The pH Scale

A pH question is usually worth 1-2 marks, but it appears on almost every paper in one costume or another: a universal indicator colour to interpret, a pH to assign to a named solution, or soil chemistry needing lime. Free marks, except that “acid is pH 1” hard-coded from primary school loses the mark when the acid in the question is weak ethanoic acid at pH 4.

The scale and what it measures (Core)

The pH scale runs from about 0 to 14 and measures how acidic or alkaline an aqueous solution is:

• pH < 7: acidic
• pH = 7: neutral
• pH > 7: alkaline

Position on the scale tracks both strength and concentration. Hydrochloric acid sits around pH 1; ethanoic acid (weak) around pH 3-4; pure water at 7; ammonia solution (weak alkali) around 10-11; sodium hydroxide near 14. Matching a sensible pH to a named solution is the standard 1-marker, so attach approximate numbers to the stock examples as you revise. The strong/weak distinction generating those numbers is Extended material covered in Acids, Bases and Indicators.

Universal indicator (Core)

Universal indicator is a mixture of dyes giving a colour spectrum across the scale:

pH	0-2	3-6	7	8-11	12-14
Colour	Red	Orange/yellow	Green	Blue	Purple

Two marks regularly hide in one observation: the colour and the interpretation. “Universal indicator turns red, pH about 1, strongly acidic” covers everything a 2-mark version can ask. Note what universal indicator is for: measuring how acidic, not just whether acidic. Litmus answers the yes/no question; universal indicator (or a pH meter, which is more accurate) puts a number on it.

pH and hydrogen ion concentration (Supplement)

Extended candidates connect the number to the particles: the higher the concentration of H+ ions, the lower the pH. That inverse relationship explains the exam comparisons:

• Equal concentrations of HCl and ethanoic acid: HCl is fully dissociated, so it has the higher H+ concentration and the lower pH.
• Diluting an acid lowers the H+ concentration, so the pH rises towards 7.
• Alkaline solutions have a high OH− concentration; neutralising removes H+ and OH− as water (H+ + OH− → H2O), pulling the pH towards 7.

A titration tracked with universal indicator tells the same story in colour: red through orange to green as alkali is added to acid, then on to purple past the end-point, one reason a sharp single-change indicator is preferred for accurate work in Titration and Solubility.

The applied favourite: soil pH

Acidic soil hosts a classic applied question: farmers add lime (calcium oxide or hydroxide) or powdered limestone (calcium carbonate) to neutralise soil acidity and raise the pH. The chemistry is just base + acid → salt + water, and the answer needs the word “neutralise” plus the direction of pH change. Crop-and-soil contexts also surface in the fertiliser work of Chemistry of the Environment.

Worked exam question

A student tests three solutions with universal indicator. Solution A turns red, solution B turns green, solution C turns purple. (a) Give the approximate pH of each solution. [3] (b) Solution A is a 0.1 mol/dm³ acid. Solution D, a different 0.1 mol/dm³ acid, turns the indicator orange. Explain the difference in terms of hydrogen ion concentration. [2] (c) Describe what happens to the pH when solution C is slowly added to solution A. [1]

Model answer: (a) A: pH 1 (accept 0-2) (1); B: pH 7 (1); C: pH 14 (accept 12-14) (1). (b) A is a strong acid, fully dissociated; D is a weak acid, only partially dissociated (1), so A has the higher concentration of H+ ions and therefore the lower pH (1). (c) The pH increases towards 7 (and beyond if excess C is added) as the acid is neutralised (1).

Mark-by-mark: (a) is direct colour-to-number conversion: ranges are accepted, but “low/high pH” without numbers is not. (b) needs the dissociation contrast and the explicit H+-to-pH link; either alone is one mark. (c) wants direction of change, with “neutralised” as the safe verb.

The mistakes that cost marks

1. One fixed pH for all acids. Weak acids sit at pH 3-6; read whether the acid is strong or weak before writing a number.
2. Reversing the H+ relationship. More H+ means lower pH. The two move in opposite directions.
3. Colour without a number (or vice versa) when the question asks for both. Pair them every time: “red, pH 1”.
4. “Litmus shows pH 4.” Litmus is two-state only: red or blue. Measuring a value needs universal indicator or a pH meter.

How examiners want it phrased

Student wording	Mark-scheme wording
”It’s really acidic"	"pH 1, strongly acidic"
"There’s more acid in it"	"The H+ ion concentration is higher, so the pH is lower"
"The acid gets cancelled out"	"The acid is neutralised: H+ + OH− → H2O, and the pH increases towards 7"
"Watered down acid is weaker"	"Diluting lowers the H+ concentration, so the pH increases” (strength is unchanged)

Every pH answer should carry a number, a colour where relevant, and (for Extended) the H+ link: three habits that take an evening to build. The section overview lives on the Acids, Bases and Salts pillar, and if you want those habits checked against real mark schemes, our free 1-hour trial lesson with a Chemistry specialist does exactly that.

Test yourself

Attach a number to every answer before clicking the mark scheme.

Q1 (2 marks). A farmer’s soil has a pH of 5. Name a substance the farmer could add and state its effect on the soil pH.

Show answer

• lime / calcium hydroxide / calcium oxide / calcium carbonate (powdered limestone) [1] • it neutralises the acid in the soil, so the pH increases towards 7 [1]

Q2 (2 marks). Universal indicator is added to aqueous ammonia. State the colour observed and the approximate pH.

Show answer

• blue [1] • approximately pH 10 (accept 8–11; ammonia is a weak alkali) [1]

Q3 (2 marks). (Extended) A sample of dilute hydrochloric acid is diluted ten times with water. State the effect on the hydrogen ion concentration and on the pH.

Show answer

• the H+ ion concentration decreases [1] • so the pH increases, towards (but not past) 7 [1]. The acid is still strong: dilution changes concentration, not strength.