Question

Why does Ti(OH)4 decompose into TiO2 and H2O? What are the rules for writing a decomposition reaction

Answer 1

Titanium is oxidizing at a +4 state, it needs to bond with two oxygen atoms to neutralize its +4 state and create a stable octet. Water is formed because two hydrogen atoms are dissociated along with an oxygen atom.

During decomposition reactions, the compound will be separated into its two individual components. The most important thing to consider is the stability of the elements/molecules/compounds formed as a result of the decomposition. Most of these will obey the octet rule and should complete their outer electron shells.

Answer 2

Since `Ti(OH)_2` is a metal hydroxide so it's product is going to have a metal oxide Ti0 and water `H_2O`.

You can read the rules here.