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CNMS ICSE - 7 Std
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Answer key

Chapter: 02. Physical And Chemical Changes

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SECTION A: OBJECTIVE TYPE QUESTIONS

1. Multiple Choice Questions (MCQs)

(b) Dissolving glucose in water (Explanation: Dissolving glucose in water absorbs heat, making the container feel cooler, which is an endothermic change.)
(​c​) Hydrogen (Explanation: Zinc reacts with dilute hydrochloric acid to form zinc chloride and hydrogen gas.)
(b) Cutting down of trees (Explanation: It is desirable for making paper/furniture but undesirable because it causes soil erosion and flooding.)
(d) Neither is attracted to the magnet (Explanation: Heating iron and sulphur forms iron sulphide, which is a new compound that loses iron’s magnetic properties.)
(a) Swinging of a clock pendulum (Explanation: It occurs at fixed intervals of time, making it a periodic change.)
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2. Fill in the Blanks

deposition
endothermic
rust
charred sugar
surface
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3. True or False Questions

False (Explanation: Photosynthesis is a chemical change, not physical, as new substances like glucose and oxygen are formed.)
True (Explanation: Steam can be cooled back into water, returning it to its original form.)
False (Explanation: Spoilage of food is harmful to humankind and is an undesirable change.)
True (Explanation: A chemical change alters both the physical and chemical properties of a substance.)
False (Explanation: Dissolving quicklime in water is exothermic; it releases heat, making the beaker hot.)
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4. Complete the Series

Irreversible change
Non-periodic change
Chemical change
Exothermic change
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SECTION B: DEFINITIONS & DIFFERENCES

5. Explanation of Terms

Sublimation: The physical process in which a solid substance changes directly into its gaseous state on heating, without passing through the intermediate liquid state.
Chemical Change: A permanent and generally irreversible change in which the chemical composition and properties of a substance are altered, resulting in the formation of one or more new substances.
Periodic Change: A change that repeats itself or occurs regularly at fixed intervals of time.
Evaporation: The slow physical process by which a liquid changes into its vapor state at any temperature below its boiling point.
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6. Identify and/or Differentiate Concepts

Evaporation vs. Boiling:
Parameter
Evaporation
Boiling
Speed of the process
It is a slow process.
It is a fast process.
Part of the liquid
It takes place only from the surface of the liquid.
It takes place throughout the entire liquid.
Effect on temperature
It causes cooling.
It does not cause cooling.
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Physical Change vs. Chemical Change Table:
Parameter
Physical Change
Chemical Change
Formation of new substances
No new substance is formed.
One or more new substances are formed.
Reversibility
It is generally temporary and reversible.
It is permanent and generally irreversible.
Change in chemical composition
No change in chemical composition.
Chemical composition is altered completely.
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SECTION C: SHORT ANSWER & PRACTICAL/OBSERVATION QUESTIONS

7. Short Answer Questions

When apple slices are kept in the open, they react with oxygen present in the air to form a new substance. This chemical reaction causes the cut surfaces to turn brown.
Cooking of food (or burning of a matchstick). It is done by humans, cannot be reversed, happens quickly, and forms new substances.
The growth of a seedling into a tree takes several years to complete, which makes it a slow change. It is irreversible because a fully grown tree cannot be changed back into a seedling under any condition.
Bring a lighted matchstick near the mouth of the test tube containing the reacting mixture. If the gas burns with a pop sound, it confirms that the evolved gas is hydrogen.
Spirit is highly volatile and vaporizes easily. When put on your palm, it absorbs heat energy (endothermic process) from your hand to change into vapor, which leaves your palm feeling cold.
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8. Diagram-Based Question

The process shown is sublimation. It is defined as the change of a solid directly into a gas on heating, or gas directly into solid on cooling (deposition).
The change is temporary and reversible. This is because no new substance is formed; only the physical state of ammonium chloride changes from solid to vapor and back to solid.
The solid ammonium chloride can be recovered by scraping it off from the cooler upper parts (the neck) of the inverted funnel where the vapors have condensed and deposited.
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9. Environmental Awareness & Everyday Use

Rainfall is desirable for farmers at the time of sowing seeds as it provides the water required for germination and crop growth. However, it is undesirable at the harvest stage because heavy rains can damage the mature crops ready to be collected.
Extracting metals chemically is highly important because raw metals are locked up in ores. Chemical processes allow us to isolate pure metals like iron, copper, and aluminium, which are essential for building infrastructure, machinery, utensils, wiring, and transportation systems.
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SECTION D: LONG ANSWER QUESTIONS

10. Long Answer Questions

The Iron and Sulphur Experiment:
(a) Mixing without heating: When mixed without heating, iron filings and yellow sulphur powder form a simple mixture. If a magnet is brought near it, the iron filings are attracted and easily separate from the yellow sulphur.
(b) Heating strongly: When heated strongly, the mixture melts and turns dark grey, forming a new substance called iron sulphide. When a magnet is brought near this dark grey product, the iron is not attracted to the magnet because it has lost its metallic magnetic properties.
(​c​) Classification:
Simple mixing is a physical change because no new substance is formed, the components retain their individual properties, and they can be separated by a simple physical method (using a magnet).
Heating the mixture is a chemical change because a brand new substance (iron sulphide) with completely different chemical properties is formed, and it cannot be reversed to get the iron and sulphur back.
The Burning Candle:
When a candle burns, both physical and chemical changes take place simultaneously:
Physical Change (Melting of Wax): The heat of the flame melts the solid wax into liquid wax. If this liquid wax drops onto a surface, it cools down and solidifies back into solid wax. No new substance is formed, and the process is easily reversible by changing the temperature.
Chemical Change (Burning of Wax): The molten wax rises up the wick, turns into vapor due to heat, and burns in the presence of oxygen. This burning process produces two entirely new substances: carbon dioxide and water vapor. This part of the change is irreversible, releases energy in the form of heat and light, and causes the candle to become smaller.
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