How to Find the Heat to Change the Mass of Ice to Steam

Melting is an endothermic process.

Molecules in a solid or a liquid are held together by intermolecular forces; overcoming these forces and breaking free of these interactions takes energy. Melting and vaporization are both endothermic processes, meaning that substances absorb heat from their surroundings as they melt and as they evaporate. If we assume the heat capacity of water is more or less constant--a fairly reasonable assumption over the 0 to 100 degree Celsius range--calculating the amount of heat we need to melt a cube of ice and evaporate it is a fairly straightforward problem.

Things You'll Need

• Pencil
• Paper
• Calculator

Instructions

1. Find the mass of the ice block you want to melt and vaporize. If you're working this problem as an assignment for homework or a quiz, this value will typically be given to you.

2. Multiply the mass of the ice block (in kilograms) by the heat of fusion of water (in kilojoules per kilogram). The heat of fusion is 334 kilojoules per kilogram. Your answer will be the amount of energy needed to melt the ice; the units should be in kilojoules.

3. Multiply the mass of the water by 4.184 joules per kilograms Kelvin (the specific heat of water) and by 100 degrees Kelvin (the difference in temperature between the freezing point of water and the boiling point). Your answer will be the amount of energy needed to heat the molten ice to the boiling point; the units should be in kilojoules.

4. Multiply the mass of the water by the latent heat of vaporization, 2257 kilojoules per kilogram. Your answer will be the amount of energy needed to vaporize the water and will be in kilojoules.

5. Add all three values together to get your final solution.

Tips & Warnings

• Note that we're assuming the boiling point of water is 100 degrees Celsius, the boiling point of water at sea level atmospheric pressure (usually a fairly reasonable assumption). If this physical change is not taking place at atmospheric pressure, you can calculate the new boiling point using the Clausius-Clayperon equation, ln P2/P1 = ( dH / R ) ( (1/T1) - (1/T2) ), where R is the ideal gas constant. Boiling will occur when the vapor pressure of the liquid is equal to atmospheric pressure--hence, if we set P2 equal to atmospheric pressure we can find T2 if we already know the change in enthalpy associated with vaporization and the vapor pressure at another temperature for the same substance.

References

• "Chemical Principles, the Quest for Insight, 4th Edition"; Peter Atkins and Loretta Jones; 2008.
• "Physics for Scientists and Engineers"; Richard Wolfson and Jay Pasachoff; 1999.

Resources

• HyperPhysics: Heat of Fusion