An element is a form of matter which cannot be split into simpler substances, e.g. iron, sulphur, oxygen, gold. There are about 106 known elements.
A compound can be split into simpler substances and is formed by the union of two or more elements in a definite proportion by weight, e.g.
1. Water can be split into hydrogen and oxygen
2. Iron sulphide can be split into iron and sulphur
3. Chalk is made of calcium, carbon and oxygen
4. Carbon Dioxide is made up of carbon and oxygen
As opposed to a compound, a mixture is a substance in which two or more substances are mixed together in any ratio without altering their properties.
1. Sand and salt
2. Sugar and water
3. Air is a mixture of nitrogen, oxygen, carbon dioxide, water vapours and other gases
4. Gun Powder is a mixture of nitrate, charcoal and sulphur
It consists of nitrogen, oxygen, water vapours, carbon dioxide and traces of argon, ozone and some other gases and dust particles. Dry air contains these gases in the following proportions, by volume
One litre of air at 0°C and 760 mm of mercury (pressure) weighs 1.2932 grams. Chemically, it is a mixture and not a compound because its composition varies from place to place.
WATER VAPOURS IN AIR
If a clean-from-outside glass containing ice is placed in the open air, the outer surface gets covered with water drops. This is due to condensation of water vapours from the atmosphere on the cooler surface of glass as the air gets saturated when its temperature falls.
CARBON DIOXIDE IN AIR
Similarly, lime water placed outside in air milky due to the absorption of carbon dioxide from the atmosphere. The milky colour so obtained is due to the formation of chalk.
CHEMICAL COMPOSITION OF WATER
Water consists of hydrogen and oxygen, in the ratio of 2:1 by volume and 1:8 by weight. When an electric current is passed through water, for every one volume of oxygen, two columns of hydrogen are produced.
Conversely, water can be prepared by combining oxygen and hydrogen in the presence of electric current. It is seen that for every one part by weight of hydrogen required to prepare water, 8 parts of oxygen by weight are required.