When hydrogen burns in oxygen, water is formed; when electricity is passed through water, hydrogen and oxygen are given out. Name the type of chemical changes involved in the two cases.
Hydrogen gas burns in oxygen, to form water involved a combination of hydrogen and oxygen gas and this type of reaction is called a combination reaction. $$H_{{2}{(g)}} + O_{{2}{(g)}}\longrightarrow H_2O_{{(liq)}}$$
When electricity is passed through water, hydrogen, and oxygen gases are given out, the chemical changes involved here are decomposition and the type of reaction is a decomposition reaction.
$$H_2O_{{(liq)}}\longrightarrow H_{{2}{(g)}} + O_{{2}{(g)}}$$