Page 138 - ICSE Chemistry 8

DECOMPOSITION REACTIONS TO FORM OXIDES

An oxide is a chemical compound containing oxygen in its molecule,
Ores
chemically combined with a metal or a non-metal. It is a compound
Ores are naturally occurring
which consists of oxygen as an anion. Oxygen is highly reac ve due to six
solid materials from which a
electrons in the outermost shell. It forms oxides by combining with metals metal can be extracted.
and non-metals. We can produce oxides of metals by hea ng their ores.
This can be done in two ways—calcina on and roas ng.

Calcination

• Calcina on is a decomposi on process in which a metallic ore is heated strongly in the absence
or limited supply of air to remove carbon dioxide and water.

• This method is employed for carbonate ores.
• Carbon dioxide is produced along with metal oxide.
Heat
CaCO CaO + CO 2
3
Calcium carbonate Calcium oxide Carbon dioxide
heat
ZnCO ZnO + CO 2
3
Zinc carbonate Zinc oxide Carbon dioxide
.
CuCO Cu(OH) heat 2CuO + CO 2 + H O
3
2
2
Copper carbonate Copper oxide Carbon dioxide Water
Roasting
• Roas ng is a decomposi on process in which a metallic ore is heated strongly in the presence of
air at a very high temperature.
• This method is employed for sulphide ores.

• Sulphur dioxide is produced along with metal oxide.
2PbS + 3O 2PbO + 2SO 2
2
Lead sulphide Oxygen Lead oxide Sulphur dioxide
4FeS + 7O 2Fe O 3 + 4SO 2
2
2
Iron sulphide Oxygen Iron(III) oxide Sulphur dioxide
2Cu S + 3O 2Cu O + 2SO 2
2
2
2
Copper (I) sulphide Oxygen Copper(I) oxide Sulphur dioxide

Thirst for Knowledge

Decomposi on of Nitrates
Metal nitrates decompose on hea ng to give metal oxides, nitrogen dioxide and gas. Examples of lead
nitrate and magnesium nitrate are given below.
2Pb(NO ) Heat 2Pb + 4NO + O 2
3 2
2
2Mg(NO ) Heat 2MgO + 4NO + O 2
2
3 2

124