Page 59 - Ripples SCIENCE 7 - TEJPUR Edition 2024 Answer Key
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E. Very short-answer questions.
1. What type of a change is crystallisation?
Ans. Crystallisation is a physical change.
2. What type of a change is involved in rusting?
Ans. Chemical change is involved in rusting.
3. Is cutting a log of wood into pieces a chemical process or a physical process?
Ans. Cutting a log of wood into pieces is a physical process.
4. Define galvanisation.
Ans. Galvanisation is the process of deposition of a thin layer of zinc on objects made
from iron.
5. Write two examples of chemical changes.
Ans. Two examples of chemical change are—digestion of food and burning of paper.
6. How will you detect the presence of carbon dioxide gas?
Ans. Turning lime water milky is a standard test to detect the presence of carbon dioxide
gas.
7. List two conditions necessary for rusting.
Ans. Two conditions necessary for rusting are :
• The presence of moisture (water vapour), and
• The presence of air (oxygen).
8. Give one example of a physical change which takes place by action of heat.
Ans. Boilling of water is a physical change which takes place by the action of heat.
F. Short-answer questions.
1. Differentiate between :
(a) Chemical change and physical change
Ans. Physical change involves a change in the physical properties of a substance (e.g, shape,
size and state) without altering its chemical properties. Hence, no new substances
are formed in a physical change. They can usually be reversed by Physical means
(e.g., melting, freezing). For example, melting of ice.
In contrast, chemical change involves a change in the chemical properties of a
substance, resulting in the formation of new substances with different properties.
These changes are often irreversible. For example, rusting of iron.
(b) Endothermic and exothermic reactions
Ans. Endothermic reactions absorb energy from their surroundings. The surroundings
get cooler because these reactions absorb heat. For example, melting of ice,
photosynthesis in plants, etc.
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