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How to Calculate the Heat Needed to Convert a Solid to a Liquid

The heat required to cause a solid to change phase and become liquid is the heat of fusion for the substance. Most people will recognize the phase shift that occurs when water in the solid form of ice gains heat from the environment and changes phases from solid to liquid. The amount of heat required for this phase change to occur depends on the amount of ice you need to melt. The temperature of the substance doesn't change during the melting process, it starts at the melting point of the substance in the solid form and ends at the same temperature with the substance in the liquid form.

Things You'll Need

Triple beam balance
Heat of fusion for the substance

Instructions

- 1
  Weigh the substance on the triple beam balance. Record this weight for use later in the calculation of the amount of heat required to melt the solid into a liquid phase.
- 2
  Look up the heat of fusion for the substance of interest. Chemistry references list the heat of fusion for all the elements and many common chemicals. Locate the heat of fusion on the list and record the value for use in the calculations. For example, assume you are interested in the heat required to melt an ice cube of water. The heat of fusion for water is 6.02 kJ per mole.
- 3
  Substitute the actual data values into the equation for the amount of heat to change phases between solid and liquid. The formula used to perform this calculation is Q = heat of fusion * grams of substance / molecular weight of the substance. Continuing the example using water as the substance, assume you have 31.8 g if ice, find the amount of heat to change it to water. Insert the data into the equation and solve, Q = (6.02 kJ/mol) * (31.8g / 18.015 g/mol) = 6.02 * 1.765 = 10.625 kJ.

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