What is a chemical reaction? How do we know one happened?
A chemical reaction is a change in which new substances with new properties are formed. The substances we start with are reactants; the new ones are products.
Signs that tell us a reaction has taken place:
- a change in colour (a shiny iron nail turns brown in copper sulphate solution),
- gas bubbles come out (zinc in dilute acid gives hydrogen),
- a change in temperature (quicklime in water gets hot),
- a solid (precipitate) forms in a solution,
- a change in state or smell (milk turning sour).
Writing a chemical equation
A word equation uses names: magnesium + oxygen → magnesium oxide. A chemical equation uses formulas: Mg + O₂ → MgO. Reactants go on the left, products on the right, and the arrow means "changes into".
An equation with correct formulas but unequal atom counts is called a skeletal equation. Mg + O₂ → MgO is skeletal: there are 2 O on the left and 1 on the right.
Balancing a chemical equation step by step
Mass cannot be created or destroyed in a reaction (law of conservation of mass). So each kind of atom must be equal on both sides.
The hit-and-trial method
- Write the skeletal equation with correct formulas.
- Make a table of atoms on each side.
- Start with the compound that has the most atoms, and the element that appears in fewest places.
- Put coefficients (numbers in front) to equalise that element. Never change subscripts: H₂O and H₂O₂ are different substances.
- Go element by element; leave H and O for the end.
- Recount everything. Use the smallest whole numbers.
Worked on screen
Fe + H₂O → Fe₃O₄ + H₂. Fe₃O₄ has the most atoms, so start with O: 4H₂O on the left. Then H: 8 on the left, so 4H₂ on the right. Then Fe: 3Fe on the left. Balanced: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
Making equations more informative: state symbols and conditions
We add the physical state after each formula: (s) solid, (l) liquid, (g) gas, (aq) aqueous, meaning dissolved in water. Example: 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g). Here (g) for water tells us steam is used.
Conditions such as heat (Δ), light, a catalyst, temperature or pressure are written above or below the arrow, for example CO(g) + 2H₂(g) → CH₃OH(l) at 340 atm.
Board exam tip: balancing questions appear almost every year, usually 1–2 marks, and "write the balanced equation with state symbols" is common in 3-mark questions.
Key formulas and definitions
- Reactants → Products
- Law of conservation of mass: total mass of reactants = total mass of products
- Coefficient × subscript = number of atoms (2H₂O has 4 H and 2 O)
- State symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water
- Δ over the arrow = heat is supplied
Worked examples
1. Balance: H₂ + Cl₂ → HCl
Left: 2 H, 2 Cl. Right: 1 H, 1 Cl. Put 2 in front of HCl: H₂ + Cl₂ → 2HCl. Now 2 H and 2 Cl on each side.
2. Balance: Mg + O₂ → MgO
Oxygen: 2 on left, 1 on right, so write 2MgO. Now Mg is 2 on the right, so write 2Mg. Answer: 2Mg + O₂ → 2MgO.
3. Balance: N₂ + H₂ → NH₃
N: 2 on left, so 2NH₃. That gives 6 H on the right, so 3H₂ on the left. Answer: N₂ + 3H₂ → 2NH₃.
4. Balance: CH₄ + O₂ → CO₂ + H₂O
C is 1 = 1. H: 4 on left, so 2H₂O. O on right = 2 (in CO₂) + 2 (in 2H₂O) = 4, so 2O₂. Answer: CH₄ + 2O₂ → CO₂ + 2H₂O.
5. Balance: Fe + H₂O → Fe₃O₄ + H₂ and add state symbols (steam is used).
O: 4 in Fe₃O₄ → 4H₂O. H: 8 → 4H₂. Fe: 3 → 3Fe. Answer: 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g).
6. Balance: Al + O₂ → Al₂O₃
O: 2 on left, 3 on right. The LCM of 2 and 3 is 6, so 3O₂ and 2Al₂O₃. Now Al is 4 on the right, so 4Al. Answer: 4Al + 3O₂ → 2Al₂O₃.
Common mistakes
- Changing a subscript to balance (writing H₂O₂ instead of 2H₂O). That makes a different substance.
- Forgetting that a coefficient multiplies every atom in the formula: 3H₂O has 6 H and 3 O.
- Leaving the answer with fractions or non-smallest numbers like 4H₂ + 2O₂ → 4H₂O. Divide by the common factor.
- Mixing up (l) and (aq): water itself is (l); salt dissolved in water is (aq).