Stepwise Heating Curves for 18.0 g H2O from -25.0 oC to 125.0 oC

Step 1:  Heat energy goes into the vibration motion of the molecule, increasing its kinetic energy.  Since temperature is the average kinetic energy, the temperature of the solid increases.  The rate of temperature increase depends on the heat capacity of the solid.  In this case, ice has a specific heat capacity of 2.09 J/g-oC.  The temperature can increase until it reaches the melting point of the solid.  In this case, that is 0.0 oC.

Step 2:  At 0.0 oC, any heat added to the solid goes into partially breaking intermolecular bonds (H-bonds for water).  The heat does not increase the kinetic energy of the molecules, so the temperature remains constant.  As long as there are H-bonds to break (as long as there is solid present), the temperature cannot increase.  We say that the solid and liquid are in equilibrium if they are both present at the same time.  The energy required to change from the solid to the liquid phase of a substance is called the heat of fusion, DHfusionand depends on the substance and the quantity.  For water, it has a value of 6.01 kJ/mole (6010 J/mole).

Step 3:  Heat energy, again, goes into the vibration motion of the molecule.  Being now in the liquid phase, energy goes into the rotational motion as well.  The kinetic energy increases, so the temperature of the liquid increases.  The rate of temperature increase now depends on the heat capacity of the liquid.  In this case, water has a specific heat capacity of 4.18 J/g-oC.  The temperature can increase until it reaches the boiling point of the liquid.  In this case, that is 100.0 oC.

Step 4:  At 100.0 oC, any heat added to the liquid goes into completely breaking intermolecular bonds (H-bonds).  The heat does not increase the kinetic energy of the molecules, so the temperature remains constant.  As long as there are H-bonds to break (as long as there is liquid present), the temperature cannot increase.  We say that the liquid and solid are in equilibrium if they are both present at the same time.  The energy required to change from the liquid to the gas phase of a substance is called the heat of vaporization, DHvaporizationand depends on the substance and the quantity.  For water, it has a value of 40.79 kJ/mole (40790 J/mole).
 

Step 5:  Heat energy, again, goes into the vibration and rotational motions of the molecule.  Being now in the gas phase, energy goes into the translational motion as well.  The kinetic energy increases, so the temperature of the gas increases.  The rate of temperature increase now depends on the heat capacity of the gas.  In this case, water vapor has a specific heat capacity of 1.84 J/g-oC.  The temperature can increase indefinitely, or until the substance decomposes (breaking covalent or ionic bonds), or the atom breaks down (forming a plasma).

The total energy needed is the sum of the five steps.  Notice the the majority of the energy is contained in the vaporization step.  This is because intermolecular forces need to be broken completely to go into the gas phase.

All Together (Animated)