[briofuel88]

What is going on when Thermal energy is converted to kinetic energy in steam??

[David Cooper]

Molecules break free of the liquid due to the high speed which they have (which is their heat energy) and they fly off at that speed. If you heat up a liquid beyond its boiling point, the molecules will no longer be held together as a liquid and will simply become a gas, unless you contain the liquid in a strong container, in which case the pressure goes up instead. Heat is simply the speed of the molecules (though you have to take into account their mass too), so there is no real conversion of thermal energy into kinetic energy - they are the same thing.