacks of decantation

Decantation is a simple separation to implement since it requires no special equipment but obtaining a complete separation and a totally clear liquid can takes a long time (sometimes several days) if the  particles mixed to water are thin and of low density.

- The separation of water and oil after shaking also corresponds to a decantation  (  the only difference being that the oil rises to the surface ).
- Decantation also occurs in nature when water is mixed with soil or sand.