Chemical changes · GCSE Chemistry
The pH scale and indicators
GCSE Chemistry revision on the pH scale and indicators: H⁺(aq) and OH⁻(aq), universal indicator colours, and the ten-fold change in hydrogen ion concentration.
Acids produce H⁺(aq). Alkalis produce OH⁻(aq). pH 7 is neutral. Each step down the pH scale is ten times more H⁺. Universal indicator is a rainbow; a probe is a number.
The important bits
What you need to know
- 1
Acids produce hydrogen ions, H⁺(aq), when they dissolve in water. Hydrochloric acid is HCl(aq) → H⁺(aq) + Cl⁻(aq).
- 2
Alkalis produce hydroxide ions, OH⁻(aq), in water. Sodium hydroxide is NaOH(aq) → Na⁺(aq) + OH⁻(aq). Soluble bases are alkalis.
- 3
The pH scale runs from 0 to 14. pH < 7 acidic, pH = 7 neutral, pH > 7 alkaline. Pure water and NaCl(aq) from a complete neutralisation are pH 7 at 25 °C.
- 4
Universal indicator is a mixture of dyes: red (strong acid), orange/yellow (weak acid), green (neutral), blue (weak alkali), purple (strong alkali).
- 5
Litmus: red in acid, blue in alkali. Phenolphthalein: colourless in acid, pink in alkali. Methyl orange: red in acid, yellow in alkali. Choose the indicator to match the titration.
- 6
A pH probe (meter) gives a numerical pH and is better for tracking a titration curve or a weak acid. Calibrate with pH 4, 7 and 10 buffers in the required practical.
- 7
On Higher tier, each decrease of 1 on the pH scale means a ten-fold increase in H⁺ concentration. pH 2 has ten times the [H⁺] of pH 3, and a hundred times pH 4.
- 8
Neutralisation is H⁺(aq) + OH⁻(aq) → H₂O(l). The spectator ions make the salt. pH moves towards 7 as H⁺ and OH⁻ cancel.
Quotations worth analysing
Short evidence. Real method.
“H⁺(aq) + OH⁻(aq) → H₂O(l)”
This is the reaction that changes pH. State symbols: aqueous ions become liquid water. Learn it until it is automatic.
“pH 0–2 strong acid; 3–6 weak acid; 7 neutral; 8–11 weak alkali; 12–14 strong alkali.”
Bands overlap in real bottles. Use them as a memory hook, then prefer a probe when the question wants a value such as 1.2 versus 3.8.
“HCl(aq) → H⁺(aq) + Cl⁻(aq)”
The acid only shows acidic properties in water, because H⁺(aq) is the species that makes pH low. Dry HCl(g) does not change dry litmus.
Go deeper
pH is about H⁺ concentration, not “how corrosive it feels”
A dilute strong acid can have a higher pH than a concentrated weak acid, but in school bottles hydrochloric acid at 0.1 mol/dm³ is about pH 1 and ethanoic acid at 0.1 mol/dm³ is about pH 3. The difference is how many H⁺ ions are actually free. Stomach acid is hydrochloric; vinegar is ethanoic. Both are acids; they are not equally ionised. Alkalis such as NaOH(aq) have very few H⁺ and many OH⁻, so pH is high. Neutral does not mean “safe”: water is pH 7, but so is a correctly made sodium chloride solution. The chemistry is the H⁺/OH⁻ balance, not the everyday word “neutral”.
Go deeper
Indicators tell you a range; probes tell you a number
Universal indicator is excellent for a first sort: acid, alkali or neither. It is a poor end-point indicator in titration because the colour changes through several hues and is hard to hit precisely. That is why titration uses methyl orange or phenolphthalein, which flip over a narrow pH range. A pH meter, rinsed and calibrated, can plot a curve: steep drop for strong acid–strong alkali, shallower for a weak acid. If the practical asks how to get a more accurate pH, say use a calibrated probe, not “add more indicator”. Indicator is itself a weak acid and can slightly change the pH if you drown the sample in it.
Go deeper
Ten-fold is a Higher-tier sentence worth practising
pH is linked to [H⁺] on a logarithmic scale. You do not need logs on every specification, but you do need “ten times”. A solution of pH 2 has 10 times the hydrogen ion concentration of pH 3. Diluting a strong acid by ten, if ionisation stays complete, raises pH by 1. Weak acids do not follow that as simply because the equilibrium shifts. If a question gives two pH values and asks which has more H⁺, pick the lower pH and, if it asks how many times more, use factors of ten per pH unit. Do not say pH 1 is “1% acid”.
See the idea in action
A beaker of 0.10 mol/dm³ HCl has pH 1 (strong acid, fully ionised). A beaker of 0.10 mol/dm³ CH₃COOH has pH about 3 (weak acid, partially ionised). Both make universal indicator red/orange, but the probe distinguishes them. Adding NaOH(aq) to either raises pH as H⁺(aq) + OH⁻(aq) → H₂O(l). At the end-point for HCl/NaOH, phenolphthalein goes from colourless to permanent pale pink and pH is about 7–8.
Exam technique
Turn knowledge into marks
Name H⁺(aq) for acids and OH⁻(aq) for alkalis before you mention colour. If the question is Higher, include the ten-fold change per pH unit. For practicals, say whether you used universal indicator, a named titration indicator, or a calibrated pH probe.
Common mistakes
Do not give these marks away
- 01
Saying acids contain OH⁻, or that pH 6 is alkaline.
- 02
Using universal indicator as if it were a precise titration end-point.
- 03
Claiming a lower pH always means a stronger acid, ignoring concentration.
What ions make a solution alkaline?
AH⁺(aq)
BOH⁻(aq)
CNa⁺(aq) only
DCl⁻(aq)
Show the answer
OH⁻(aq). Alkalis produce hydroxide ions in water. Sodium ions are spectators in NaOH(aq); it is OH⁻ that raises pH above 7.
Quick questions
If this is the bit you searched
What is the pH scale GCSE Chemistry?
A scale from 0 to 14 that measures how acidic or alkaline a solution is. Below 7 is acidic, 7 is neutral, above 7 is alkaline. It is linked to H⁺ concentration.
What colour is universal indicator in acid alkali and neutral?
Red to orange/yellow in acids, green when neutral, blue to purple in alkalis. Stronger acids are redder; stronger alkalis are more purple.
What ions do acids and alkalis produce in water?
Acids produce H⁺(aq). Alkalis produce OH⁻(aq). Neutralisation is H⁺(aq) + OH⁻(aq) → H₂O(l).
How much does H+ change for one pH unit?
On Higher tier, each decrease of 1 on the pH scale is a ten-fold increase in hydrogen ion concentration.