Chemical equations can be made more informative by expressing following characteristics of the reactants and products.
1. Expressing the physical state :
The different states, i.e. gaseous, liquid and solid states are represented by the notations (g), (l) and (s) respectively. If the substance is present as a solution in water the word aqueous is written.
2. Expressing the heat changes :
Q is heat energy which is shown with plus ’+’ sign on product side for exothermic reactions and minus sign on product side for endothermic reactions.
e.g.: a) C(s) + O2(g) → CO2(g) + Q (exothermic reaction)
N2(g) + O2(g) → 2NO(g) ” Q (endothermic reaction)
3. Expressing the gas evolved :
If a gas evolved in a reaction, it is denoted by an upward arrow ↑ or (g).
e.g.: Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)↑
4. Expressing precipitate formed:
If a precipitate is formed in the reaction, it is denoted by downward arrow ↓.
e.g.: AgNO3(aq) + NaCl(aq) → AgCl(s) ↓ + NaNO3(aq)
