Acids and Bases — pH Scale

This lesson covers acids, bases, the pH scale and indicators as required by the AQA GCSE Combined Science Trilogy specification (8464). You need to understand the definitions of acids and bases, how to measure pH, and identify substances as acidic, neutral or alkaline.

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What Is an Acid?

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

Common Acids

Acid	Formula	Source / Use
Hydrochloric acid	HCl	Stomach acid, cleaning
Sulfuric acid	H₂SO₄	Car batteries, fertilisers
Nitric acid	HNO₃	Making fertilisers, explosives
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
• Sour taste (never taste chemicals in the lab)

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What Is a Base?

A base is a substance that neutralises an acid. Bases include metal oxides and metal hydroxides.

• If a base dissolves in water, it is called an alkali.
• An alkali produces hydroxide ions (OH⁻) in aqueous solution.

Common Bases and Alkalis

Base	Formula	Soluble?	Classification
Sodium hydroxide	NaOH	Yes	Alkali
Potassium hydroxide	KOH	Yes	Alkali
Calcium hydroxide	Ca(OH)₂	Slightly	Alkali (limewater)
Copper oxide	CuO	No	Insoluble base
Zinc oxide	ZnO	No	Insoluble base
Magnesium oxide	MgO	No	Insoluble base

Exam Tip: All alkalis are bases, but not all bases are alkalis. An alkali must dissolve in water. Copper oxide is a base but NOT an alkali because it is insoluble.

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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	Battery acid (pH 0–1), stomach acid (pH 1–2)
3–6	Weak acid	Lemon juice (pH 2–3), vinegar (pH 3), black coffee (pH 5)
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 (pH 13–14), sodium hydroxide solution

Key Points

• The lower the pH, the more acidic the solution and the higher the concentration of H⁺ ions.
• The higher the pH, the more alkaline the solution and the higher the concentration of OH⁻ ions.
• pH 7 is neutral — the concentrations of H⁺ and OH⁻ are equal.

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Measuring pH

Indicators

An indicator is a substance that changes colour in acidic or alkaline conditions.

Indicator	Colour in Acid	Colour in Neutral	Colour in Alkali
Litmus	Red	Purple	Blue
Methyl orange	Red	Orange	Yellow
Phenolphthalein	Colourless	Colourless	Pink
Universal indicator	Red–orange–yellow	Green	Blue–purple

Universal Indicator

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

graph LR
    A["pH 0–2<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

pH Meters and pH Probes

A pH meter (or pH probe connected to a data logger) gives a precise numerical reading of pH. This is more accurate than using universal indicator, which only gives an approximate value.

Exam Tip: Universal indicator gives an approximate pH (to the nearest whole number). A pH meter gives a precise reading (e.g. 4.3). If asked which method is more accurate, always choose the pH meter.

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Ions in Acidic and Alkaline Solutions

Acidic Solutions

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

$\text{HCl}(aq) \rightarrow \text{H}^+(aq) + \text{Cl}^-(aq)$HCl(aq)→H+(aq)+Cl−(aq)

$\text{H}_2\text{SO}_4(aq) \rightarrow 2\text{H}^+(aq) + \text{SO}_4^{2-}(aq)$H2​SO4​(aq)→2H+(aq)+SO42−​(aq)

$\text{HNO}_3(aq) \rightarrow \text{H}^+(aq) + \text{NO}_3^-(aq)$HNO3​(aq)→H+(aq)+NO3−​(aq)

Alkaline Solutions

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

$\text{NaOH}(aq) \rightarrow \text{Na}^+(aq) + \text{OH}^-(aq)$NaOH(aq)→Na+(aq)+OH−(aq)

$\text{KOH}(aq) \rightarrow \text{K}^+(aq) + \text{OH}^-(aq)$KOH(aq)→K+(aq)+OH−(aq)

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Acids and the Salt They Produce

The acid used determines the type of salt formed:

Acid	Salt Type	Example
Hydrochloric acid (HCl)	Chloride	Sodium chloride (NaCl)
Sulfuric acid (H₂SO₄)	Sulfate	Copper sulfate (CuSO₄)
Nitric acid (HNO₃)	Nitrate	Potassium nitrate (KNO₃)

Exam Tip: To name the salt, take the metal from the base and the ending from the acid. HCl → chloride, H₂SO₄ → sulfate, HNO₃ → nitrate.

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Common Mistakes

Mistake	Correction
Saying "all bases are alkalis"	Only soluble bases are alkalis — insoluble bases (e.g. CuO, MgO) are not alkalis
Thinking pH 1 means "slightly acidic"	pH 1 is a strong acid — the lower the number, the more acidic
Confusing litmus with universal indicator	Litmus only tells you acid or alkali; universal indicator estimates the pH value
Not linking H⁺ concentration to pH	More H⁺ ions = lower pH = more acidic

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Worked Example

Question: A student tests three solutions with universal indicator. Solution A turns red, Solution B turns green, and Solution C turns purple. State the approximate pH and classify each solution.