Chemical Equations - GitHub Pages It is not uncommon to include a phase label with each formula— (s) for solid, (ℓ) for liquid, (g) for gas, and (aq) for a substance dissolved in water, also known as an aqueous solution. If we included phase labels for the reactants and products, under normal environmental conditions, the reaction would be as follows: H2(g) + O2(g) → H2O (ℓ) Note