Acids and Alkalis

This lesson covers acids, alkalis, the pH scale and indicators as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the difference between acids and alkalis, how to measure pH, and — for higher tier — the distinction between strong and weak acids.

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What Are Acids?

An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. The presence of H⁺ ions is what gives all acidic solutions their characteristic properties.

Common Acids

Acid	Formula	Where It Is Found
Hydrochloric acid	HCl	Stomach acid
Sulfuric acid	H₂SO₄	Car batteries
Nitric acid	HNO₃	Making fertilisers
Citric acid	C₆H₈O₇	Citrus fruits
Ethanoic acid	CH₃COOH	Vinegar

Properties of Acids

• pH less than 7
• Turn blue litmus paper red
• Turn universal indicator yellow, orange or red
• React with metals, bases and carbonates
• Taste sour (do NOT taste chemicals in the lab)

Exam Tip: When asked what all acids have in common, the key answer is that they all produce H⁺ ions (hydrogen ions) in aqueous solution.

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What Are Alkalis?

An alkali is a soluble base that produces hydroxide ions (OH⁻) when dissolved in water. Not all bases dissolve in water — those that do are called alkalis.

Common Alkalis

Alkali	Formula	Use
Sodium hydroxide	NaOH	Making soap
Potassium hydroxide	KOH	Alkaline batteries
Calcium hydroxide	Ca(OH)₂	Treating acidic soil
Ammonia solution	NH₃ (aq)	Cleaning products

Properties of Alkalis

• pH greater than 7
• Turn red litmus paper blue
• Turn universal indicator blue or purple
• Feel soapy to the touch
• React with acids in neutralisation reactions

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The pH Scale

The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is.

pH Range	Description	Examples
0–2	Strong acid	Stomach acid (pH 1–2), battery acid
3–6	Weak acid	Lemon juice (pH 2–3), vinegar (pH 3), rain water (pH 5–6)
7	Neutral	Pure water, sodium chloride solution
8–11	Weak alkali	Baking soda (pH 8–9), soap (pH 9–10)
12–14	Strong alkali	Oven cleaner, sodium hydroxide solution

Key Points About the pH Scale

• The lower the pH, the more acidic the solution.
• The higher the pH, the more alkaline the solution.
• A pH of 7 is neutral — neither acidic nor alkaline.
• Each step on the pH scale represents a tenfold change in H⁺ ion concentration.

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Indicators

An indicator is a substance that changes colour depending on whether it is in an acidic or alkaline solution. Indicators help us measure or estimate pH.

Common Indicators

Indicator	Colour in Acid	Colour in Alkali	Colour at Neutral
Litmus	Red	Blue	Purple
Methyl orange	Red	Yellow	Orange
Phenolphthalein	Colourless	Pink	Colourless

Universal Indicator

Universal indicator is a mixture of several indicators that produces a continuous range of colours corresponding to the pH scale:

graph LR
    A["pH 1–2<br/>Red<br/>Strong acid"] --> B["pH 3–4<br/>Orange<br/>Weak acid"]
    B --> C["pH 5–6<br/>Yellow<br/>Weak acid"]
    C --> D["pH 7<br/>Green<br/>Neutral"]
    D --> E["pH 8–9<br/>Blue<br/>Weak alkali"]
    E --> F["pH 10–11<br/>Indigo<br/>Alkali"]
    F --> G["pH 12–14<br/>Purple<br/>Strong alkali"]

    style A fill:#d32f2f,color:#fff
    style B fill:#e65100,color:#fff
    style C fill:#f9a825,color:#000
    style D fill:#2e7d32,color:#fff
    style E fill:#1565c0,color:#fff
    style F fill:#283593,color:#fff
    style G fill:#4a148c,color:#fff

Exam Tip: Universal indicator gives a range of colours and can estimate pH. Litmus only tells you acid (red) or alkali (blue) — it does not give a pH number.

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Acids and Alkalis in Terms of Ions

When an acid dissolves in water, it releases H⁺ ions:

• HCl(aq) → H⁺(aq) + Cl⁻(aq)
• HNO₃(aq) → H⁺(aq) + NO₃⁻(aq)
• H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq)

When an alkali dissolves in water, it releases OH⁻ ions:

• NaOH(aq) → Na⁺(aq) + OH⁻(aq)
• KOH(aq) → K⁺(aq) + OH⁻(aq)

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Strong and Weak Acids (Higher Tier)

This section is assessed on the higher tier only in the Edexcel GCSE Combined Science specification (1SC0).

Strong Acids

A strong acid is one that completely dissociates (ionises) in water. Every molecule of a strong acid breaks apart to release H⁺ ions.

Examples:

• Hydrochloric acid — HCl → H⁺ + Cl⁻ (100% ionised)
• Sulfuric acid — H₂SO₄ → 2H⁺ + SO₄²⁻
• Nitric acid — HNO₃ → H⁺ + NO₃⁻

Weak Acids

A weak acid only partially dissociates in water. Only a small fraction of its molecules release H⁺ ions; the rest remain as whole molecules. An equilibrium is set up.

Examples:

• Ethanoic acid — CH₃COOH ⇌ CH₃COO⁻ + H⁺ (reversible)
• Citric acid — partially ionises
• Carbonic acid — H₂CO₃ ⇌ H⁺ + HCO₃⁻

Key Distinction

Feature	Strong Acid	Weak Acid
Ionisation	Complete (100%)	Partial (small %)
Arrow in equation	→ (one-way)	⇌ (reversible)
pH (same concentration)	Lower (e.g. pH 1)	Higher (e.g. pH 3–4)
H⁺ concentration	High	Low
Conductivity	Higher	Lower
Rate of reaction	Faster	Slower

Exam Tip: Do NOT confuse strong/weak with concentrated/dilute. Strong/weak refers to how much an acid ionises. Concentrated/dilute refers to how much acid is dissolved in a given volume of water. You can have a concentrated weak acid or a dilute strong acid.

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Summary

• An acid produces H⁺ ions in aqueous solution; an alkali produces OH⁻ ions.
• The pH scale runs from 0 (most acidic) to 14 (most alkaline), with 7 being neutral.
• Indicators change colour in acidic and alkaline solutions; universal indicator gives a colour for each pH value.
• Strong acids fully dissociate in water; weak acids only partially dissociate (higher tier).
• Do not confuse strong/weak with concentrated/dilute — they describe different things.