How does the amount of heat required to change water from a solid to a liquid compare with the amount needed to change from a liquid to a gas?

Answer 1

Very simply, the amount of heat required to change state is exactly the same for solid to liquid (ice to water) and liquid to gas (water to steam) for a given substance, in this case water.

The basic measurement of heat is the BTU (British Thermal Unit). 1 BTU is the quantity of heat required to raise 1 pound of water 1 degree Fahrenheit at atmospheric pressure. Keep in mind that there are two 'types' of heat to consider when contemplating the change of state of a substance: Sensible heat and latent heat. Clearly, sensible heat we can measure directly with a thermometer. Latent heat is a calculated quantity. When water undergoes the change of state from water to steam (liquid to gas) it does so through the gain of heat quantity. That heat gain that causes the change of state is latent heat, i.e; the actual heat gain is unmeasurable by thermometer since the temperature of that water is 212 deg f as it undergoes the change of state to steam, and that steam is also 212 deg f. Any increase in the temperature of the steam is said to 'superheat' the steam. As an example, steam fed boilers are nearly always supplied with 'superheated' steam at temperatures near 600 deg f.

We see the addition of heat quantity as sensible heat during the rise of the waters temperature to 212 deg f (we are able to measure the rise in temperature), then we see the addition of heat quantity as latent heat during the change of state to gas from liquid (no change in the measurable temperature during the change of state), and again the addition of heat quantity as sensible heat in the process of 'superheating' the steam to any temperature above 212 deg f.

If you can quantify the amount of water in pounds, you can quantify the amount of heat in BTU's.