A balanced symbol equation shows the formulas of reactants and products, with the same number of each type of atom on both sides, because atoms are conserved. You balance an equation by changing the big numbers in front of formulas, never the small numbers inside them. State symbols show the states: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
The relative formula mass, Mr, of a compound is the sum of the relative atomic masses of all the atoms in its formula. For CO₂, Mr = 12 + 16 + 16 = 44. In a balanced equation, the total Mr of the reactants equals the total Mr of the products. If a reaction seems to lose mass, a gas has usually escaped; if it seems to gain mass, a gas from the air has reacted.
Worked example
Balance: Mg + O₂ → MgO
- There are 2 O atoms on the left and 1 on the right, so put 2 in front of MgO.
- Now there are 2 Mg on the right, so put 2 in front of Mg.
Answer:2Mg + O₂ → 2MgO.
Key words
- symbol equation
- shows reactants and products with formulas
- balanced
- same number of each atom on both sides
- state symbol
- (s), (l), (g) or (aq)
- Mr
- relative formula mass: sum of the atomic masses
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NGSS-referenced: HS-PS1-7