CAPF AC 2020 Q21: Science & Technology – Physics
At triple point the substance co-exists in
1. Liquid phase
2. Solid phase
3. Vapour phase
Select the correct answer using the code given below:
- (a) 1 only
- (b) 1 and 2 only
- (c) 2 and 3 only
- (d) 1, 2 and 3
Answer: (d) 1, 2 and 3
Explanation
Triple point
How to think: the word triple itself means three.
So all three phases must be present.
The triple point is the one temperature and pressure at which the solid, liquid and vapour phases of a substance exist together in equilibrium.
| Phase in the question | Present at the triple point? |
|---|---|
| 1. Liquid phase | Yes |
| 2. Solid phase | Yes |
| 3. Vapour phase | Yes |
Triple point of water:
| Fact | Value |
|---|---|
| Temperature | 273.16 K (0.01 °C) |
| Pressure | About 0.006 atm |
| Phases together | Ice, water and water vapour |
| Use | Long used as the fixed point for the Kelvin scale |
Answer: (d) 1, 2 and 3.
Why each option is right or wrong
- (a) Incorrect. The liquid phase is present, but so are the other two.
- (b) Incorrect. Liquid and solid alone describe the melting point, not the triple point.
- (c) Incorrect. Solid and vapour alone describe sublimation, not the triple point.
- (d) CORRECT. At the triple point solid, liquid and vapour co-exist in equilibrium.
Key point
Triple point: the single temperature and pressure where solid, liquid and vapour co-exist. For water: 273.16 K and about 0.006 atm.
Remember
Triple point = solid + liquid + vapour together; water at 273.16 K.
Common mistakes
- Confusing the triple point with the critical point, where liquid and gas become alike.
- Writing the triple point of water as 273.15 K; that is the ice point, the triple point is 273.16 K.
- Thinking a substance has many triple points for these three phases; it has one.
Exam tip
Read the word: triple means three phases. No other fact is needed for this question.
Sources
- NCERT Class XI Physics — Thermal Properties of Matter (triple point)
- NCERT Class XI Physics — Thermodynamics