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An acid is a substance that produces hydrogen ions (H+) when dissolved in water, while an alkali produces hydroxide ions (OH-). Free GCSE Chemistry notes.
Acids, Alkalis and Neutralisation is part of Reactions of acids in GCSE Edexcel Higher Chemistry.
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GCSE Edexcel Higher
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An acid is a substance that produces hydrogen ions (H+) when dissolved in water, while an alkali produces hydroxide ions (OH-). Free GCSE Chemistry notes.
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Use this lesson to stay inside Reactions of acids while connecting the detail back to the wider Topic 3: Chemical changes topic.
Read the core explanations from Acids, Alkalis and Neutralisation before testing yourself from memory.
You taste sourness in lemons and feel the sting of soap in your eyes. These aren't random sensations - they're your body detecting specific particles floating in solution. The key to understanding acids and alkalis lies in recognising what these substances actually release when they dissolve in water.
Picture hydrochloric acid dissolving in water. The HCl molecules don't stay whole - they split apart, releasing H+ ions into the solution. This is what makes the solution acidic.
Sodium hydroxide works the opposite way. When it dissolves, it releases OH- ions that make the solution alkaline. These hydroxide ions are what make soap feel slippery on your skin.
When you mix an acid with an alkali, something remarkable happens. The H+ ions from the acid meet the OH- ions from the alkali, and they react together. This isn't just mixing - it's a chemical reaction that produces something completely different.
Notice how the H+ and OH- ions combine to form H₂O - water. This is why we call it neutralisation: the acidic and alkaline properties cancel each other out. The solution becomes neutral (pH 7) when equal amounts of H+ and OH- ions react.
Worked example: Complete Neutralisation Reaction
Write the full equation for the reaction between hydrochloric acid and sodium hydroxide.
Answer: $HCl_{(aq)} + NaOH_{(aq)} \rightarrow H_2O_{(l)} + NaCl_{(aq)}$
The pH scale measures how many H+ ions are in solution. More H+ ions means lower pH (more acidic). Fewer H+ ions means higher pH (more alkaline). During neutralisation, we're removing H+ ions by making them react with OH- ions to form water.
Picture starting with a strong acid (pH 1) and gradually adding alkali. Each drop of alkali removes some H+ ions from solution. The pH slowly rises. When you've added exactly the right amount of alkali to react with all the H+ ions, you reach pH 7 - the neutralisation point.
Worked example: Calculating pH During Neutralisation
25 cm³ of 0.1 M HCl is neutralised by 25 cm³ of 0.1 M NaOH. What is the final pH?
Answer: pH = 7 (neutral solution)
Neutralisation isn't just a laboratory curiosity - it's happening around you constantly. Your stomach produces hydrochloric acid, and when you take an antacid tablet, you're performing a neutralisation reaction to reduce the acidity.
Farmers use lime (calcium hydroxide) to neutralise acidic soil, making it suitable for crops. Industrial processes use neutralisation to treat acidic waste water before it's released into rivers. Even your toothpaste contains mild alkalis to neutralise the acids produced by bacteria in your mouth.
Worked example: Antacid Neutralisation
An antacid tablet contains 0.5g of magnesium hydroxide. How much stomach acid (0.1 M HCl) can it neutralise?
Answer: 172 cm³ of 0.1 M HCl can be neutralised
Neutralisation questions appear frequently in GCSE exams, often worth 4-6 marks. The key to success is recognising the different ways examiners can test this topic and knowing exactly what they want to see in your answers.
**Common Mistake**: Many students write 'H₂ + O₂ → H₂O' instead of 'H⁺ + OH⁻ → H₂O'. Remember, neutralisation is about ions reacting, not elements combining. The H⁺ comes from the acid and the OH⁻ comes from the alkali.
**Mark-winning phrases**: Use 'hydrogen ions from the acid' and 'hydroxide ions from the alkali' to show you understand where each ion comes from. Examiners love to see this level of detail in explanations.
Exam precision note: In Acids, Alkalis and Neutralisation - The pH scale and neutralisation - Reactions of acids, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
Neutralisation can be described at the ion level as H+ ions reacting with OH- ions to form water. The other ions are spectators, which is why many acid-alkali reactions can be simplified to the same ionic idea even when the named substances are different.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-22.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
The ionic equation for neutralisation is H⁺ + OH⁻ → H₂O. Hydrogen ions from the acid react with hydroxide ions from the alkali to form water, and the other ions are spectators.
An acid is a substance that produces hydrogen ions (H+) when dissolved in water, while an alkali produces hydroxide ions (OH-). The more H+ ions present, the more acidic the solution; the more OH- ions, the more alkaline.
Examples of neutralisation include taking an antacid tablet to reduce stomach acidity, farmers using lime (calcium hydroxide) to neutralise acidic soil, treating acidic waste water before it is released into rivers, and toothpaste containing mild alkalis to neutralise acids from bacteria.
Alkalis produce hydroxide ions (OH⁻) when dissolved in water. Sodium hydroxide releases OH⁻ ions when it dissolves, and the more OH⁻ ions present, the more alkaline the solution.
Acids produce hydrogen ions (H⁺) when dissolved in water. For example, HCl molecules split apart in water, releasing H⁺ ions, and the more H⁺ ions present, the more acidic the solution.
The equation for hydrochloric acid and sodium hydroxide is HCl(aq) + NaOH(aq) → H₂O(l) + NaCl(aq). Water forms from the H⁺ and OH⁻ ions, and the salt NaCl forms from the remaining ions.
For an antacid with 0.5g of magnesium hydroxide (Mr 58), moles = 0.5 ÷ 58 = 0.0086 mol. Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O, so it neutralises 0.0172 mol HCl, which is 0.0172 ÷ 0.1 = 0.172 dm³ = 172 cm³ of 0.1 M HCl.
A fully neutralised solution is pH 7 because equal amounts of H⁺ and OH⁻ ions have reacted to form water. The acidic and alkaline properties cancel each other out, so the solution becomes neutral.
When alkali is added to a strong acid at pH 1, each drop removes some H⁺ ions from solution, so the pH slowly rises. When exactly enough alkali has been added to react with all the H⁺ ions, the pH reaches 7.
The products of neutralising an acid with an alkali are a salt and water. In HCl + NaOH, water forms from H⁺ and OH⁻ ions and the salt NaCl forms from the remaining ions.
Toothpaste contains mild alkalis that neutralise the acids produced by bacteria in the mouth. Antacid tablets also work by neutralising hydrochloric acid in the stomach.
When equal moles of HCl and NaOH react, the final pH is 7. For example, 25 cm³ of 0.1 M HCl contains 0.0025 mol, the same as 25 cm³ of 0.1 M NaOH, so the 1:1 ratio gives complete neutralisation.