Answer is: chemical.
If the arrangement of particles is changed, that is chemical change or chemical reaction (new substance is formed).
In physical change, the bonds between oxygen and hydrogen can not be separated.
For example, balanced chemical reaction: 2Mg(s) + O₂(g) → 2MgO(s).
Chemical changes (chemical synthesis) is when a substance combines with another (in this example magnesium and oxygen) to form a new substance.
