Page 97 - Chemistry_Form_2

Chemistry
for Secondary Schools

Some decomposition reactions are caused by light. For example, white silver
chloride breaks down when exposed to light to give tiny black crystals of silver
and chlorine gas.

FOR ONLINE READING ONLY

Decomposition reactions are common in daily life. Some important processes
that are related to decomposition reactions include:

(a) Digestion of food: This is a biological decomposition reaction in which
Complex food molecules (carbohydrates, proteins, and fats) break down
into simpler substances in the body through enzymatic reactions. The body
absorbs and uses simpler substances for energy, growth, and repair. For

example, starch in food is broken down into simple sugars by the amylase
enzyme as shown in the following equation:

This reaction occurs in the stomach and intestines during digestion.
(b) Spoilage of food: Organic matters in food decompose over time because of
activities of microorganisms such as bacteria and fungi, leading to decay
and fermentation. For example, when milk spoils, lactose breaks down into
lactic acid due to bacterial actions, resulting in a sour taste. The following
equation shows its decomposition reaction.

(c) Thermal decomposition of limestone: In the cement industry, limestone
(calcium carbonate) is heated to form lime (calcium oxide) and carbon
dioxide as shown in the following reaction:

Combustion reactions
A combustion reaction is a chemical process in which a substance reacts with
oxygen to produce heat and light. For example, burning of fuels such as wood,
coal, diesel and petrol is a combustion reaction.

General formula:

90 Student’s Book Form Two

01/08/2025 11:20:59
CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 90
CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 90 01/08/2025 11:20:59