5.1 Substances in the Earth’s crust

The Earth’s crust consists of many different substances.

oxygen 45% silicon 27%

aluminium 8%iron 6%

calcium 5%magnesium 3%

sodium 2.5%potassium 1.5%

all the other metals and non-metals 2%

5.2 What is a compound?

A compound is a substance containing two or more elements chemically joined together.

Compound Elements which make up the compoundWater Hydrogen and oxygenSilicon dioxide Silicon and oxygenSugar Carbon, hydrogen and oxygenTable salt Sodium and chlorine

5.3 Making and breaking the compound water

When hydrogen combines with oxygen to form the compound water, a lot of energy is given out.

Hydrogen + oxygen water

Electrical energy can be used to break down water into hydrogen and oxygen.

Water hydrogen + oxygen

Electricalenergy

video 5.3

5.4 What is a mixture?

A mixture contains two or more elements or compounds not chemically joined together.

Mixture CompositionSilver coins Copper and nickelSoda water Mainly water and carbon dioxideMuddy water Mud and waterVinegar Mainly water and ethanoic acidAir Nitrogen, oxygen and other gases

5.5 Comparing properties of carbon/oxygen mixture and carbon dioxide

Property ortest

Carbon/ oxygenmixture

Carbon dioxide

Appearance Black solid incolourless gas

Colourless gas

Inserting aglowing splint

Relights a glowingsplint

Extinguishes aglowing splint

Adding limewater No change Turns limewatermilky

Molecule ( 分子 ) Some atoms can join together to

form a group called a molecule.

Molecule of an element

Molecule of a compound

5.5 Comparing properties of carbon/oxygen mixture and carbon dioxide

When carbon and oxygen are mixed together without heating, the carbon atoms and oxygen molecules are still present in the mixture.

When carbon is heated in oxygen, carbon atoms and oxygen atoms join together to form carbon dioxide molecules.

The carbon dioxide molecules are different from the carbon atoms and oxygen molecules.

Properties of carbon dioxide are different from those of carbon and oxygen.

ani 5.5

5.6 Comparing properties of iron/sulphur mixture and

iron(II) sulphide

Property or test Iron/ sulphurmixture

Iron(II) sulphide

Appearance Yellowish black solidmixture

Dark brown solid

Effect of magnet Attracts the iron only Not attractedAdding water Iron and most

sulphur sinkSinks as a singlesubstance

Adding dilutehydrochloric acid

Iron reacts to givehydrogen

Reacts to give a gaswith bad smell

Iron and sulphur still retain their original properties in the mixture. However, when they combine to form a compound, the compound shows new properties.

Comparing properties of hydrogen/oxygen mixture and water

Appearance Hydrogen : colourless gas Oxygen : colourless gas Water : colourless liquid

Comparing properties of hydrogen/oxygen mixture and water(chemical properties)

Inserting a burning splint Hydrogen : a ‘pop’ sound Oxygen : burn more violently Water : extinguish the fire

5.7 Useful web links

Materials on mixtures http://library.thinkquest.org/3659/atommole

Comparison of mixtures and compounds http://www.chem.vt.edu

/RVGS/ACT/notes/Mixtures_and_Compounds.html