A symbol equation can also show the physical state of each substance. A state symbol is written in brackets straight after the formula. (s) means solid, (l) means liquid, (g) means gas and (aq) means aqueous.
Aqueous means dissolved in water. So HCl(aq) is hydrogen chloride dissolved in water, which is hydrochloric acid. Do not confuse (aq) with (l). Pure water is a liquid, so it is written H2O(l), while a solution of salt in water is NaCl(aq). Steam is water as a gas, H2O(g). The state depends on the temperature, so state symbols describe the conditions of the reaction.
Worked examples: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g). Here solid magnesium reacts with the acid solution, and the gas given off is hydrogen, seen as bubbles. When calcium carbonate is heated, CaCO3(s) → CaO(s) + CO2(g). The carbon dioxide is a gas, and the calcium oxide stays as a solid. Hydrogen burning in oxygen gives 2H2(g) + O2(g) → 2H2O(l).