Page 140 - Chemistry_Form_2
P. 140
Chemistry
for Secondary Schools
Action of heat on salts
Different salts behave differently when heated. Some salts will decompose after
slight heating, yet others have to be heated strongly to decompose. Some salts do
not decompose at all, even with strong heating.
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Action of heat on carbonates
Upon heating, metal carbonates decompose to form metal oxides and carbon
dioxide gas. The decomposition reaction can be represented by the following
equation, where M represents a particular metal:
Sodium and potassium carbonates do not decompose even when strongly heated.
The carbonates of magnesium and calcium decompose to give magnesium oxide
and calcium oxide, respectively, with the evolution of carbon dioxide gas.
Other metal carbonates decompose easily on heating. For example, green
copper(II) carbonate decomposes easily upon heating to give black copper(II)
oxide and carbon dioxide gas.
Zinc carbonate decomposes easily on heating to form zinc oxide and carbon
dioxide gas. The zinc oxide produced is yellow when hot and white when cold.
Lead(II) carbonate decomposes when heated to give lead(II) oxide and carbon
dioxide gas.
Ammonium carbonate behaves so differently when heated. It decomposes slowly
at room temperature to produce ammonia gas, water, and carbon dioxide.
A strong pungent smell is produced due to the presence of ammonia gas. The
evolved gas turns limewater milky, indicating the presence of carbon dioxide. It
also turns a blue cobalt(II) chloride paper pink indicating the presence of water
Student’s Book Form Two 133
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CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 133 01/08/2025 11:21:10
CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 133
