Year 10 Chemistry Question?

aq means that the compound is aqueous. This tells us that the compound has been broken down to its ions. MgCl2(aq) for example would be Mg2+ and 2Cl-. In terms of balancing reactions this is used in the ionic and the net ionic equations where everything needs to be broken down to be canceled out. Basically, aq means that the compound is dissolved in water, so the ions of the compound are floating around in the solution.