Elements, Compounds and Mixtures.

Chapter 16

Classification of Substances

• Elements

• Molecules

• Compounds

• Mixtures.

Elements

• Elements are substances made up of only one type of atom.

• Example Hydrogen, Gold.

Periodic Table of Elements

Molecule

• A molecule is made up of two or more atoms chemically combined.

Compounds

• Compounds are made up of two or more different types of atoms chemically combined.

Properties of Compounds

• When a compound is formed during a chemical reaction its properties differ from those of the elements from which it is made.

• Example: Water is made from hydrogen and oxygen.

Hydrogen flammable gas

Oxygen colourless gas.

• Diagram of Salt, water, sucrose.

Mixtures

• A mixture consists of two or more different substances mingled together but not chemically combined.

• Examples: Sea – water and salt

Air – nitrogen, oxygen …

Soil – sand, clay, humus …

Compare mixtures and Compounds.

Mixture Compound

The amounts of the substances in the mixture can vary.

The elements in a compound are always present in the same fixed proportion.

A mixture contains two or more substances

A compound is a single substance.

The properties of a mixture are similar to those of the substances in a mixture.

The properties of a compound are different to those of the elements which reacted to form it.

There are practically no energy changes when a mixture is made

Heat is usually given out or taken in when a compound is formed.

It is usually easy to separate the components of a mixture.

It is usually difficult to separate the components of a compound.

iron particlesKey:

strong bonds between iron particles

one iron particle

iron particlesheld together by strong bonds

iron particles

iron particles are attracted to a magnet

iron particlesdiagram

sulfur particlesKey:

strong bonds between sulfur particles

one sulfur particle

sulfur particles held together by strong bonds

sulfur particles

sulfur particles are not attracted to a magnet

sulfur particlesdiagram

iron particles

sulfur particles

A mixture of iron and sulfur particles

strong bonds between sulfur particles

strong bonds betweeniron particles

What’s this?

A mixture of iron and sulfur particles

A magnet attracts the iron particles

but not the sulfur particles.

The mixture is separated

mixture diagram

one sulfur particle one iron particle

strong bonds between iron and sulfur particles

What’s this?A compound between iron and sulfur particles

strong bonds between iron and sulfur particles

A compound between iron and sulfur particles

The magnet cannot separate iron and sulfur particles in a compound

compound diagram

ReferencesSteve Lewis for the Royal Society of Chemistry

Photographs by Peter Hollamby

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