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.

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Worked Example: Predicting pH Changes on Dilution

A student starts with 10 cm³ of hydrochloric acid at pH 1 and dilutes it step by step with distilled water. Each tenfold dilution changes the hydrogen ion concentration by a factor of ten, which means the pH rises by one unit each time.

Step	Dilution	[H⁺] (mol/dm³)	pH
Start	Undiluted	0.1	1
1	Diluted by 10	0.01	2
2	Diluted by 100	0.001	3
3	Diluted by 1000	0.0001	4

This table illustrates why the pH scale is logarithmic. A pH 1 solution is not simply "twice as acidic" as pH 2 — it contains ten times as many H⁺ ions per unit volume. For an Edexcel exam answer, write: "Each decrease of one pH unit represents a tenfold increase in hydrogen ion concentration."

Common Mistake: Students often write that pH 3 has "three times more" H⁺ ions than pH 6. It is actually one thousand times more (10³). Always multiply by powers of ten.

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Comparing Acids at the Same Concentration

Imagine you have 0.1 mol/dm³ solutions of hydrochloric acid (strong) and ethanoic acid (weak). The concentrations are identical, yet the two solutions behave very differently because of the degree of ionisation.

Property	0.1 mol/dm³ HCl	0.1 mol/dm³ CH₃COOH
pH	~1	~3
[H⁺]	0.1 mol/dm³	~0.001 mol/dm³
Electrical conductivity	High	Low
Rate of reaction with Mg	Vigorous fizzing	Slow fizzing
Equation	HCl → H⁺ + Cl⁻	CH₃COOH ⇌ CH₃COO⁻ + H⁺

The hydrochloric acid ionises completely, releasing many H⁺ ions. The ethanoic acid only partially ionises — most of the molecules remain intact. The equilibrium lies far to the left for weak acids.

graph LR
    A["HCl molecules"] -->|"100% ionise"| B["H⁺ + Cl⁻<br/>(many ions)"]
    C["CH3COOH molecules"] -->|"~1% ionise"| D["H⁺ + CH3COO⁻<br/>(few ions)"]
    C -->|"~99% remain"| E["CH3COOH<br/>(intact molecules)"]

    style A fill:#c0392b,color:#fff
    style B fill:#27ae60,color:#fff
    style C fill:#2980b9,color:#fff
    style D fill:#27ae60,color:#fff
    style E fill:#8e44ad,color:#fff

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Choosing the Right Indicator

Different titrations require different indicators because each indicator changes colour over a specific pH range.

Indicator	pH Range	Best Used For
Methyl orange	3.1–4.4	Strong acid + strong/weak base titrations with a sharp end point near pH 4
Phenolphthalein	8.3–10.0	Strong acid + strong base, or weak acid + strong base titrations
Litmus	5.0–8.0	Rough identification of acid/alkali — NOT for titration
Universal indicator	1–14	Estimating pH — NOT for titration (too broad)

Common Mistake: Students sometimes choose universal indicator for a titration. This is wrong — a titration needs a sharp colour change at the end point, and universal indicator gives a slow, gradual change through many colours.

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Real-World Contexts

• Stomach acid and antacids: The stomach contains HCl at pH 1–2. Indigestion tablets contain weak bases (e.g. magnesium hydroxide) to neutralise excess acid gently without over-correcting.
• Acid rain: Rain normally has pH 5–6 due to dissolved CO₂. True acid rain (pH 3–4) is caused by sulfur dioxide and nitrogen oxides from burning fossil fuels, which dissolve to form sulfuric and nitric acids.
• Swimming pools: Pool water is maintained at pH 7.2–7.6. Operators add hydrochloric acid to lower pH or sodium carbonate to raise it.

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Answering at different grade levels

Grade 3–4 response: "Acids have a pH below 7. Alkalis have a pH above 7. Litmus paper turns red in acid and blue in alkali."

Grade 5–6 response: "An acid releases hydrogen ions (H⁺) in water and has a pH below 7. An alkali is a soluble base that releases hydroxide ions (OH⁻) in water and has a pH above 7. Universal indicator gives a colour across the full pH scale, while litmus only tells you whether a solution is acidic or alkaline."

Grade 7–9 response: "Acidity is determined by the concentration of H⁺ ions. A strong acid (e.g. HCl) fully dissociates, so all its molecules release H⁺, whereas a weak acid (e.g. ethanoic acid) only partially dissociates, with an equilibrium between molecules and ions. At equal concentration, the strong acid has a lower pH and higher electrical conductivity. Each unit on the pH scale represents a tenfold change in [H⁺], because the scale is logarithmic. Strong/weak refers to the extent of ionisation, not the amount of acid dissolved — concentration is a separate variable."

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Exam-Style Question Walkthrough

"A student tests two colourless liquids with universal indicator. Liquid A gives a green colour. Liquid B gives an orange colour. Suggest the pH of each liquid and identify whether each is acidic, alkaline or neutral. (2 marks)"

• Liquid A — green — pH 7 — neutral. (1 mark)
• Liquid B — orange — pH 3–4 — weakly acidic. (1 mark)

Follow-up: "Explain why universal indicator is more useful than litmus paper for this test." Universal indicator gives a range of colours corresponding to specific pH values on the pH scale, whereas litmus only shows whether a solution is acidic or alkaline — not how strongly.

Exam Tip: Always quote the colour AND the corresponding pH. Markers want the link, not just a colour.

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Quick-Fire Revision Facts

• An acid produces H⁺ ions in aqueous solution.
• An alkali produces OH⁻ ions in aqueous solution.
• Neutral solutions have a pH of 7 — pure water is the classic example.
• The pH scale runs from 0 (most acidic) to 14 (most alkaline).
• Universal indicator is a mixture that gives a continuous colour range across the pH scale.
• Phenolphthalein is colourless in acid and pink in alkali.
• A strong acid fully ionises; a weak acid partially ionises.
• Reactivity with magnesium is greater for stronger acids at the same concentration.
• A pH 2 acid has one hundred times the hydrogen ion concentration of a pH 4 acid.

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Edexcel alignment: This content is aligned with Edexcel GCSE Combined Science (1SC0) Chemistry Topic 4 Chemical changes / Topic 5 Extracting metals — specifically CC9 Acids and alkalis. Assessed on Chemistry Paper 1.