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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


Mixtures and solutions

Heterogeneous mixtures

1) What is a mixture?

Before studying the heterogeneous mixtures we must first know  what's a mixture:
A mixture consists of at least two substances that are brought together in the same receptacle.
Here are some examples:
- Sand and water.
- Oil and vinegar
- Air and water vapor
- Flour, water and salt


2) Example of a heterogeneous mixture

Before defining a heterogeneous mixture, we will see an example that will help to understand more easily this definition.
This example is that of water and oil.

The following figure shows a mixture of water and oil.
Heterogeneous mixture of oil and water

Step 1: Water and oil are introduced into a test tube. Oil stays above the water (it floats) because oil is less dense than water (a phenomenon explained by the buoyancy).
Step 2: After shaking the oil is dispersed in water as droplets (more or less small depending on the force of agitation)
At this point it is said that oil and water form an emulsion.
Step 3: After waiting, all oil droplets finally rise to the surface and re-form the initial layer of oil.

3) Definition of a heterogeneous mixture

A mixture is said to be heterogeneous if at least two of its constituents are visible to the naked eye even after shaking.

These mixtures may consist of:
- Two liquids such as water and oil or water and alcohol.
- A liquid and a solid as the water and sand, water and salt or water and flour.
- A liquid and a gas as in a lemonade exposed to air.