8 min readConcept sheetPart I (1 mark), Part II (2 marks: define pH) and Part III (4 marks: ionic product of water and pH + pOH = 14)

Acids, bases, pH and the ionic product of water

Acids give H⁺ and bases give OH⁻. pH is −log₁₀[H⁺], and the ionic product of water is 1 × 10⁻¹⁴ at 25 °C, so pH + pOH = 14.

pHpH = −log₁₀[H⁺]; pOH = −log₁₀[OH⁻]

Ionic product of water (Kw)Kw = [H⁺][OH⁻] = 1 × 10⁻¹⁴ mol² L⁻² at 25 °C

Key relationpH + pOH = 14 at 25 °C

pH scaleBelow 7 acidic, 7 neutral, above 7 basic

Acids vs bases

AcidsGive H⁺ ions in water: HCl → H⁺ + Cl⁻. Taste sour, turn blue litmus red, pH below 7. Examples: hydrochloric acid, sulphuric acid, citric acid in lemon, tartaric acid in tamarind.

BasesGive OH⁻ ions in water: NaOH → Na⁺ + OH⁻. Taste bitter, feel soapy, turn red litmus blue, pH above 7. Bases that dissolve in water are alkalis: NaOH, KOH, Ca(OH)₂.

Square brackets, as in [H⁺], mean concentration in mol L⁻¹.

Ionic product of water and pH + pOH = 14

1
Water ionises very slightly: H₂O ⇌ H⁺ + OH⁻
2
The ionic product of water is Kw = [H⁺][OH⁻]
3★
At 25 °C, Kw = 1 × 10⁻¹⁴ mol² L⁻²
4
In pure water [H⁺] = [OH⁻], so each is √(10⁻¹⁴) = 1 × 10⁻⁷ mol L⁻¹. Pure water is neutral, pH 7.
5
Take −log₁₀ of both sides of [H⁺][OH⁻] = 10⁻¹⁴: −log[H⁺] + (−log[OH⁻]) = −log(10⁻¹⁴)
6★
So pH + pOH = 14

★ key step

The pH scale

The term pH was introduced by S. P. L. Sorensen in 1909. pH = −log₁₀[H⁺].
The scale runs from 0 to 14 at 25 °C.
pH < 7: acidic. pH = 7: neutral. pH > 7: basic (alkaline).
The lower the pH, the stronger the acid. The higher the pH, the stronger the base.
One pH unit means a tenfold change in [H⁺]: pH 3 has ten times more H⁺ than pH 4.
A universal indicator shows different colours across the scale, from red for strong acids to violet for strong bases.

In real life

The sour taste of tamarind and lemon comes from acids, and the slippery feel of soap comes from a base. Your stomach makes hydrochloric acid to digest food, and antacid tablets contain bases such as magnesium hydroxide.

Exam angle

1 marks: “The negative logarithm to base 10 of the hydroxyl ion concentration is called ______.” pOH (pOH = −log₁₀[OH⁻]).

2 marks: “Define pH.” pH is the negative logarithm to base 10 of the hydrogen ion concentration: pH = −log₁₀[H⁺].

4 marks: “What is the ionic product of water? Show that pH + pOH = 14.” H₂O ⇌ H⁺ + OH⁻; Kw = [H⁺][OH⁻] = 1 × 10⁻¹⁴ at 25 °C. Take −log of both sides to get pH + pOH = 14.

Remember it

Water's product is 10⁻¹⁴, so pH and pOH always add up to 14.