1.22   use the state symbols (s), (l), (g) and (aq) in chemical equations to represent solids, liquids, gases and aqueous solutions respectively

(s) stands for solid  :  (l) stands for liquid  :  (g) stands for gas  :  (aq) stands for aqueous solution

Don’t get confused between a LIQUID and a SOLUTION IN WATER (aqueous).  Pure water itself is a liquid.  Ethanol is a liquid, but can mix with (dissolve in) water.  NaCl(s) means solid salt, NaCl(l) means molten (liquid) salt and NaCl(aq) means salty water (an aqueous solution).

As applied to the example equation above, you would be expected to know Mg is a solid and H2 is a gas.  You should also know that HCl is an aqueous solution (Section 2.11).  From Section 4.6 you should know that common chlorides are soluble, so would expect MgCl2 to be aqueous.

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

TIP       Watch out for clues in questions as well, to tell you which things are in which state.