Strong and concentrated are different.

An Arrhenius acid increases aqueous hydrogen-ion concentration and an Arrhenius base increases hydroxide-ion concentration. Strong acids ionise essentially completely under the stated dilute conditions; weak acids ionise only partly. Concentration describes how much solute is present per volume, not the extent of ionisation.

pH = −log₁₀[ $H^{+}$ ] makes each unit a tenfold concentration change. At 25 °C neutrality corresponds to pH 7 for pure water. Dilution of an ordinary acidic solution moves its pH toward neutrality; it does not make a strong acid become weak.

INTERACTIVE MODEL

A logarithmic acidity scale

Dilution factor1

READ THE GRAPH

Explore the relationship

Fully dissociated monoprotic acid; concentration remains far above 10⁻⁷ M so water can be neglected.

pH after dilution

Tap or focus a plotted point to read its values.

View plotted data as a table

Explore the relationship
Series	Initial pH	pH after dilution
pH after dilution	1	1
pH after dilution	1.05	1.05
pH after dilution	1.1	1.1
pH after dilution	1.15	1.15
pH after dilution	1.2	1.2
pH after dilution	1.25	1.25
pH after dilution	1.3	1.3
pH after dilution	1.35	1.35
pH after dilution	1.4	1.4
pH after dilution	1.45	1.45
pH after dilution	1.5	1.5
pH after dilution	1.55	1.55
pH after dilution	1.6	1.6
pH after dilution	1.65	1.65
pH after dilution	1.7	1.7
pH after dilution	1.75	1.75
pH after dilution	1.8	1.8
pH after dilution	1.85	1.85
pH after dilution	1.9	1.9
pH after dilution	1.95	1.95
pH after dilution	2	2
pH after dilution	2.05	2.05
pH after dilution	2.1	2.1
pH after dilution	2.15	2.15
pH after dilution	2.2	2.2
pH after dilution	2.25	2.25
pH after dilution	2.3	2.3
pH after dilution	2.35	2.35
pH after dilution	2.4	2.4
pH after dilution	2.45	2.45
pH after dilution	2.5	2.5
pH after dilution	2.55	2.55
pH after dilution	2.6	2.6
pH after dilution	2.65	2.65
pH after dilution	2.7	2.7
pH after dilution	2.75	2.75
pH after dilution	2.8	2.8
pH after dilution	2.85	2.85
pH after dilution	2.9	2.9
pH after dilution	2.95	2.95
pH after dilution	3	3
pH after dilution	3.05	3.05
pH after dilution	3.1	3.1
pH after dilution	3.15	3.15
pH after dilution	3.2	3.2
pH after dilution	3.25	3.25
pH after dilution	3.3	3.3
pH after dilution	3.35	3.35
pH after dilution	3.4	3.4
pH after dilution	3.45	3.45
pH after dilution	3.5	3.5
pH after dilution	3.55	3.55
pH after dilution	3.6	3.6
pH after dilution	3.65	3.65
pH after dilution	3.7	3.7
pH after dilution	3.75	3.75
pH after dilution	3.8	3.8
pH after dilution	3.85	3.85
pH after dilution	3.9	3.9
pH after dilution	3.95	3.95
pH after dilution	4	4

Probe along the horizontal axis1pH after dilution: 1

Explore: Compare dilution factors of 1 and 10 for the same starting pH.

\mathrm{pH}=-\log_{10}[\mathrm H^+]

WORKED EXAMPLE

A strong monoprotic acid has [ $H^{+}$ ] = 1.0 × $10^{-3}$ mol $L^{-1}$ .

pH = −log₁₀( $10^{-3}$ ) = 3.
A tenfold dilution gives approximately pH 4, where water’s contribution is negligible.

Assumed knowledge

Powers of ten and concentration.

Learning checkpoints & source

YOUR LEARNING CHECKPOINT

Use the Arrhenius model for aqueous acids and bases.
Relate pH to hydrogen-ion concentration.

QCAA Chemistry · Unit 2 · Aqueous solutions and acidity ↗

READY TO TRY IT?

Strong and concentrated are different.

A logarithmic acidity scale

Explore the relationship

A strong monoprotic acid has [H+H^{+}H+] = 1.0 × 10−310^{-3}10−3 mol L−1L^{-1}L−1.

Put the idea to work.

A strong monoprotic acid has [ $H^{+}$ ] = 1.0 × $10^{-3}$ mol $L^{-1}$ .