Acids and Bases

This lesson covers acids and bases, a fundamental topic in the AQA GCSE Chemistry specification (4.4.2). You need to understand the properties of acids and alkalis, measure pH using indicators and pH meters, write equations for reactions of acids, and understand the role of hydrogen ions (H+) and hydroxide ions (OH-) in determining acidity and alkalinity. This knowledge is essential for neutralisation, salt preparation, and understanding chemical reactions in everyday life.

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What Are Acids?

An acid is a substance that produces hydrogen ions (H+) when dissolved in water. It is the hydrogen ions that give acids their characteristic properties.

Common Acids

Acid	Formula	Type	Found in
Hydrochloric acid	HCl	Strong	Stomach acid; laboratory reagent
Sulfuric acid	H2SO4	Strong	Car batteries; industrial manufacture
Nitric acid	HNO3	Strong	Fertiliser manufacture
Citric acid	C6H8O7	Weak	Citrus fruits (lemons, oranges)
Ethanoic acid	CH3COOH	Weak	Vinegar
Carbonic acid	H2CO3	Weak	Fizzy drinks (dissolved CO2)

Properties of Acids

• Have a pH of less than 7
• Turn blue litmus paper red
• Turn universal indicator red/orange/yellow
• React with metals to produce a salt + hydrogen
• React with metal carbonates to produce a salt + water + carbon dioxide
• React with bases and alkalis to produce a salt + water (neutralisation)
• Taste sour (but never taste chemicals in the lab!)

Exam Tip: Always remember that acids produce H+ ions in solution. The more H+ ions present, the lower the pH and the stronger the acid. This is a key concept that connects acids, pH, and neutralisation. When writing about acids, refer to H+ ions to gain full marks.

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What Are Bases and Alkalis?

A base is a substance that can neutralise an acid. Bases are metal oxides and metal hydroxides.

An alkali is a soluble base — a base that dissolves in water to produce hydroxide ions (OH-).

Term	Definition	Example
Base	A substance that neutralises an acid; a metal oxide or hydroxide	Copper oxide (CuO) — insoluble base
Alkali	A soluble base that produces OH- ions in water	Sodium hydroxide (NaOH) — soluble in water

Common Alkalis

Alkali	Formula	Use
Sodium hydroxide	NaOH	Laboratory reagent; soap-making
Potassium hydroxide	KOH	Fertiliser; electrolyte in batteries
Calcium hydroxide	Ca(OH)2	Limewater (test for CO2); neutralising acidic soil
Ammonia solution	NH3(aq)	Cleaning products; fertiliser manufacture

Properties of Alkalis

• Have a pH of more than 7
• Turn red litmus paper blue
• Turn universal indicator blue/purple
• Feel soapy or slippery
• React with acids to produce a salt + water (neutralisation)

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The pH Scale

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

pH value	Classification	Examples
0–2	Strong acid	Hydrochloric acid, sulfuric acid
3–6	Weak acid	Vinegar (pH 3), lemon juice (pH 2–3), carbonic acid (pH 4–5)
7	Neutral	Pure water, sodium chloride solution
8–11	Weak alkali	Baking soda solution (pH 8–9), soap (pH 9–10)
12–14	Strong alkali	Sodium hydroxide, potassium hydroxide

graph LR
    A["pH 0-1<br/>Strong Acid<br/>e.g. HCl"] --> B["pH 2-3<br/>Acid<br/>e.g. Lemon juice"]
    B --> C["pH 4-6<br/>Weak Acid<br/>e.g. Vinegar"]
    C --> D["pH 7<br/>Neutral<br/>e.g. Water"]
    D --> E["pH 8-10<br/>Weak Alkali<br/>e.g. Soap"]
    E --> F["pH 11-14<br/>Strong Alkali<br/>e.g. NaOH"]

    style A fill:#ff0000,color:#fff
    style B fill:#ff6600,color:#fff
    style C fill:#ffcc00,color:#000
    style D fill:#00cc00,color:#fff
    style E fill:#0066ff,color:#fff
    style F fill:#660099,color:#fff

Measuring pH

pH can be measured using:

• Universal indicator — a mixture of dyes that changes colour across the pH range (red = acid, green = neutral, blue/purple = alkali)
• Litmus paper — turns red in acid, blue in alkali (only tells you acid or alkali, not the pH value)
• pH meter — an electronic device that gives a precise numerical pH reading
• pH probe connected to a data logger — for continuous monitoring

Exam Tip: Universal indicator gives an approximate pH value (by matching the colour to a colour chart), but a pH meter gives a more accurate numerical reading. If a question asks for the "most accurate" way to measure pH, the answer is a pH meter or pH probe, not universal indicator.

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Hydrogen Ions and Hydroxide Ions

The pH of a solution depends on the concentration of hydrogen ions (H+):

• Acidic solutions have a high concentration of H+ ions (and low concentration of OH- ions)
• Alkaline solutions have a high concentration of OH- ions (and low concentration of H+ ions)
• Neutral solutions have equal concentrations of H+ and OH- ions

Relationship Between pH and H+ Concentration

• Each decrease of 1 on the pH scale represents a tenfold increase in H+ ion concentration.
• For example, a solution with pH 3 has ten times more H+ ions than a solution with pH 4, and one hundred times more H+ ions than a solution with pH 5.

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Reactions of Acids

1. Acid + Metal → Salt + Hydrogen

Example: magnesium + hydrochloric acid → magnesium chloride + hydrogen

• Only metals above hydrogen in the reactivity series react with dilute acids.
• The gas produced can be tested with a burning splint — hydrogen burns with a squeaky pop.

2. Acid + Metal Oxide → Salt + Water

Example: hydrochloric acid + copper oxide → copper chloride + water

• This is a neutralisation reaction.
• The metal oxide acts as a base.

3. Acid + Metal Hydroxide → Salt + Water

Example: sulfuric acid + sodium hydroxide → sodium sulfate + water

• This is also a neutralisation reaction.
• The ionic equation for all neutralisation reactions is: H+(aq) + OH-(aq) → H2O(l)

4. Acid + Metal Carbonate → Salt + Water + Carbon Dioxide

Example: hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide

• The gas produced (CO2) can be tested with limewater — it turns milky/cloudy.

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Naming Salts