The purpose to balance a chemical equation is for mass conservation. The total mass of reactants should be equal to the total mass of products. In another words, the number of atoms for each individual atoms should remain constant.
To consider the following chemical equation:
4Fe(s) + 3O2(g) —> 2Fe2O3(s)
The symbols in brackets represent the physical states. We can say 4 moles of solid iron react with 3 moles of oxygen gas to produce 2 mole of iron(III) oxide solid. Other symbols of physical states include: l for liquid and aq for aqueous solution.
It should notice that the difference between liquid state and aqueous solution. A liquid state can be produce due to heating of one solid, and this liquid is still a pure substance. An aqueous solution will be formed when solute dissolves in water, and it is a mixture. For example, NaCl(l) is molten sodium chloride, and represent when solid sodium chloride was heated, it will be melt to produce a liquid state. NaCl(aq) represents that solid sodium chloride dissolves in water to form a transparent solution.
| Symbol in chemical equation | Physical state |
| s | solid |
| l | liquid |
| g | gas |
| aq | aqueous solution (water is solvent) |
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