---
title: "Define saturation temperature, critical point, etc."
subject: "Fundamental of Thermodynamics & Heat Transfer"
chapter: 3
key: "semester_2/fundamental_of_thermodynamics_and_heat_transfer:saturation-temp-critical-point-defs"
source: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/saturation-temp-critical-point-defs
site: BCT Engineering
---

# Define saturation temperature, critical point, etc.

Fundamental of Thermodynamics & Heat Transfer · Chapter 3: Properties of Common Substances

- Asked in: 2073 Bhadra, 4 marks
- Video: [T-v Diagrams and PROPERTY TABLES for Thermodynamics in 13 Minutes!](https://www.youtube.com/watch?v=iZbnY0QWHe8) — Less Boring Lectures
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/saturation-temp-critical-point-defs
- Whole chapter: https://www.bctengineer.com/md/semester_2/fundamental_of_thermodynamics_and_heat_transfer/chapter-3

## Question

Define the following terms: saturation temperature, superheated vapor, moisture content, critical point and specific heat at constant pressure.

## Short answer

Saturation temperature is the phase-change temperature at a given pressure; superheated vapor is above its saturation temperature; moisture content is 1−x; critical point is where liquid and vapor phases become identical; specific heat at constant pressure c_p=(∂h/∂T)_P is the energy to raise unit mass one degree at constant pressure.

## Answer

- **Saturation temperature:** the temperature at which a pure substance changes phase (boils or condenses) at a given pressure. For water at 101.325 kPa it is 100 °C; it rises with pressure.
- **Superheated vapour:** vapour at a temperature above the saturation temperature for its pressure. It is a single-phase state; degree of superheat = T − T_sat.
- **Moisture content:** the mass fraction of liquid in a liquid-vapour mixture = 1 − x, where x is quality.
- **Critical point:** the point at the top of the saturation dome where the saturated liquid and saturated vapour states become identical; above it, liquid and vapour cannot be distinguished. For water P_c = 22.09 MPa, T_c = 374.1 °C, v_c = 0.00316 m³/kg.
- **Specific heat at constant pressure (c_p):** the energy required to raise the temperature of a unit mass of a substance by one degree while the pressure is constant: c_p = (∂h/∂T)_P (kJ/kg·K).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is saturation temperature?
   A. Temperature at which it changes phase at 1 atm only
   B. Temperature at which a pure substance changes phase at a given pressure
   C. Temperature of the surroundings
   D. The highest temperature of the liquid
   Answer: B. It depends on pressure and rises with it (water 100 C at 101.325 kPa). (Key point: Saturation temperature)

2. What defines the critical point?
   A. The point where quality is 0.5
   B. The point where the substance freezes
   C. Saturated liquid and vapour states become identical at the top of the dome
   D. The lowest pressure of boiling
   Answer: C. Above it, liquid and vapour cannot be distinguished. (Key point: Critical point)

3. How is cp defined?
   A. Energy to raise unit mass by one degree at constant volume, (du/dT)v
   B. The energy to vaporise unit mass
   C. The ratio of cp to cv
   D. Energy to raise unit mass by one degree at constant pressure, (dh/dT)P
   Answer: D. cp = (dh/dT) at constant pressure, in kJ/kg K. (Key point: Specific heat cp)

4. Moisture content equals:
   A. 1 - x
   B. x
   C. 1/x
   D. x vfg
   Answer: A. It is the liquid mass fraction of the mixture. (Key point: Moisture content)

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