Curriculum fit: GCSE Chemistry and Combined Science (chemical changes: acids and alkalis; titration calculations are mainly Higher). US: high school physical science (chemical reactions).
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What Is It?
Acids release hydrogen ions (H⁺) in water, and alkalis release hydroxide ions (OH⁻). The pH scale measures how acidic or alkaline a solution is: below 7 is acidic, 7 is neutral and above 7 is alkaline.
When an acid reacts with a base, they neutralise each other and form a salt. Indicators, such as universal indicator, change colour to show the pH.
Key Formulas and Ideas
- Acid + base → salt + water.
- Acid + metal carbonate → salt + water + carbon dioxide.
- Acid + metal → salt + hydrogen.
- Hydrochloric acid makes chlorides, sulfuric acid makes sulfates and nitric acid makes nitrates.
Worked Examples
Neutralisation
Write the equation for hydrochloric acid reacting with sodium hydroxide.
- Acid + base → salt + water.
- HCl + NaOH → NaCl + H₂O.
✓ Answer HCl + NaOH → NaCl + H₂O
Acid and Metal
Name the products when magnesium reacts with sulfuric acid.
- Metal + acid → salt + hydrogen.
- The salt is a sulfate.
✓ Answer Magnesium sulfate and hydrogen
Titration Calculation (Higher)
25.0 cm³ of 0.100 mol/dm³ NaOH is neutralised by 20.0 cm³ of HCl. Find the concentration of the acid.
- Moles of NaOH = 0.025 × 0.100 = 0.0025 mol.
- The reaction is 1 : 1, so moles of HCl = 0.0025 mol.
- Concentration = 0.0025 ÷ 0.020 = 0.125.
✓ Answer 0.125 mol/dm³
Common Mistakes to Avoid
- Confusing a strong acid with a concentrated acid.
- Writing hydrogen as H instead of H₂.
- Forgetting that carbon dioxide is also made when an acid reacts with a carbonate.
- Not converting cm³ to dm³ in concentration calculations.
Practice Questions
Frequently Asked Questions
Is a lower pH always more dangerous?
Strong acids and strong alkalis can both be hazardous. Concentration and the substance matter, not pH alone.
What is the difference between strong and weak acids?
A strong acid ionises completely in water. A weak acid only partly ionises. Strength is different from concentration.
Keep Going With These Guides
The Reactivity Series: Displacement Reactions and Extraction
Order metals by reactivity, predict displacement reactions and link the reactivity series to how metals are extracted, with worked examples.
GCSEHigh SchoolBalancing Chemical Equations: A Step-by-Step Method
Learn how to balance equations by counting atoms, with worked examples including combustion, plus practice questions with explained answers.
GCSEHigh SchoolMoles and Molar Mass: Calculations Explained Step by Step
Convert between mass and moles using Mr, use balanced equations to find reacting masses and calculate concentration, with worked examples.
GCSEA-LevelHigh SchoolAPSources and Review
Reviewed by StudyMorrow editorial review on 5 Nov 2026.