Neutralization

Summary: Neutralisation is the reaction between H+(aq) and OH-(aq) to form H2O(l). The ionic equation H+ + OH- → H2O applies to ALL neutralisation reactions. Applications include treating acidic soil, indigestion remedies, industrial waste treatment, and insect sting relief. Tags: igcse chemistry acids-bases Created: 2026-07-14 Last Updated: 2026-07-16

Learning Objectives

By the end of this topic, you should be able to:

• Write the ionic equation for ALL neutralisation reactions: H+(aq) + OH-(aq) → H2O(l)
• Explain how neutralisation is used to treat acidic soil (lime — CaO/CaCO3)
• Describe the use of antacid tablets for indigestion (Mg(OH)2, CaCO3, NaHCO3)
• Explain how pH is regulated in swimming pools
• Explain the difference between treating bee stings (acidic) and wasp stings (alkaline)
• Describe how acidic industrial waste is neutralised before disposal

Content

1. The Universal Ionic Equation for Neutralisation

When ANY acid is neutralised by ANY base (alkali), the essential reaction is:

H+(aq) + OH-(aq) → H2O(l)

This is the ionic equation for neutralisation. The other ions present (spectator ions) combine to form the salt.

Why this equation applies to ALL neutralisation reactions:

• ALL acids contain H+ ions in solution
• ALL alkalis contain OH- ions in solution
• When mixed, H+ and OH- combine to form water molecules
• The other ions (e.g., Na+, Cl-, SO4 2-) remain in solution as the salt

Examples showing H+ + OH- → H2O is always the core:

Exam tip: Always write H+ + OH- → H2O as the ionic equation for neutralisation, even if the word equation is more complex. This is worth an easy mark.

2. Neutralisation of Acids with Carbonates and Metal Oxides

Neutralisation is NOT just about H+ + OH- → H2O. Acids can also be neutralised by metal oxides and carbonates:

With Metal Oxides: Acid + Metal oxide → Salt + Water

Example: 2HCl(aq) + CuO(s) → CuCl2(aq) + H2O(l) Ionic: 2H+(aq) + CuO(s) → Cu2+(aq) + H2O(l)

With Carbonates: Acid + Carbonate → Salt + Water + Carbon dioxide

Example: 2HCl(aq) + CaCO3(s) → CaCl2(aq) + H2O(l) + CO2(g) Ionic: 2H+(aq) + CaCO3(s) → Ca2+(aq) + H2O(l) + CO2(g)

In both cases, the H+ ions from the acid are consumed — the acid is neutralised.

3. Application 1: Treating Acidic Soil

The Problem: Soil becomes acidic due to:

• Acid rain (from SO2 and NO2 emissions)
• Excessive use of nitrogen-based fertilisers (ammonium salts)
• Natural decomposition of organic matter

Acidic soil reduces crop yields because most plants cannot grow well in acidic conditions.

The Solution: Add lime to neutralise the acid in the soil.

Forms of lime used:

Limestone (CaCO3) is the most commonly used because:

• It is cheap and readily available
• It is insoluble so it does not wash away quickly
• It releases calcium ions which are beneficial for plant growth

4. Application 2: Antacid Tablets for Indigestion

The Problem: The stomach produces hydrochloric acid (HCl) to kill bacteria and digest food. Excess acid causes indigestion (acid reflux, heartburn, stomach discomfort).

The Solution: Antacid tablets contain mild bases that neutralise the excess stomach acid.

Common active ingredients in antacids:

Why weak bases are used (not strong alkalis like NaOH):

• Strong alkalis like NaOH would damage the stomach lining
• Antacids need to be mild enough to neutralise excess acid without making the stomach alkaline
• Excess alkalinity can be as harmful as excess acidity

5. Application 3: Treating Acidic Industrial Waste

The Problem: Many industrial processes produce acidic effluent (liquid waste). Discharging acidic waste into rivers would:

• Kill aquatic life (fish cannot survive in acidic water)
• Damage the ecosystem
• Contaminate drinking water supplies

The Solution: Neutralise acidic waste before disposal.

• Slaked lime (Ca(OH)2) or limestone (CaCO3) is added to the acidic waste
• The pH is monitored until it reaches neutral (pH ~7)
• The neutralised waste (containing harmless salts) can then be safely discharged

Why lime is commonly used: It is cheap, readily available, and produces harmless calcium salts (like CaCl2 or CaSO4).

6. Application 4: Bee Stings vs Wasp Stings

Bee stings are ACIDIC — they contain methanoic acid (formic acid).

Wasp stings are ALKALINE — they contain an alkaline substance.

Treatment — the neutralisation approach:

Never treat them the wrong way around — adding acid to a bee sting or alkali to a wasp sting would make the pain worse!

7. Application 5: pH Regulation in Swimming Pools

The Problem: Swimming pool water must be maintained at a specific pH (typically 7.2-7.6) because:

• If pH is too low (acidic): stings eyes, corrodes metal fittings, irritates skin
• If pH is too high (alkaline): reduces the effectiveness of chlorine disinfectant, causes scale formation on pool surfaces, makes water cloudy

Factors that change pool pH:

• Chlorine added for disinfection (can be acidic or alkaline depending on the type)
• Rainwater (slightly acidic from dissolved CO2)
• Swimmers’ sweat and urine (can be acidic or basic)
• Make-up water from the mains supply

The Solution: Regular testing and adjustment:

• If pH is too low (acidic): add sodium carbonate (Na2CO3) or sodium hydrogencarbonate (NaHCO3)
• If pH is too high (alkaline): add dilute hydrochloric acid (HCl) or sodium hydrogen sulfate (NaHSO4)

Worked Examples (Exam-Style)

Example 1: The Ionic Equation

Question: Write the ionic equation for the reaction between hydrochloric acid and sodium hydroxide. [1 mark]

Solution: H+(aq) + OH-(aq) → H2O(l) ✓

Example 2: pH of Mixture

Question: 25 cm3 of 1.0 mol/dm3 HCl is mixed with 25 cm3 of 1.0 mol/dm3 NaOH. What will be the pH of the resulting solution? Explain your answer. [2 marks]

Solution:

• pH = 7 (neutral) ✓
• The acid and alkali have equal concentrations and volumes, so they exactly neutralise each other: H+ + OH- → H2O ✓

Example 3: Antacid

Question: Explain how magnesium hydroxide tablets relieve indigestion. Include a balanced equation. [3 marks]

Solution:

• The stomach contains excess hydrochloric acid which causes indigestion ✓
• Magnesium hydroxide neutralises the acid: Mg(OH)2 + 2HCl → MgCl2 + 2H2O ✓
• This reduces the acidity in the stomach, relieving the pain ✓

Example 4: Bee vs Wasp

Question: Explain why different treatments are used for bee stings and wasp stings. [3 marks]

Solution:

• Bee stings are acidic — treated with an alkali (e.g., baking soda) to neutralise ✓
• Wasp stings are alkaline — treated with a weak acid (e.g., vinegar) to neutralise ✓
• Using the wrong treatment would make the pain worse / not neutralise the sting ✓

Practice Questions

1. Write the ionic equation for any neutralisation reaction. [1 mark]

2. Explain why calcium carbonate (limestone) is added to acidic soil. Include a word equation. [3 marks]

3. Magnesium hydroxide is used in antacid tablets. State two reasons why a strong alkali such as sodium hydroxide is NOT used. [2 marks]

4. A student is stung by a wasp. State whether the sting is acidic or alkaline, and suggest a suitable household treatment. [2 marks]

5. Explain why it is important to maintain the pH of swimming pool water within a specific range. [3 marks]

6. Acidic industrial waste must be neutralised before disposal. Name a suitable substance that could be used and explain why it is chosen. [3 marks]

Key Facts to Memorise

• Universal ionic equation for neutralisation: H+(aq) + OH-(aq) → H2O(l)
• Acidic soil → treat with lime (CaO, Ca(OH)2, or CaCO3)
• Indigestion (excess stomach acid) → antacid tablets: Mg(OH)2, CaCO3, NaHCO3
• Weak bases used in antacids — strong alkalis would damage the stomach
• Bee sting = acidic (methanoic acid) → treat with alkali (baking soda / NaHCO3)
• Wasp sting = alkaline → treat with acid (vinegar / CH3COOH)
• Swimming pool pH: 7.2-7.6
• Industrial waste neutralised with lime (Ca(OH)2 or CaCO3) before disposal

Common Misconceptions

Key Concepts from Past Papers

Definitions You MUST Know (Exact Mark Scheme Wording)

• Neutralisation: the reaction between an acid and a base to form a salt and water / the reaction between H+ ions and OH- ions to form water
• Antacid: a substance that neutralises excess stomach acid
• Lime: calcium oxide (CaO) or calcium hydroxide (Ca(OH)2) used to neutralise acidic soil

Recurring Mark Scheme Answers

• “H+ + OH- → H2O is the ionic equation for neutralisation”
• “Calcium carbonate neutralises acid in the soil, raising the pH”
• “Antacids contain weak bases; strong alkalis would damage the stomach”
• “Bee stings are acidic; wasp stings are alkaline”
• “Acidic industrial waste is neutralised with slaked lime (calcium hydroxide) before disposal to prevent harm to aquatic life”
• substances required:
• steam / water
• conditions: one mark each for any two of:
• 6000 kPa / 60 atm (pressure)
• acid catalyst
• salt carbon dioxide water
• add (hydrochloric) acid
• test gas given off with limewater

Common Question Types

• Short recall (state/give/name): “Write the ionic equation for neutralisation”
• Extended writing (describe/explain/suggest): “Explain how you would treat a bee sting”
• Multiple choice / tick-box: “Which substance is used in antacid tablets?”
• Application: “A farmer finds his soil is too acidic. Suggest a suitable treatment”
• Comparison: “Compare the treatments for bee and wasp stings”

Exam Tips

• This topic appears in 34 papers in the database
• Total Q+A entries: 86
• Average marks per question: 1.1
• H+ + OH- → H2O is arguably the single most important equation in the IGCSE Chemistry syllabus — it is tested across multiple topics
• For application questions, always identify the acid/base nature first, THEN explain the neutralisation, THEN give the specific substance and reaction
• When explaining why a weak base is used (not a strong one), mention the harm a strong alkali would cause
• The bee/wasp sting question is a classic — expect it as a 2-3 mark question testing whether you know which is which

Keywords from Past Papers

acid, add, aqueous, precipitate, indicator, carbon, dioxide, white, sodium, hydroxide, colour, nitrogen, limewater, temperature, point

Related Notes

• Acids and Bases — Properties of acids and bases, strong vs weak acids
• Indicators and pH — pH scale, universal indicator, litmus
• Making Salts — Preparing salts by neutralisation (titration, excess solid)
• Qualitative Analysis — Testing for ions, gas tests
• Air and Combustion — Acid rain (SO2 + water → acid rain)
• IGCSE-Chem-Index — Full IGCSE Chemistry index

Sources

• OpenStax Chemistry 2e — [Chapter 14: Acid-Base Equilibria (Neutralization Reactions)], Rice University (free, CC BY 4.0)
• BBC Bitesize GCSE Chemistry — [Neutralisation], BBC (free educational resource)
• Cambridge IGCSE Chemistry 0620 — Syllabus Section 7: Acids, Bases and Salts (Neutralisation), Cambridge Assessment International Education
• CK-12 Chemistry for High School — [Chapter 21: Neutralization Reactions], CK-12 Foundation (free, CC BY-NC 3.0)

Past Paper Sources

• 0620/31 May/June 2015: Q11(c)(i) (0m), Q66(a)(i) (0m), Q66(a)(ii) (0m)
• 0620/32 Feb/March 2015: Q66(e)(ii) (2m), Q66(b)(ii) (1m), Q77(d)(i) (0m)
• 0620/32 Feb/March 2017: Q22(f)(ii) (1m), Q44(d)(i) (1m), Q55(f)(i) (1m) (+1 more)
• 0620/32 Feb/March 2018: Q88(b)(ii) (3m), Q22(a)(iv) (2m), Q77(b)(i) (1m) (+1 more)
• 0620/32 Feb/March 2019: Q44(b)(ii) (1m)
• 0620/32 Feb/March 2020: Q55(d)(iii) (2m), Q66(a)(ii) (1m)
• 0620/32 Feb/March 2021: Q66(a)(ii) (1m)
• 0620/32 Feb/March 2022: Q88(d)(ii) (2m), Q44(a)(ii) (0m)
• 0620/32 Feb/March 2023: Q33(b)(ii) (1m), Q88(c)(iii) (1m)
• 0620/32 May/June 2018: Q33(b)(i) (1m), Q33(b)(ii) (1m)
• 0620/32 May/June 2020: Q88(e)(i) (1m)
• 0620/33 May/June 2016: Q88(c)(ii) (1m), Q88(c)(iii) (0m)