A. Choose the Correct Options
1. A: Biogas is produced by
decomposition of plant and animal waste (organic matter).
B: Biogas is used as fuel to cook food.
o Answer: (c) Both A and B are chemical
changes.
§ Reason: Decomposition of organic matter
(A) is a chemical change, and burning biogas (B) is also a chemical change
(combustion).
2. A change in which heat energy
_______________ is called an endothermic change.
o Answer: (c) is absorbed
3. A change that is useful for us is
called a(n) _______________ change.
o Answer: (d) desirable
4. _______________ is a chemical
change.
o Answer: (b) Burning wood
5. Ripening of fruits is a(n)
_______________ change.
o Answer: (d) all of these (slow,
irreversible, chemical)
6. Amar has classified a few changes
as given in the table. Which of them are classified incorrectly?
o Answer: (b) II only
§ Reason: Melting of ice is endothermic,
not exothermic.
B. Classify These Changes in More Than One Way
Classification
of Changes
|
Changes |
Reversible/Irreversible |
Desirable/Undesirable |
Physical/Chemical |
|
1.
Burning a candle |
Irreversible |
Desirable |
Chemical |
|
2.
Breaking a brick |
Irreversible |
Undesirable |
Physical |
|
3.
Dissolving salt in water |
Reversible |
Desirable |
Physical |
|
4.
Tearing paper |
Irreversible |
Undesirable |
Physical |
|
5.
Digestion of food |
Irreversible |
Desirable |
Chemical |
|
6.
Curdling of milk |
Irreversible |
Desirable/Undesirable* |
Chemical |
*Curdling
of milk can be desirable (e.g., making yogurt) or undesirable (e.g., spoilage).
C. Short-Answer Questions
1. Why are melting and boiling
regarded as physical changes? Explain with examples.
o Melting and boiling are physical
changes because they involve a change in the state of matter (solid →
liquid or liquid → gas) without forming a new substance. The chemical
composition remains the same.
§ Example: Melting of ice (solid → liquid
water) or boiling of water (liquid → steam).
2. Rajat took water in a steel glass
and dissolved some glucose in it. He observed that the glass felt cold. Is this
an endothermic or an exothermic change? Why?
o This is an endothermic change
because heat energy is absorbed from the surroundings (the steel glass)
to dissolve the glucose, causing the glass to feel cold.
3. Cutting wood is an irreversible
change. How will you classify it as a physical or a chemical change? Give a
reason to support your answer.
o Cutting wood is a physical
change because it only changes the shape and size of the wood. No
new substance is formed, and the chemical composition remains the same.
4. Why is evaporation considered a
physical and an endothermic change? Explain.
o Evaporation is a physical
change because it involves a change in the state of matter (liquid → gas)
without forming a new substance.
o It is endothermic because
it absorbs heat from the surroundings to convert liquid into vapor
(e.g., sweat evaporating cools the body).
5. Why is shaping of clay on a
potter's wheel considered a physical change?
o Shaping clay is a physical
change because it only alters the shape and appearance of the clay.
No new substance is formed, and the chemical composition remains unchanged.
6. Write any two differences between
endothermic and exothermic changes.
o Endothermic Change:
1. Absorbs heat energy from the
surroundings.
2. Causes a drop in the temperature
of the surroundings (e.g., melting ice).
o Exothermic Change:
0. Releases heat energy into the
surroundings.
1. Causes a rise in the temperature
of the surroundings (e.g., burning wood).
D. Long-Answer Questions
1. The formation of clouds is a
physical change. Why?
o The formation of clouds involves
the condensation of water vapour into tiny water droplets or ice
crystals. This is a physical change because:
§ Only the state of matter
changes (gas → liquid).
§ No new substance is formed; the
water remains chemically the same (H₂O).
§ The process is reversible
(water can evaporate again).
2. How will you prove that
dissolving quicklime in water is an exothermic change?
o Experiment:
1. Take a beaker of water and
measure its initial temperature.
2. Add a spoonful of quicklime
(calcium oxide, CaO) to the water and stir.
3. Observe the temperature of the
water.
o Observation: The temperature of the water rises,
indicating that heat is released into the surroundings.
o Conclusion: Since heat is evolved,
dissolving quicklime in water is an exothermic change.
3. Write any five differences
between physical and chemical changes.
Differences
Between Physical and Chemical Changes
|
Physical Change |
Chemical Change |
|
No new
substance is formed. |
A new
substance is formed. |
|
Chemical
composition remains the same. |
Chemical
composition changes. |
|
Usually
reversible (e.g., melting ice). |
Usually
irreversible (e.g., burning wood). |
|
Energy
may or may not be involved. |
Energy
is absorbed or released. |
|
Only
physical properties change (e.g., shape, size, state). |
Chemical
properties change (e.g., reactivity, color). |
4. Why is rusting an undesirable
chemical change? Give any four ways to protect iron objects from rusting.
o Undesirable: Rusting weakens iron objects,
reduces their lifespan, and causes economic losses (e.g., damaged
infrastructure).
o Ways to Prevent Rusting:
1. Painting: Coats iron to prevent contact
with moisture and air.
2. Oiling/Greasing: Forms a protective layer that
repels water.
3. Galvanization: Coating iron with zinc to
prevent rusting.
4. Alloying: Mixing iron with other metals
(e.g., chromium in stainless steel) to make it corrosion-resistant.
5. Describe an experiment to show
that sublimation is a physical change.
o Experiment:
1. Take a few crystals of iodine or
camphor in a China dish.
2. Place a funnel inverted over the
dish and plug its mouth with cotton wool.
3. Heat the dish gently using a
burner.
o Observation:
§ Violet vapors of iodine (or
camphor) rise and deposit as solid on the cool inner surface of the
funnel.
o Conclusion:
§ Sublimation (solid → gas) and
deposition (gas → solid) are physical changes because:
§ No new substance is formed.
§ The change is reversible
(iodine can sublime back to solid).
§ Only the state of matter changes.
E. Case-Based Questions
I. Rusting of Iron
1. The type of change shown here is:
o Answer: (b) irreversible and chemical
change
2. Is this change desirable or
undesirable? Give a reason.
o Undesirable because rusting weakens iron
objects, reduces their usability, and leads to economic losses (e.g., replacing
rusted iron structures).
3. Give any two simple ways to avoid
this change.
o Painting iron objects to prevent contact
with moisture and air.
o Galvanization (coating iron with zinc) to
protect it from rusting.
II. Reaction of Marble with Acid (Limewater Test)
1. Which type of change is involved
in the given reaction? Give reasons to support your answer.
o Chemical change because:
§ A new substance (carbon
dioxide, CO₂) is formed when marble (calcium carbonate, CaCO₃) reacts with
hydrochloric acid (HCl).
§ The limewater turns milky
due to the formation of calcium carbonate (CaCO₃) precipitate, confirming the
production of CO₂.
2. Why does limewater turn milky on
passing carbon dioxide through it?
o Limewater (calcium hydroxide,
Ca(OH)₂) reacts with carbon dioxide (CO₂) to form insoluble calcium
carbonate (CaCO₃), which appears as a white precipitate, making the
limewater turn milky.
§ Reaction:
Ca(OH)₂ + CO₂ → CaCO₃ (white precipitate) + H₂O
3. Name the new substance formed
with a new composition and new properties in this reaction.
o Calcium carbonate (CaCO₃) is the new substance formed.
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