Chemical equations show the state of each substance using a state symbol in brackets after the formula. The symbol (s) means solid, (l) means liquid and (g) means gas. The symbol (aq) means aqueous, which describes a substance dissolved in water to make a solution.
For example, when ice melts and then boils the changes are H2O(s) to H2O(l) to H2O(g). In the reaction of magnesium with hydrochloric acid, Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g), magnesium is a solid, the acid and the salt are dissolved in water, and hydrogen is a gas.
Always check what the substance is doing. A solid reactant is labelled (s), and a reactant that is dissolved in water is labelled (aq). A gas given off is labelled (g). You should be able to add the correct state symbols to the equations you meet in this course.