Latent heat
When solid melt or liquid evaporate, the particle absorb extra heat (maybe we can add another property to ptypes[PT_NUM]?).
When liquid freezed or gas liquefy, it gives off extra heat.
Which is called Enthalpy of vaporization and Enthalpy of fusion.


Why needed

• Realistic simulate fun
• Courage players try to design cooling system with water or something instead of steady LIFE, which is not so fun
• Slow down the evaporation process. With realistic heat, I found my water pot takes 2 minutes to warm up to 100C, then all the water turned to steam in 2 seconds
• Fix some bug caused by new realistic heat in 74.0 beta
  
• It's cool!

Agree with @egezort : Realistic heat should have an option, including heat capacity (available in beta) and latent heat, it break saves.


*edit*

What's more
Other reactions should also generate or absorb heat to re-balance the realistic heat, for example look at this thread https://powdertoy.co.uk/Discussions/Thread/View.html?Thread=13192
The bug was because combustion do not generate heat, just generate hot FIRE particles, whose heat capacity is too low to heat up the surrounding objects.
Changes that should be made, or tweak:

• Enthalpy of vaporization (Look above)
  
• Enthalpy of fusion (Look above)
• Combustion should generate heat
• Conductive metal with current
• Thermite reaction
• Radioactive reaction
  
• Explosion (C-4, etc. Anybody noticed that BOMB in beta is much more powerful than which in released?)
  
• Enthalpy change of solution (Salt in water, etc.)
• ACID/CAUS eat something
• (More to add...)
  

This is a HUGE project! Realistic heat!
We should wait patiently, give @simon more time to finish this.