Page 37 - ICSE Chemistry 8

Characteristics of a Chemical Change

• A chemical change is permanent and irreversible.
• The composi on of a substance changes during a chemical change.

• During a chemical change, one or more new substances with new sets of proper es are formed.
• There is an exchange of energy during a chemical change. Thinking Fountain

• A chemical change is accompanied by a change in mass. Why is boiling of an egg
a chemical change?
Examples of Chemical Change

Germina on of seeds, cooking of food, curdling of milk, ripening of fruits and vegetables, ro ng of eggs,
burning of wood, burning of papers and fuels are some very common examples of chemical change.

Categorising Chemical Changes

Let’s study about diff erent types of chemical changes.

Digestion of Food

Food undergoes many chemical changes during the process of diges on. Complex molecules such as
proteins, carbohydrates, fats, etc. breakdown into simpler molecules that have diff erent structures. The
original molecules cannot be obtained again from these molecules. Hence, diges on of food is a chemical
change.

Rusting of Iron

A piece of iron when le out in moist air comes in contact with oxygen to produce a dull bri le brown
substance called rust. This process of forma on of rust on the iron surface is called rus ng of iron. The
rust formed has diff erent proper es from that of iron and it cannot be converted back to iron by simple
physical methods. Hence, rus ng of iron is a chemical change.

Cooking of Food

During cooking of food, raw materials such as grains and Thirst for Knowledge
vegetables are heated to form new substances. The cooked
Burning of a candle is one example
food cannot be converted back to raw ingredients. Hence,
where both physical and chemical
cooking of food is a chemical change.
changes occurs together. When a
Burning of Paper candle burns, its wax melt but again
solidifi es on cooling. This is a physical
The paper burns immediately when brought near a fl ame. change. However, when some of the
The burnt paper (ash) is a new substance and it cannot be wax burns with the fl ame two new
changed to what it originally was. Therefore, burning of substances are formed. These are
paper is a chemical change. carbon dioxide and water vapour. This
is a chemical change.
Rotting of Eggs

Over the me, eggs become more alkaline and the sulphur present in them reacts with hydrogen to form
hydrogen sulphide producing an unpleasant ro en eggs odour. This is an irreversible chemical change.

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