What makes bond break apart from heat?
You have a two part question so I'll answer both of them:  






  
  Question - What makes bonds break apart from heat? 
  


"If you start to heat the system, two things can happen:


The atoms gain enough energy to move into higher, excited states in the potential well. Eventually the additional energy may be high enough to make the total energy positive. The nuclei can escape the potential well - the bond is broken.  
The electrons that participated in forming the bond, i.e. those on orbitals overlapping both nuclei, get excited into different, higher energy orbitals. This makes the potential well shallower, and the nuclei can escape it easier - the bond becomes weaker and easier to break. Eventually the potential well can even become so shallow, that no bound state can exist." - Source  





  
  Question - Why do alkali metals ($\ce{Li, Na, K ...}$) break bonds in water ($\ce{H_2O}$)?  
  


"All alkali metals melt as a part of the reaction with water. Water molecules ionise the bare metallic surface of the liquid metal, leaving a positively charged metal surface and negatively charged water ions. The attraction between the charged metal and water ions will rapidly increase the surface area, causing an exponential increase of ionisation. When the repulsive forces within the liquid metal surface exceeds the forces of the surface tension, it vigorously explodes."- Source  



I hope that my answers were useful to you.