---
title: "Define compressed liquid, degree of superheat, etc."
subject: "Fundamental of Thermodynamics & Heat Transfer"
chapter: 3
key: "semester_2/fundamental_of_thermodynamics_and_heat_transfer:compressed-liquid-superheat-quality-defs"
source: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/compressed-liquid-superheat-quality-defs
site: BCT Engineering
---

# Define compressed liquid, degree of superheat, etc.

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

- Asked in: 2073 Magh, 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/compressed-liquid-superheat-quality-defs
- Whole chapter: https://www.bctengineer.com/md/semester_2/fundamental_of_thermodynamics_and_heat_transfer/chapter-3

## Question

Define the following terms: compressed liquid, degree of superheat, quality, enthalpy and specific heat at constant volume.

## Short answer

Compressed liquid is liquid below saturation temperature at its pressure. Degree of superheat is T−T_sat. Quality is the vapor mass fraction; enthalpy h=u+Pv. Specific heat at constant volume c_v=(∂u/∂T)_v.

## Answer

- **Compressed (sub-cooled) liquid:** liquid at a temperature below the saturation temperature for its pressure (or at a pressure above the saturation pressure for its temperature). It is not about to vaporise.
- **Degree of superheat:** the difference between the actual temperature of a superheated vapour and the saturation temperature at the same pressure: ΔT_sup = T − T_sat.
- **Quality (dryness fraction), x:** the ratio of the mass of vapour to the total mass of a saturated liquid-vapour mixture, x = m_g/(m_f + m_g). It lies between 0 (saturated liquid) and 1 (saturated vapour).
- **Enthalpy:** a property defined as H = U + PV (kJ); specific enthalpy h = u + Pv. It represents the total energy of a flowing fluid per unit mass (internal energy plus flow work).
- **Specific heat at constant volume (c_v):** the energy required to raise the temperature of a unit mass of a substance by one degree while its volume is constant: c_v = (∂u/∂T)_v.

## Check yourself

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

1. What is a compressed liquid?
   A. Liquid at Tsat exactly
   B. Liquid below Tsat at its pressure (or above Psat at its temperature)
   C. Liquid with quality 0.5
   D. Vapour above Tsat
   Answer: B. It is not about to vaporise. (Key point: Compressed liquid)

2. How is degree of superheat found?
   A. Tsat - T
   B. T/Tsat
   C. T - Tsat at the same pressure
   D. T - Tc
   Answer: C. It is the actual temperature minus saturation temperature. (Key point: Degree of superheat)

3. What is enthalpy?
   A. H = U - PV
   B. H = U + T S
   C. H = Q - W
   D. H = U + PV
   Answer: D. Specific enthalpy is h = u + Pv, including flow work. (Key point: Enthalpy)

4. How is cv defined?
   A. (du/dT) at constant volume
   B. (dh/dT) at constant pressure
   C. (dQ/dP) at constant T
   D. Energy to vaporise unit mass
   Answer: A. cv is energy per unit mass per degree at constant volume. (Key point: Specific heat cv)

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