Interconversions between the three states of matter
4 marks · NormalThe table shows the melting and boiling points of three substances at standard pressure.
| Substance | Melting point (°C) | Boiling point (°C) |
|---|---|---|
| A | –15 | 58 |
| B | 112 | 212 |
| C | 75 | –10 |
At room temperature (20°C), name the physical state of each substance (A, B, and C).
Explain why substance C cannot exist as a liquid at room temperature.
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(a) A: liquid; B: solid; C: gas
(b) Because C's boiling point is lower than its melting point, so there is no temperature at which it can be a liquid. It changes directly from solid to gas.
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Marking points
B1 (a) · 1 mark · Correctly identifies the state of A as liquid at 20°C.
B2 (a) · 1 mark · Correctly identifies the states of B (solid) and C (gas) at 20°C.
M1 (b) · 1 mark · Explains there is no temperature range where C is a liquid (boiling point is below melting point).
A1 (b) · 1 mark · States that C changes directly from solid to gas (sublimation).
Worked solution
a) A: Melting point is –15°C, boiling point is 58°C; at 20°C it is liquid. B: Melting point is 112°C, so at 20°C it is solid. C: Boiling point is below melting point, so at 20°C it is a gas.
b) C cannot exist as a liquid at any temperature, because its boiling point (–10°C) is below its melting point (75°C), so it changes directly from solid to gas (sublimation).