Neutralisation Reactions and Salt Formulae Revision Notes | GCSE AQA Higher Chemistry

1. What Happens When Acids Meet Their Match

Picture this: you've got stomach acid burning your throat, so you take an antacid tablet. Within minutes, the burning stops. You've witnessed neutralisation in action — acids and bases cancelling each other out to form something completely different.

Neutralisation isn't about destruction — it's about transformation. When acids meet bases, they don't disappear. They rearrange to form salts and water, following predictable patterns that examiners love to test.

2. The Three Types of Neutralisation

Acids are picky about their partners. They'll neutralise with three different types of substances, but each reaction has its own signature products. Here's the complete picture:

Type 1: Acid + Alkali → Salt + WaterExample: $HCl + NaOH \rightarrow NaCl + H_2O$The classic neutralisation. Alkalis are soluble, so this happens in solution.

Type 2: Acid + Base → Salt + WaterExample: $H_2SO_4 + CuO \rightarrow CuSO_4 + H_2O$Bases include insoluble metal oxides and hydroxides. The reaction still works, but you might see a colour change if a coloured metal oxide dissolves, but fizzing is characteristic of acid-carbonate reactions rather than acid-base reactions.

Type 3: Acid + Metal Carbonate → Salt + Water + Carbon DioxideExample: $HCl + CaCO_3 \rightarrow CaCl_2 + H_2O + CO_2$The extra product is carbon dioxide gas — you'll see bubbling.

3. Cracking the Salt Formula Code

Every salt has a two-part name that tells you exactly what it's made from. The first part comes from the base, the second part comes from the acid. Once you know this pattern, you can predict any salt formula.

Here's the key insight: salts are ionic compounds. The metal (or ammonium) forms the positive ion, and the acid provides the negative ion. You need to balance the charges to get the correct formula.

The charges must balance. If calcium ($Ca^{2+}$) meets chloride ($Cl^-$), you need two chlorides to balance one calcium: $CaCl_2$. Think of it like a dance — every positive charge needs a negative partner.

Worked example: Working Out Salt Formulae

What salt forms when magnesium oxide reacts with sulfuric acid?

1. Magnesium: $Mg^{2+}$ Sulfuric acid: $SO_4^{2-}$ — Magnesium always forms 2+ ions, sulfuric acid provides sulphate ions
2. $Mg^{2+}$ has charge +2 $SO_4^{2-}$ has charge -2 — The charges are equal and opposite
3. One $Mg^{2+}$ balances one $SO_4^{2-}$ — Since charges are equal, we need 1:1 ratio
4. $MgSO_4$ — Magnesium sulphate - no subscripts needed when ratio is 1:1

Answer: MgSO₄ (magnesium sulphate)

4. Writing Balanced Neutralisation Equations

Now you understand the products, let's write the full equations. The pattern is always the same: identify reactants, predict products, then balance. But there's a systematic approach that prevents mistakes.

Worked example: Balancing Neutralisation Equations

Write the balanced equation for nitric acid neutralising calcium hydroxide

1. $HNO_3 + Ca(OH)_2 \rightarrow$ — Nitric acid + calcium hydroxide
2. $HNO_3 + Ca(OH)_2 \rightarrow Ca(NO_3)_2 + H_2O$ — Salt (calcium nitrate) + water
3. Left: 1H, 1N, 5O, 1Ca Right: 2H, 2N, 7O, 1Ca — Not balanced - need more acid molecules
4. $2HNO_3 + Ca(OH)_2 \rightarrow Ca(NO_3)_2 + 2H_2O$ — Now: 4H, 2N, 8O, 1Ca on both sides

Answer: 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O

5. Real-World Applications

Neutralisation isn't confined to test tubes. Your stomach produces hydrochloric acid for digestion, but too much causes heartburn. Antacid tablets contain metal carbonates or hydroxides that neutralise excess acid.

Farmers use lime (calcium oxide) to neutralise acidic soil, making nutrients more available to plants. Industrial waste often contains acids that must be neutralised before disposal to prevent environmental damage.

Even bee stings and wasp stings work differently — bee stings are acidic (neutralise with baking soda), while wasp stings are alkaline (neutralise with vinegar).

6. Exam Technique and Common Mistakes

Neutralisation questions appear in multiple formats on AQA papers. You might need to complete equations, predict products, or explain observations. The key is recognising the reaction type from the reactants.

When writing salt formulae, examiners check your understanding of ionic charges. Show your working by writing the ions separately first, then combining them. This prevents charge-balancing errors and earns method marks.

AQA distinguishes between bases and alkalis. A base is any substance that neutralises an acid. Alkalis are bases that dissolve in water, so all alkalis are bases, but not all bases are alkalis. Metal oxides and many metal hydroxides are bases, while soluble metal hydroxides such as sodium hydroxide are alkalis. This distinction matters because acid + alkali reactions usually make salt and water, whereas acid + carbonate reactions also make carbon dioxide.