Page 61 - Ripples SCIENCE 7 - TEJPUR Edition 2024 Answer Key

Ans. Evaporation.

8. Classify the following reactions in two categories with energy either absorbed or

released in these reactions :
(a) Volcanic eruption

(b) Photosynthesis by green plants

(c) Bursting of crackers

(d) Boiling of milk

(e) Composting of dry leaves and organic matter into manure
Ans. Energy observed Energy Released

1. Boiling of milk 1. Volcanic eruption.
2. Photosynthesis by green plants.
3. Bursting of crackers.

4. Composting of dry leaves and organic matter
into manure.

G. Long-answer questions.
1. Explain how we can prevent rusting.

Ans. Rusting can be prevented by the following methods.

(a) Painting, greasing or oiling regularly prevents rusting of iron objects. These

prevent the surfaces of iron objects from coming in direct contact with the air
& moisture.

(b) Galvanisation is the process of deposition of a thin layer of zinc on iron objects.

Zinc reacts with air and moisture to form a layer of zinc oxide. This layer prevents

rusting.
(c) Rusting can be prevented by using alloys. An alloy is a substance made by mixing

two or more metals or a metal and a non-metal. This process is called alloying.

2. Explain the term crystallisation with the help of an activity.

Ans. Crystallisation is the process of obtaining crystals of a pure substance from its
solution. In this process, the solid is allowed to dissolve in water. The solution is then

heated, so the water evaporates and a pure substance is left behind. This process is

a physical change.

Let us understand the process of crystallisation with the help of an experiment.
Experiment

Aim: To make pure crystals of copper sulphate from its solution

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