Heat of vaporization is the amount of heat energy required to change the state of a substance from a liquid into a vapor or gas. It is also known as enthalpy of vaporization, with units typically given in joules (J) or calories (cal).

## Heat of Vaporization Problem

This sample problem demonstrates how to calculate the amount of energy required to turn a sample of water into steam:

What is the heat in joules required to convert 25 grams of water into steam? What is the heat in calories?

What you know: Heat of vaporization of water = 2257 J/g = 540 cal/g

Note: You won't be expected to know enthalpy or heat values; they will be given in a problem or can be looked up in a table.

## How to Solve

You can solve this problem using either joules or calories for heat.

Part I:

Use the formula q = m·ΔH_{v }in which q = heat energy, m = mass, and ΔH_{v} = heat of vaporization.

q = (25 g)x(2257 J/g)

q = 56425 J

Part II:

q = m·ΔH_{f}

q = (25 g)x(540 cal/g)

q = 13500 cal

## Answer

The amount of heat required to change 25 grams of water into steam is 56425 joules or 13500 calories.

A related example illustrates how to calculate the energy when water changes from solid ice into steam.