At triple point for water, which of the following term is not equal to zero?

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SSC JE ME Previous Paper 17 ( Held on: 24 Jan 2018 Morning)
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  1. Enthalpy
  2. Entropy
  3. Internal energy
  4. None of these

Answer (Detailed Solution Below)

Option 1 : Enthalpy
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Explanation:

Since, the properties like internal energy, enthalpy and entropy of a system cannot be directly measured. They are related to change in the energy of the system.

Hence, we can determine Δu, Δh, Δs but not the absolute values of these properties.

⇒ Therefore, It is necessary to choose a reference state to which, these properties are arbitrary assigned some numerical values.  

So for water, the triple point (T = 0.01°C & P = 611 Pa) is selected as reference a state, where the “Internal energy” (u) and “Entropy” (s) of saturated liquid are assigned a zero value.

#Note: h = u + Pv

At triple point, u = 0, but p × ν ≠ 0

Therefore h ≠ 0 at triple point.
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