How will the amount of thermal energy required to change each substance from a liquid to a gas differ?

a) water and mercury will require the same amount of energy to change from liquid to gas.
b) mercury will require a greater amount of energy to change from liquid to gas than water.
c) neither water nor mercury will require any energy to change from liquid to gas as long as they are each at their boiling temperatures.
d) water will require a greater amount of energy to change from liquid to gas than mercury.