Apply electrical resistance heating (I^2R = V^2/R) to a substance at or just barely below its freezing point. All of the resistance heating will go into changing the substance from a solid to a liquid without a rise in temperature --- into latent heating not sensible heating. Vaporization generally entails a larger latent heat than freezing, but since vapors typically occupy much large volumes per unit mass than solids or liquids for many purposes it may be less convenient.
If energy is like money in the bank, then energy conversion is like withdrawing funds to make a specific purchase. Previously, we used electrical energy to heat water, and noted the efficiency of this operation. In this lesson we will again use electrical energy to heat water, this time until we boil the water and convert it into steam. This takes more energy because we must change the state of the water molecules into a higher energy form - steam. For each gram of liquid water at its boiling point that we convert into steam at the same temperature, we need 540. calories of energy. This is called the latent heat of vaporization or latent heat of boiling for water. We will use a microwave oven to heat our water, and note the efficiency of this method compared to the immersion heater used to make tea in the previous lesson