Elements, compounds and mixtures

Elements

• Elements cannot be broken down into anything simpler by either chemical or physical means.

• The smallest part of an element is the atom.

• Atoms of each element are identical but different from those of all other elements.

Eg; Sulphur only contains one type of atom, S.

Iron filings only contain one type of atom, Fe.

Compounds

• Compounds consist of two or more particles joined together by chemical bonds.

• When elements react to form compounds they give, take or share electrons to form chemical bonds.

• Their particles can only be separated by breaking the chemical bonds by carrying out a chemical reaction.

Eg; Salt, NaCl, consists of sodium atoms chemically combined with chlorine atoms.

The composition of a compound is constant. Salt always has one ion of sodium combined with one ion of chlorine.

Molecules

Molecules consist of two or more atoms joined together by a chemical bond.

The atoms may be the same eg Oxygen.

Or they may be different eg carbon dioxide.

Two atoms of oxygen joined by a chemical bond.

One atom of carbon joined by chemical bonds to two atoms of oxygen.

Diatomic molecules

Molecules that consist of two atoms are known as diatomic.

Many gases are diatomic.

Oxygen Hydrogen

Nitrogen Chlorine

Mixtures

• Mixtures are made up of particles that have been physically mixed together, but are not linked in any way.

• Mixtures have a variable composition.• Their different components can be easily

separated by physical methods.• Mixtures can be made up of elements and/or

compounds.• Eg; Air is a mixture of gases, mainly nitrogen

(N2) and oxygen (O2).

Sulphur and iron filings can be physically added together to form a mixture.

Eg; Sand (SiO2) and salt can be combined to make a mixture.

Air is a mixture of gases

Separating mixtures Mixtures can be separated into their components using physical means.

Eg; A mixture of sulphur and iron filings can be separated using a magnet.

Salt can be separated from sand by adding water, filtering and evaporating.

Mixtures of liquids can be separated using fractional

distillation.

The lowest boiling point liquid appears first.

Chromatography

• Chromatography (“colour writing”) is used to separate coloured liquids.

• The mixture is “spotted” on to filter paper, then placed in contact with a solvent.

• Different liquids are soluble to different extents in the solvent.

• So as the solvent passes up the filter paper it separates the components of the mixture.