States of Matter and Phase Changes - Worksheet | Printable and ... - Free Printable

Let’s solve each question step by step with clear explanations.

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Q.7. Which part of the graph shows that the substance was undergoing phase change?

✔ Correct Answer: b) L

Explanation:
In a temperature vs. time graph for heating or cooling a substance, phase changes occur when temperature remains constant even though heat is being added or removed. This is because the energy is used to break or form intermolecular bonds (changing phase), not to increase/decrease kinetic energy (temperature).

Looking at the graph:
- K: Temperature is rising → substance is heating up (solid or liquid, depending on context).
- L: Horizontal line → temperature is constant → this is a phase change (likely melting or freezing).
- M: Temperature rising again → substance is heating in a new phase (e.g., liquid after melting).
- N: Temperature falling → cooling down, possibly condensing or freezing.

So, L is the only flat section — indicating a phase change.

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Q.8. The amount of matter __________ during a phase change.

✔ Correct Answer: b) stays the same

Explanation:
Phase changes (melting, freezing, vaporization, condensation, sublimation, deposition) are physical changes, not chemical. No atoms are created or destroyed — only their arrangement and energy state changes.

Mass (amount of matter) is conserved in physical changes. So, whether ice melts to water or water boils to steam, the mass remains the same.

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Q.9. A substance has a melting point of -10 °C, and a boiling point of 60 °C. What will be the state of this substance at -27 °C?

✔ Correct Answer: a) Solid

Explanation:
- Melting point = -10 °C → below this temp, substance is solid.
- Boiling point = 60 °C → above this temp, substance is gas.
- Between -10 °C and 60 °C → substance is liquid.

Given temperature: -27 °C

Since -27 °C < -10 °C, the substance is below its melting point → it must be in the solid state.

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Q.10. Which of the following phase changes requires the subtraction of heat energy?

✔ Correct Answer: d) Condensation

Explanation:
“Subtraction of heat energy” means heat is released — the process is exothermic.

Let’s review:

- Melting → solid → liquid → absorbs heat (endothermic)
- Vaporization → liquid → gas → absorbs heat (endothermic)
- Sublimation → solid → gas → absorbs heat (endothermic)
- Condensation → gas → liquid → releases heat (exothermic) → ✔ This is the correct answer

Condensation occurs when gas molecules lose energy and come together to form liquid — so heat is subtracted (removed) from the system.

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✔ Final Answers:

Q.7: b) L
Q.8: b) stays the same
Q.9: a) Solid
Q.10: d) Condensation