Chemical equations: reactants, products and states

A sentence written in formulas

A chemical equation shows a chemical change with symbols and formulas. Reactants go on the left, products on the right, and an arrow points from reactants to products.

A correct equation is balanced (same number and kind of atoms on both sides), uses correct formulas, gives the mole ratio, and shows the direction of change.

State symbols show the physical state: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water. Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g): one mole of each.

Some reactions go both ways; these reversible reactions use a double arrow: N₂ + 3H₂ ⇌ 2NH₃. When AgNO₃ and NaCl solutions mix, insoluble AgCl comes out as a precipitate, AgCl(s).

An equation is written only after experiments confirm the products and their formulas.

Key terms

Reactants

Substances that react.

Products

Substances that form.

State symbol

Showing the physical state.

Reversible reaction

Reactions where products re-form reactants.

Short questions with model answers

1. Q1. Check that 4Al(s) + 3O₂(g) → 2Al₂O₃(s) is balanced. (p. 60)

   • Al: 4 = 2 × 2
   • O: 3 × 2 = 2 × 3

   Balanced: 4 Al and 6 O on each side.

2. Q2. What does Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g) tell you? (p. 61)

   • Reactants: solid zinc, sulphuric acid solution
   • Products: zinc sulphate solution, hydrogen gas
   • 1 : 1 : 1 : 1

   1 mol Zn reacts with 1 mol H₂SO₄ to give 1 mol ZnSO₄ and 1 mol H₂.

Common mistakes

• ✗ Al + O₂ → Al₂O₃ is fine as written.

  ✓ Atoms must balance: 4Al + 3O₂ → 2Al₂O₃.

• ✗ (aq) means a liquid.

  ✓ (l) is liquid; (aq) means dissolved in water.

• ✗ Every reaction only goes forward.

  ✓ Reversible reactions go both ways and use ⇌.

MCQs

1. 1. In an equation, reactants are written:

   1. (a) On the right
   2. (b) On the left
   3. (c) Above the arrow
   4. (d) Anywhere
   Show answer

   (b) p. 60.

2. 2. The symbol (aq) means:

   1. (a) Liquid
   2. (b) Gas
   3. (c) Dissolved in water
   4. (d) Solid
   Show answer

   (c) p. 60: aqueous.

3. 3. A double arrow ⇌ shows a reaction that is:

   1. (a) Reversible
   2. (b) Very fast
   3. (c) Unbalanced
   4. (d) Ionic
   Show answer

   (a) p. 61: N₂ + 3H₂ ⇌ 2NH₃.

4. 4. In AgNO₃(aq) + NaCl(aq), the precipitate is:

   1. (a) NaNO₃
   2. (b) AgNO₃
   3. (c) NaCl
   4. (d) AgCl
   Show answer

   (d) p. 61: AgCl is insoluble.

5. 5. In 4Al + 3O₂ → 2Al₂O₃, how many oxygen atoms are on each side?

   1. (a) 3
   2. (b) 6
   3. (c) 2
   4. (d) 5
   Show answer

   (b) 3 × 2 = 6 and 2 × 3 = 6.

Quick revision

• Reactants on the left, products on the right, arrow showing the direction.
• Balance atoms, use correct formulas, and add state symbols (s), (l), (g), (aq).
• ⇌ marks a reversible reaction; an insoluble product is a precipitate (s).