Acids, Bases & the pH Scale

What Are Acids?

An acid is a substance that, when dissolved in water, releases hydrogen ions (H⁺) into the solution. The more H⁺ ions released, the stronger the acid. Acids have several recognisable properties:

Sour taste (e.g. vinegar, lemon juice)
React with metals to produce hydrogen gas: Acid + Metal → Salt + H₂
React with carbonates to produce carbon dioxide: Acid + Carbonate → Salt + H₂O + CO₂
Turn blue litmus paper red
Corrosive — can damage skin and materials at high concentrations
pH less than 7

Common acids and their formulas:

Acid name	Formula	Found in / used for
Hydrochloric acid	HCl	Stomach acid, cleaning metals
Sulfuric acid	H₂SO₄	Car batteries, fertiliser production
Nitric acid	HNO₃	Fertilisers, explosives manufacture
Ethanoic (acetic) acid	CH₃COOH	Vinegar, food preservation
Citric acid	C₆H₈O₇	Lemons, fizzy drinks
Carbonic acid	H₂CO₃	Carbonated water and soft drinks

What Are Bases & Alkalis?

A base is a substance that can neutralise an acid. When dissolved in water, a base releases hydroxide ions (OH⁻). Bases that dissolve in water are called alkalis. Properties of bases:

Bitter taste
Slippery or soapy feel (like soap on skin)
Turn red litmus paper blue
Can be corrosive at high concentrations
pH greater than 7

Common bases and alkalis:

Name	Formula	Found in / used for
Sodium hydroxide (caustic soda)	NaOH	Soap making, drain cleaners
Potassium hydroxide	KOH	Soap making, batteries
Calcium hydroxide (slaked lime)	Ca(OH)₂	Treating acidic soil, cement
Ammonia solution	NH₃(aq)	Cleaning products, fertilisers
Sodium bicarbonate (baking soda)	NaHCO₃	Baking, antacids, fire extinguishers
Magnesium hydroxide	Mg(OH)₂	Antacids (milk of magnesia)

Alkali vs Base: All alkalis are bases, but not all bases are alkalis. A base only becomes an alkali if it is soluble in water. Copper oxide (CuO) is a base but not an alkali — it does not dissolve in water.

The pH Scale

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

pH 0–6: Acidic (lower pH = more acidic, more H⁺ ions)
pH 7: Neutral (pure water — equal H⁺ and OH⁻ ions)
pH 8–14: Alkaline/basic (higher pH = more alkaline, more OH⁻ ions)

The pH scale is logarithmic — a solution with pH 4 is 10× more acidic than pH 5, and 100× more acidic than pH 6.

pH of common substances:

Substance	Approx. pH	Classification
Battery acid (H₂SO₄)	0–1	Very strong acid
Stomach acid (HCl)	1–2	Strong acid
Lemon juice	2–3	Acid
Vinegar	2.5–3.5	Weak acid
Coffee	5	Weak acid
Rainwater	5.6	Slightly acidic
Pure water	7	Neutral
Blood	7.4	Very slightly alkaline
Seawater	8	Slightly alkaline
Baking soda solution	8.5–9	Weak base
Milk of magnesia	10.5	Base
Ammonia solution	11–12	Strong base
Bleach (NaOCl)	12–13	Strong base
Sodium hydroxide	13–14	Very strong base

Indicators

An indicator is a substance that changes colour depending on whether it is in an acidic, neutral, or alkaline solution. Indicators are used to test the pH of a solution.

Indicator	In acid	Neutral	In base (alkali)
Litmus	Red	Purple	Blue
Universal indicator	Red → orange → yellow	Green	Blue → violet
Phenolphthalein	Colourless	Colourless	Pink/magenta
Methyl orange	Red/orange	Orange	Yellow

Universal indicator is a mixture of several indicators that gives a full range of colours across the pH scale, making it possible to estimate pH values rather than just distinguishing acid from base.

Neutralisation

Neutralisation is the reaction between an acid and a base to produce a salt and water. The H⁺ ions from the acid combine with the OH⁻ ions from the base to form water:

Acid + Base → Salt + Water

HCl + NaOH → NaCl + H₂O

H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O

HCl + KOH → KCl + H₂O

The salt produced depends on which acid and which base react. The name of the salt comes from the metal of the base + the anion of the acid:

Acid	Base	Salt produced	Salt name
HCl	NaOH	NaCl	Sodium chloride
HCl	KOH	KCl	Potassium chloride
H₂SO₄	NaOH	Na₂SO₄	Sodium sulfate
HNO₃	NaOH	NaNO₃	Sodium nitrate
HCl	Ca(OH)₂	CaCl₂	Calcium chloride

Everyday Uses of Acids and Bases

Stomach acid (HCl) — digests food and kills bacteria
Vinegar (ethanoic acid) — food preservation, cooking
Baking soda (NaHCO₃) — reacts with acids in baking to release CO₂, making dough rise; also used as an antacid
Lime (Ca(OH)₂) — added to acidic soil to neutralise it and improve crop growth
Soap and shampoo — slightly alkaline; helps remove oils
Antacids (Mg(OH)₂, CaCO₃) — neutralise excess stomach acid to relieve heartburn
Sulfuric acid — used in manufacturing fertilisers, plastics, dyes, and explosives

Acid rain: When fossil fuels burn, SO₂ and NO₂ are released into the atmosphere. These gases dissolve in rainwater to form sulfuric acid and nitric acid, lowering the pH of rain to 4–5. Acid rain damages forests, corrodes buildings, and acidifies lakes and rivers.

Answer all questions in your notebook. Write balanced equations where required. Show all reasoning and working clearly.

Question 1

A learner tests four unknown solutions (A, B, C, D) with litmus indicator and universal indicator. The results are shown below:

Solution A: red litmus → red; universal indicator → orange (pH ≈ 4)
Solution B: red litmus → blue; universal indicator → blue (pH ≈ 9)
Solution C: no colour change with litmus; universal indicator → green (pH = 7)
Solution D: red litmus → blue; universal indicator → violet (pH ≈ 13)

(a) Identify each solution as acidic, neutral, or alkaline.
(b) Rank solutions A, B, C, and D from most acidic to most alkaline.
(c) Which solution is a strong base? Explain your reasoning using pH.
(6 marks)

Question 2

Write balanced word equations AND balanced formula equations for the following neutralisation reactions:

(a) Hydrochloric acid + potassium hydroxide
(b) Sulfuric acid + sodium hydroxide
(c) Nitric acid + calcium hydroxide

For each reaction, name the salt produced. (3 marks per reaction — 9 marks total)

Question 3

A farmer notices that her crops are growing poorly. She tests the soil and finds it has a pH of 4.5.

(a) Is the soil acidic, neutral, or alkaline?
(b) What substance could the farmer add to the soil to raise its pH? Give the chemical formula of this substance.
(c) What type of reaction takes place when this substance reacts with the acidic soil?
(d) Explain why most crops grow best in soil with a pH close to 7.
(4 marks)

Question 4

Explain the difference between an acid and a base in terms of the ions they produce in water. Then explain what a salt is and how it forms during a neutralisation reaction. Use an example equation in your answer. (5 marks)

Question 5

The pH of stomach acid is approximately 2. Antacid tablets contain magnesium hydroxide (Mg(OH)₂).

(a) What type of substance is Mg(OH)₂? (acid / base / neutral)
(b) Write a balanced formula equation for the reaction between hydrochloric acid (HCl) and magnesium hydroxide (Mg(OH)₂). (Hint: Mg forms a 2+ ion)
(c) After taking the antacid, the stomach pH rises to 5. Has the stomach become more or less acidic? Explain using the pH scale.
(d) Would the stomach contents now be acidic, neutral, or alkaline?
(5 marks)

Acids, Bases & the pH Scale

What Are Acids?

What Are Bases & Alkalis?

The pH Scale

Indicators

Neutralisation

Everyday Uses of Acids and Bases

Virtual pH Meter — Interactive Beaker