Acids & Bases
Discover what makes a substance acidic or basic, how the pH scale measures acidity, how indicators detect acids and bases, and how neutralisation reactions work in everyday life.
What Are Acids and Bases?
Acids are substances that produce hydrogen ions (H⁺) when dissolved in water. They taste sour (like vinegar and citrus) and can corrode metals. Bases are substances that accept H⁺ ions — they produce hydroxide ions (OH⁻) in water. Bases that dissolve in water are called alkalis. They feel slippery and taste bitter.
Acids
- Produce H⁺ ions in water
- pH less than 7
- Taste sour
- Turn blue litmus red
- React with metals to produce hydrogen gas
- React with carbonates to produce CO₂
Examples: hydrochloric acid (HCl), sulfuric acid (H₂SO₄), vinegar (ethanoic acid), citric acid (lemon juice)
Bases / Alkalis
- Produce OH⁻ ions in water (alkalis)
- pH greater than 7
- Taste bitter
- Turn red litmus blue
- Feel slippery/soapy
- React with acids in neutralisation
Examples: sodium hydroxide (NaOH), ammonia solution (NH₃), baking soda, bleach, toothpaste
Important distinction: All alkalis are bases, but not all bases are alkalis. A base is any substance that neutralises an acid; an alkali is a base that dissolves in water to form OH⁻ ions.
The pH Scale
The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is. A pH of 7 is neutral (pure water). Each whole number change on the pH scale represents a 10-fold change in hydrogen ion concentration — so pH 5 is 10 times more acidic than pH 6, and 100 times more acidic than pH 7.
The pH Scale
| Substance | Approx. pH | Classification |
|---|---|---|
| Battery acid (H₂SO₄) | 0–1 | Strongly acidic |
| Lemon juice / citric acid | 2–3 | Acidic |
| Cola drink | 3–4 | Weakly acidic |
| Rainwater (natural) | 5–6 | Slightly acidic |
| Pure water | 7 | Neutral |
| Blood / baking soda | 7.4 / 8–9 | Slightly alkaline |
| Ammonia solution | 11 | Alkaline |
| Sodium hydroxide (NaOH) | 13–14 | Strongly alkaline |
Indicators
An indicator is a substance that changes colour depending on whether a solution is acidic or alkaline. Indicators are used in labs (and in everyday life) to test the pH of solutions without expensive equipment.
Litmus
The most common indicator. Turns red in acid and blue in alkali. Purple in neutral. Good for a quick acid/alkali test but does not measure pH precisely.
Universal Indicator
A mixture of several indicators. Shows a range of colours (red → orange → yellow → green → blue → purple) to give an approximate pH value from 0–14.
Phenolphthalein
Colourless in acid, turns bright pink/magenta in alkali. Very useful for titrations to detect the exact point of neutralisation.
Red Cabbage Juice
A natural indicator! Contains anthocyanin pigments. Turns red/pink in acid, purple when neutral, and green/yellow in alkali.
In the lab: A pH meter gives the most accurate digital reading. Universal indicator paper (pH paper) is a quick and inexpensive alternative that gives an approximate reading by comparing colour to a chart.
Neutralisation Reactions
When an acid and a base react together, they neutralise each other, producing a salt and water. The H⁺ ions from the acid combine with the OH⁻ ions from the base to form water molecules.
HCl + NaOH → NaCl + H₂O
Everyday Applications of Neutralisation
Treating Acid Indigestion
Stomach acid (HCl, pH ~2) can cause heartburn. Antacid tablets contain bases like calcium carbonate (CaCO₃) or magnesium hydroxide (Mg(OH)₂) that neutralise the excess acid and relieve discomfort.
Treating Acidic Soil
Farmers add agricultural lime (calcium oxide or calcium carbonate) to acidic soil to raise the pH to the neutral range (pH 6–7), which is ideal for most crops.
Treating Bee and Wasp Stings
Bee stings are acidic (pH ~5); applying baking soda (alkaline) helps neutralise the sting. Wasp stings are alkaline; applying vinegar (acidic) can help neutralise the pain.
Toothpaste
Toothpaste is slightly alkaline (pH 8–9). It neutralises the acids produced by bacteria in the mouth after eating, helping prevent tooth decay.
Key Vocabulary
pH Scale
A scale from 0–14 that measures the concentration of H⁺ ions. pH < 7 is acidic, pH = 7 is neutral, pH > 7 is alkaline. Each unit represents a 10-fold change.
Indicator
A substance that changes colour according to the pH of a solution, allowing acids and alkalis to be identified. Examples include litmus, universal indicator, and phenolphthalein.
Neutralisation
The reaction between an acid and a base that produces a salt and water. The H⁺ and OH⁻ ions combine: H⁺ + OH⁻ → H₂O. The resulting solution has a pH closer to 7.
Alkali
A base that dissolves in water to produce hydroxide ions (OH⁻). All alkalis are bases, but not all bases are alkalis. Examples: NaOH, KOH, ammonia solution.
Worked Examples
A solution turns universal indicator orange. What does this tell you about its pH?
Recall: Universal indicator colours: red (1–2) → orange (3–4) → yellow (5–6) → green (7) → blue (8–9) → purple/violet (10–14).
Orange corresponds to a pH of approximately 3–4.
Conclusion: The solution is weakly acidic, similar in pH to vinegar or orange juice. It contains more H⁺ ions than OH⁻ ions.
Write the word equation for the reaction between hydrochloric acid and sodium hydroxide.
Identify reactants: hydrochloric acid (HCl) + sodium hydroxide (NaOH)
Recall: Acid + Base → Salt + Water
Identify salt: HCl produces Cl⁻ ions; NaOH provides Na⁺ ions → salt is sodium chloride (NaCl)
Word equation: Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water
HCl + NaOH → NaCl + H₂O
A farmer's soil has a pH of 4.5. What should they add, and why?
pH 4.5 is acidic — most crops grow best at pH 6–7.
Solution: Add agricultural lime (calcium carbonate or calcium oxide) — a base that will neutralise the soil acidity.
Why: The base reacts with the H⁺ ions in the soil, raising the pH closer to neutral. This improves nutrient availability and plant growth.
Knowledge Check
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Key Concepts Summary
- ✓ Acids produce H⁺ ions in water (pH < 7); bases produce OH⁻ ions (pH > 7).
- ✓ The pH scale (0–14) is logarithmic — each unit is a 10-fold change in H⁺ concentration.
- ✓ Indicators change colour to show whether a solution is acidic, neutral, or alkaline.
- ✓ Neutralisation: Acid + Base → Salt + Water. The H⁺ and OH⁻ ions combine to form water.
- ✓ Everyday applications include antacids, soil treatment, toothpaste, and treating insect stings.
- ✓ All alkalis are bases, but not all bases are alkalis (only those that dissolve in water).