Why LiOH is not stable to heat?

Answer 1

LiOH is less stable than the rest of the group 1 hydroxides . All group 1 hydroxides will decompose

2MOH -> M2O + H2O

As you go down the group the stability of the hydroxides increases, KOH forms dimeric molecules in the vapour, the reason is the polarising power decreases as you go down the group, Li is small and more polarising than the rest. This effect makes the oxide more stable and at the same time destabilises the hydroxide. There is good site (UK) for this trend stuff which is at high school level- and puts the arguments in a way some examiners want to hear.

Another argument is the diagonal effect Li is more like Mg in its chemistry. In fact Li is a really interesting element and the hydroxide is well studied now as it is used as a CO2 scrubber in spacecraft.