What kind of reaction is the "water-splitting" reaction? $\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{H}_{2}(g)+1 / 2 \mathrm{O}_{2}(g)$ (a) an acid-base reaction (b) a metathesis reaction (c) a redox reaction (d) a precipitation reaction [Section 4.4]

The reactants of the reaction are: \[ \mathrm{H}_{2}\mathrm{O}(l) \] The products of the reaction are: \[ \mathrm{H}_{2}(g) + \frac{1}{2}\mathrm{O}_{2}(g) \] Now that we have the reactants and products, it is time to evaluate the reaction based on the given types.

We now analyze each reaction type: (a) An acid-base reaction is a reaction in which an acid reacts with a base, and the products are a salt and water. In this case, the only reactant is water. The reaction is not an acid-base reaction since there is no acid and base involved in the reaction. (b) A metathesis reaction (also called a double displacement reaction) involves the exchange of two ionic species between two compounds. In this case, there is only one compound, water, with no ions. Thus, this reaction is not a metathesis reaction. (c) A redox reaction is a reaction that involves the transfer of electrons. In the given reaction, hydrogen (H) is changing states from +1 in water (as part of the hydroxyl group) to 0 in the hydrogen gas molecule (\(\mathrm{H}_2\)). Oxygen (O) is also changing states, from -2 in the water to 0 in the oxygen gas molecule (\(\mathrm{O}_2\)). Due to these changes in the oxidation states, the water-splitting reaction is a redox reaction. (d) A precipitation reaction is a reaction in which two soluble salts react to form an insoluble salt (the precipitate) and a soluble salt. There are no salts involved in this reaction, so it is not a precipitation reaction. Therefore, based on our analysis, the water-splitting reaction is:

(c) a redox reaction