The meaning of an equation
A chemical equation summarises a reaction using formulae. Reactants are written on the left, products on the right, joined by the yield arrow →. Because atoms are neither created nor destroyed (the law of conservation of mass), an equation must be balanced: the number of atoms of each element must be equal on both sides.
Key idea
2H2(g) + O2(g) → 2H2O(l)
Balance by placing numbers (coefficients) in front of formulae — never change a subscript inside a formula.
State symbols
Add state symbols to show the physical state: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water).
Mole ratio calculations
The coefficients give the mole ratio in which substances react. This lets you predict the mass or volume of product from a given amount of reactant.
Worked example
What mass of magnesium oxide forms when 6 g of magnesium burns completely? (Ar: Mg = 24, O = 16)
2Mg(s) + O2(g) → 2MgO(s)
Moles of Mg = 6 ÷ 24 = 0.25 mol.
Mole ratio Mg : MgO = 2 : 2 = 1 : 1, so moles of MgO = 0.25 mol.
Mass of MgO = 0.25 × 40 = 10 g.
Remember
- Balance atoms, not molecules — adjust coefficients only.
- Include state symbols where asked.
- Coefficients = mole ratio; convert reactant to moles first.