How does liquid convert to gas on getting thermal heat energy? Say for example, when we heat, water converts to steam gas. How does it happen? What happens underneath giving rise to breaking of bond between molecules in liquid state and spreading them in gas state..?
 A: When you continue to supply thermal energy to a liquid, the mean energy of the molecules (temperature of the system) is increased. Above a certain threshold, the thermal agitation of the molecules are sufficiently large so that a phase change may start.
During the period of the phase change, the two phases (liquid and solid) coexist and temperature does not change. As soon as the whole system has converted to gas the temperature of the gas will continue to increase.
If you want to know more elaborately then ask for it and I will add the details. But it will get technical.
A: The bond that holds water as a liquid is a simple static electricity bond. it has a strength and will 'break' with sufficient energy. this happens all the time. water evaporates, when a random chance of circumstances through thermal agitation and exterior pressure are at the right amount the molecule leaves the liquid and goes flying off as a gas. the higher the temperature the higher the probability. 
you can even see this static affinity using a comb and a slow even stream of water out of the tap. take a comb through your hair and move it towards the stream and watch it 'bend'  
A: By heating the liquid the heat energy absorbed by the molecules and then it tends to vibrate more compared to its ground state. Due to the heavier vibrations the atoms moves more far apart from its equilibrium position. Now the phenomenon called  "phase change" occures. Then the liquid is transformed into gaseous state.
