When an acid reacts with a substance, one of the products that is always formed is water (H2O). This is because acids are compounds that, when dissolved in water, release hydrogen ions (H+) into the solution. When these hydrogen ions react with other substances, they often form water molecules as a byproduct. For example, when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), the products are water (H2O) and sodium chloride (NaCl).