CDS 2026 (I) General Knowledge, question 12

Topic: Physics

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Q12

Which one of the following gives the triple point of water ?

  1. (a) Temperature 273.16 K273.16\text{ K} and Pressure 1 Pa1\text{ Pa}
  2. (b) Temperature 273.16 K273.16\text{ K} and Pressure 6.11×10−3 Pa6.11\times 10^{-3}\text{ Pa}Correct answer
  3. (c) Temperature 0 K0\text{ K} and Pressure 103 Pa10^{3}\text{ Pa}
  4. (d) Temperature 373 K373\text{ K} and Pressure 6.11×10−3 Pa6.11\times 10^{-3}\text{ Pa}

Solution.

Answer: (b) Temperature 273.16 K273.16\text{ K} and Pressure 6.11×10−3 Pa6.11\times 10^{-3}\text{ Pa}

  1. The triple point of water is the one temperature and pressure at which ice, liquid water and water vapour exist together.
  2. It is at 273.16 K273.16\text{ K} and about 611 Pa611\text{ Pa}, which is 6.11×10−3 bar6.11\times 10^{-3}\text{ bar}.
  3. Only (b) has both the correct temperature and the correct figure 6.11×10−36.11\times 10^{-3}, so it is the closest option.

Trap: The unit printed in (b) is wrong: the pressure is 6.11×10−36.11\times 10^{-3} bar (611 Pa), not Pa. (a) has the right temperature but the wrong pressure.