Who this is for: Cambridge IGCSE Chemistry (0620) students who want the characteristic properties of acids and bases — pH, indicators, reactions with metals and carbonates, and neutralisation — to become reliable marks instead of general statements that miss the chemistry.
What query it owns: how to understand and revise the characteristic properties of acids and bases in Cambridge IGCSE Chemistry.
Why this is safe: this page owns the acids and bases revision-guide angle, while Tutopiya’s Characteristic Properties of Acids and Bases subtopic page owns the learning resource and the free Acids and Bases quiz owns the practice.

Acids and bases are among the most frequently tested topics in Cambridge IGCSE Chemistry (0620). Examiners expect you to describe the characteristic reactions of acids, use the pH scale and indicators correctly, write neutralisation equations, and explain the difference between strong and weak acids in terms of ionisation. This guide covers the syllabus content, the reaction patterns that earn marks, and the question types that appear every year.

Key takeaways

• Acids produce H⁺ ions in aqueous solution; bases neutralise acids; alkalis are soluble bases producing OH⁻ ions.
• The pH scale runs from 0 (strong acid) to 14 (strong alkali); pH 7 is neutral.
• Acids react with metals (above hydrogen in reactivity series), metal oxides/hydroxides, carbonates and ammonia.
• Neutralisation: acid + base/alkali → salt + water.

What are the characteristic properties of acids and bases?

Acids are proton (H⁺) donors in aqueous solution. They turn blue litmus red, have pH below 7, and react with metals, metal oxides, metal hydroxides and carbonates to form salts. Bases neutralise acids; alkalis are soluble bases that produce OH⁻ ions and have pH above 7. Universal indicator gives a continuous colour change across the pH scale. Neutralisation produces a salt and water.

You can read the full explanation, worked examples and notes on Tutopiya’s Characteristic Properties of Acids and Bases subtopic page before you attempt questions.

The core ideas you must master

Idea	What it means	How the exam uses it
pH scale	Measure of acidity/alkalinity	”State the pH of a strong acid.”
Indicators	Substance that changes colour with pH	”Name an indicator that turns red in acid.”
Acid + metal	Salt + hydrogen gas	”Write the equation for Mg + HCl.”
Acid + carbonate	Salt + water + CO₂	”Describe the test for carbon dioxide.”
Neutralisation	H⁺ + OH⁻ → H₂O	”Write the ionic equation for neutralisation.”

Characteristic reactions of acids — the patterns to learn

Reaction	Products	Observations
Acid + reactive metal	Salt + H₂	Effervescence (bubbles of hydrogen)
Acid + metal oxide	Salt + water	Solid dissolves; no gas
Acid + metal hydroxide	Salt + water	Neutralisation; temperature may rise
Acid + carbonate	Salt + water + CO₂	Effervescence; CO₂ turns limewater milky
Acid + ammonia	Ammonium salt	No water produced

How to use the pH scale and indicators — step by step

1. pH 0–6: acidic (lower = stronger acid).
2. pH 7: neutral (pure water).
3. pH 8–14: alkaline (higher = stronger alkali).
4. Litmus: red in acid, blue in alkali.
5. Universal indicator: full colour range — compare to a pH colour chart.

Once you have worked through a few, test yourself with the free Acids and Bases quiz — it tells you fast whether the reaction patterns have actually stuck.

Acids and bases in past-paper wording: command words that matter

Command word / phrase	What the question wants	Typical acids/bases stem
Describe	Observations and products	”Describe what happens when acid is added to calcium carbonate.”
Write an equation	Balanced symbol equation	”Write the equation for the reaction of NaOH with HCl.”
Explain	Reason using ions	”Explain why an alkali has pH above 7.”
State	Short factual answer	”State the products of neutralisation.”
Identify	Name ion or substance	”Identify the ion responsible for acidity.”

Worked exam-style stems (how to answer the wording)

1. “Describe the reaction between dilute hydrochloric acid and magnesium ribbon.” Effervescence (bubbles of hydrogen gas) observed; magnesium dissolves; solution forms magnesium chloride. Equation: Mg + 2HCl → MgCl₂ + H₂. Mark-scheme reward: effervescence + hydrogen + salt named.
2. “Write the ionic equation for the neutralisation of hydrochloric acid with sodium hydroxide.” H⁺(aq) + OH⁻(aq) → H₂O(l). Reward: correct ions and water product.
3. “Explain why a solution of hydrochloric acid conducts electricity but a solution of ethanol does not.” HCl ionises in water to produce H⁺ and Cl⁻ ions that carry charge; ethanol does not produce ions in solution. Reward: ions in HCl + no ions in ethanol.

When you can recognise the wording instantly, work the full set on the Acids, Bases and Salts topical past paper questions and the Acids and Bases quiz to lock the patterns in.

How acids and bases connect to the rest of the syllabus

Acids and bases link to Oxides (acidic, basic and amphoteric oxides), Preparation of Salts (titration and precipitation) and Redox (reactions with metals). When you are ready to mix topics, the Cambridge IGCSE Chemistry resource hub links every Acids, Bases and Salts subtopic.

Common mistakes students make

• Saying all acids contain oxygen (HCl does not).
• Confusing bases and alkalis (all alkalis are bases, but not all bases are alkalis).
• Writing H₂O as a product when acid reacts with ammonia (product is ammonium salt only).
• Forgetting to test for CO₂ with limewater in carbonate reactions.
• Stating pH 7 is acidic (it is neutral).

When you need more support

If acids and bases questions keep costing marks — especially equations and ionic neutralisation — work through the Acids, Bases and Salts topical past paper questions and the Acids and Bases quiz, then get focused help from a Cambridge IGCSE Chemistry tutor.

Frequently asked questions

Are acids and bases hard in Cambridge IGCSE Chemistry? The reaction patterns are fixed — marks are lost when students mix up products (especially with carbonates and ammonia) or confuse bases with alkalis.

What is the difference between a base and an alkali? A base neutralises an acid. An alkali is a soluble base that produces OH⁻ ions in aqueous solution.

What are the products of neutralisation? Salt and water.

How do I revise acids and bases effectively? Learn the five acid reaction patterns, practise ionic equations, then take the Acids and Bases quiz.

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