Latent Heat Calculator

Created by Luis Fernando
Last updated: Aug 24, 2022

Welcome to the latent heat calculator, a tool created to calculate the heat energy corresponding to phase transition processes, such as melting or evaporation. You can use this calculator for three purposes:

  • Calculating the latent heat to cause a specific phase change in an amount of substance.
  • Calculating the specific latent heat, given the substance mass and total latent heat; and
  • Calculate the mass required corresponding to an amount of latent heat in a particular substance.

If you still don't know what we're talking about, don't worry! The following sections briefly explain the concept of latent heat and the formulas for the latent heat of vaporization or fusion.

What is latent heat?

When we transfer heat to a substance, its temperature increases according to its specific heat capacity. The energy needed to change the temperature is known as sensible heat.

As we add heat, this temperature increase stops once we reach the phase change temperature (boiling point for liquids and melting point for solids). At this point, the heat we add won't change the temperature but will cause a phase change by breaking internal structures in the substance. After this change occurs, the temperature change will be possible again.

In general, latent heat refers to the energy necessary to cause a phase change in a thermodynamic system (usually a body of mass). The latent heat will be different depending on the type of phase change. For example, we need about 22647 kJ of energy to evaporate 10 kilograms of liquid water, while we need about 3340 kJ to melt 10 kg of ice. Considering those differences, we can define two types of latent heat:

  • Latent heat of fusion: that is the heat required to melt an amount of substance, taking it from solid to liquid state. It's also known as the enthalpy of fusion.
  • Latent heat of vaporization: the heat required to evaporate an amount of substance, taking it from liquid to gas state. It's also known as the enthalpy of vaporization.

Latent heats of fusion and vaporization also refer to the energy necessary to achieve the opposite process. For example, the energy required to evaporate 1 kg of water is the same energy that water has to release to return from vapor to liquid.

Check the enthalpy calculator to learn more about this thermodynamic property.

Specific latent heat

Heating 10 kg of water is not the same as heating 1 kg. As recording energy necessary to heat different amounts of different substances is not practical, we need to define an intensive property that allows for latent heat energy calculation for any quantity of substance.

Specific latent heat is the energy needed to change the phase of 1 kg of a particular substance. While latent heat has energy units (kJ, usually), specific latent heat has energy units per unit mass (kJ/kg).

In the following graph, you can look at the evolution of the water heated from −100 °C to 200 °C and the different amounts of energy involved. You can note that when reaching the melting and boiling points (0 and 100°C, respectively), the heat won't cause a temperature change until a phase change occurs.

Now, let's look at the latent heat equation behind this calculator.

Water temperature vs. heat added
Water temperature vs. heat added. Attribution: Cmglee, CC BY-SA 4.0, via Wikimedia Commons.

The equation for latent heat calculation

To calculate the latent heat, we multiply the amount of substance by the specific latent heat. That is, in essence, the latent heat equation:

Q = m × L

where:

  • Q — Latent heat, in kilojoules (kJ);
  • m — Mass of the body, in kilograms (kg); and
  • L — Specific latent heat, in kilojoules (kJ);

For example, suppose we want to calculate the latent heat necessary to melt 10 kg of oxygen and convert them into liquid. In that case, we use its specific latent heat of vaporization in the formula above in the following way:

Q = m × L = 10 kg × 213 kJ/kg
Q = 2130 kJ

Remember that, as mentioned before, the energy to take one substance from one state to another is the same energy to achieve the opposite process. Therefore, the latent heat of vaporization is the heat necessary to evaporate liquid oxygen but also to take it from gas to liquid.

Liquid oxygen in a beaker
Liquid oxygen (pale blue liquid) in a beaker. Before using latent heat to condense the oxygen gas, we must use sensible heat to take it down to its boiling point of -182.96°C. Attribution: Staff Sgt. Nika Glover, U.S. Air Force, Public domain, via Wikimedia Commons.

You can scroll through the different substances of this heat energy calculator and look for the different latent heats of vaporization and fusion.

Luis Fernando
Mass
oz
Substance
Water (vaporization)
Specific latent heat
kJ/kg
Latent heat
J
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