A pure substance:

is made up of one sort of molecules (or atoms).

A mixture:

Is made up from more than one sort of molecules (atoms).

Example: Oxygen gas is a pure substance because it is made up of one sort of molecule.

Example: Carbon dioxide gas is a pure substance because it is made up of one sort of molecule.

Air is a mixture of the pure substances like oxygen, nitrogen, carbon dioxide, water, etc.

So there are many sorts of molecules in air: the O2 molecules, N2 molecules, CO2 molecules, H2O molecules, etc.

Nitrogen

oxygen

hydrogen

water

carbon dioxide

Just mixing elements together will not usually give a compound.

For example,

Hydrogen is mixed with oxygen

A mixture of hydrogen and oxygen

Hydrogen is mixed with oxygen

water

No water is formed for the simple mixing of hydrogen gas and oxygen gas.

In water, the hydrogen and oxygen atoms are joined together in every water molecule.

Mixture of hydrogen and oxygen.

Compound water: it contains individual water molecules.

Each water molecule is made up of two hydrogen atoms and one oxygen atom. It is a liquid.

This mixture is a gas.

In the mixture, both hydrogen and oxygen still own their properties.

e.g. the mixture is a gas, like hydrogen and oxygen.

The properties of the compound (water) are entirely different from those of hydrogen and oxygen.

E.g. water is a liquid while both hydrogen and oxygen are gases.

1. Two or more elements (e.g.a mixture of hydrogen and oxygen.)

2. Two or more compounds (e.g. a mixture of salt and sugar)

3. A combination of one or more elements and compounds (e.g. a mixture of oxygen and carbon dioxide)

When a substance is dissolved in a liquid, the mixture is called a solution.

1. The substance is called the solute.

2. The liquid is called the solvent.

A solid + a liquid solution

solvent solute

Example:

Sugar is soluble in water. Add a spoon of sugar into a beaker of water and stir. The final mixture is a solution. Sugar is the solute and water is the solvent.

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