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Question

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

This question was previously asked in
CDS 1 2026 Maths Question Paper (12-Apr-2026)
The correct answer is

Temperature 273.16 K and Pressure 611 Pa

The triple point of water is a fundamental concept in thermodynamics and it represents the unique set of conditions at which water can coexist in its solid, liquid, and gaseous phases simultaneously. This is a specific and well-defined point, both in terms of temperature and pressure.

Let's analyze the given options:

  1. Temperature 273.16 K and Pressure 1 Pa: Although the temperature is correct, the pressure is too low. The standard pressure at the triple point is much higher.
  2. Temperature 273.16 K and Pressure 611 Pa: This is the scientifically accepted value for the triple point of water. At 273.16 K and 611 Pa, water can simultaneously exist in the solid, liquid, and vapor phases.
  3. Temperature 0 K and Pressure 103 Pa: Temperature of 0 K is absolute zero, where all molecular motion ceases. This cannot be the triple point of any substance.
  4. Temperature 373 K and Pressure 6.11 x 10-3 Pa: This temperature is equivalent to the boiling point of water at 1 atmospheric pressure, not the triple point. 373 K is much higher than the triple point temperature.

Based on this analysis, the correct answer is: Temperature 273.16 K and Pressure 611 Pa.

This value is universally recognized in scientific and engineering contexts for the calibration of temperature measurement instruments, among other applications.

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Important Questions from Thermodynamics

  1. A system that does NOT allow exchange of heat with its surrounding is called

  2. Which of the following statements correctly describes the thermodynamic classification of entropy?
  3. A mass of $10 \text{ kg}$ is suspended vertically by a rope from the roof. A horizontal force is applied on the rope at a point $P$. The point $P$ is $1 \text{ m}$ vertically below the roof attachment point, and the length of the rope segment from the roof to $P$ is $2 \text{ m}$. If the suspended mass is in equilibrium, what is the tension in the upper part of the rope (from roof to $P$)? (Take $g = 10 \text{ ms}^{-2}$)
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