ChemistryDifficulty 2-4

pH

pH is a number that expresses how acidic or basic a water-based solution is. It is the negative base-10 logarithm of the hydronium ion concentration, so each step of one pH unit is a tenfold change. At 25 °C a neutral solution has pH 7, acidic solutions are below 7 and basic solutions above 7; the neutral point itself shifts with temperature.

Temperature and the neutral point

Water's self-ionisation is endothermic, so Kw rises with temperature: IAPWS gives pKw = 14.95 at 0 °C, 13.99 at 25 °C and 12.25 at 100 °C. The neutral pH, pKw/2, therefore falls from about 7.47 at 0 °C to about 6.1 at 100 °C (ScienceVerse calculation), passing 6.31 at 80 °C, while pure water remains neutral throughout.

\mathrm{pH}_{\text{neutral}} = \tfrac{1}{2}\,\mathrm{p}K_w(T)

Neutrality means [H₃O⁺] = [OH⁻], so pH = pOH = pKw/2.

Natural waters

Water in contact with air absorbs CO₂, which raises its hydronium ion concentration to about 2.0 × 10⁻⁶ M — a pH of about 5.70. On a much larger scale, ocean uptake of atmospheric CO₂ is lowering seawater pH.

Common misconception: The simulation's fixed pKw = 14 is a 25 °C assumption, not a universal constant.

Full explanation — the complete reference version every reading depth is based on

The definition

\mathrm{pH} = -\log_{10}[\mathrm{H_3O^+}], \qquad [\mathrm{H_3O^+}] = 10^{-\mathrm{pH}}

Concentrations in mol/L. This is exactly what the pH & acid–base simulation computes.

Because pH uses a logarithm, it compresses an enormous range of concentrations into small numbers. Going from pH 7 to pH 6 means ten times more hydronium ions; from pH 7 to pH 4 means a thousand times more.

pH, pOH and neutrality

Water itself ionises very slightly. At 25 °C its ion-product constant is Kw = 1.0 × 10⁻¹⁴, which makes pH + pOH = 14.00. In pure water the hydronium and hydroxide concentrations are equal at 1.0 × 10⁻⁷ M, giving pH 7.00.

\mathrm{pH} + \mathrm{pOH} = 14.00 \quad (25\,^{\circ}\mathrm{C})

Acidic (pH < 7 at 25 °C): more H₃O⁺ than OH⁻.
Neutral (pH = 7 at 25 °C): equal H₃O⁺ and OH⁻.
Basic (pH > 7 at 25 °C): more OH⁻ than H₃O⁺.

Worked examples (ScienceVerse calculations)

0.1 M hydrochloric acid, a strong acid that ionises essentially completely: [H₃O⁺] = 0.1 M, so pH = −log(0.1) = 1.
0.1 M sodium hydroxide: [OH⁻] = 0.1 M, pOH = 1, so pH = 14 − 1 = 13 at 25 °C.
Air-saturated water, with [H₃O⁺] ≈ 2.0 × 10⁻⁶ M from dissolved carbon dioxide: pH = −log(2.0 × 10⁻⁶) ≈ 5.70.

Common misconception: 'Neutral means pH 7' is only true at 25 °C. Pure water is about pH 6.31 at 80 °C, and — taking half of IAPWS's pKw values (ScienceVerse calculation) — about 7.5 at 0 °C and 6.1 at 100 °C, yet it is still neutral, because its hydronium and hydroxide concentrations remain equal.

Common misconception: pH 4 is not 'a bit more acidic' than pH 5 — it has ten times the hydronium ion concentration.

Why pH matters

pH affects most chemical and biological processes in water: the optimum for most aquatic organisms is between pH 6.5 and 8. The ocean absorbs about 30 percent of the carbon dioxide released into the atmosphere, and that uptake is lowering the ocean's pH — ocean acidification — which makes it harder for oysters, corals and other shell-builders to form their shells and skeletons.

Info: Real pH meters respond to hydrogen ion activity rather than concentration, so the concentration-based definition used here and in the simulation is an idealisation.

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Connections

Prerequisites

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Acids and Bases

Guided learning path

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Related concepts

Acids and Bases — Measures

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pH & Acid-Base

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Red Cabbage pH Indicator

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Sources and methodology

pH is defined as the negative base-10 logarithm of the hydronium ion concentration, pH = −log[H₃O⁺], so [H₃O⁺] = 10⁻ᵖᴴ mol/L. (awaiting scientific review)
- 8.3: pH and pOH (Chemistry: Physical Principles), CC BY 4.0 — Textbook
Because pH is a logarithmic scale, a change of one pH unit (for example from 7 to 6) corresponds to a tenfold change in hydrogen ion concentration. (awaiting scientific review)
- CADDIS Volume 2: pH — Government or standards body
At 25 °C the ion-product constant of water is Kw = 1.0 × 10⁻¹⁴, so pH + pOH = 14.00 and a neutral solution has pH 7.00. (awaiting scientific review)
- 8.3: pH and pOH (Chemistry: Physical Principles), CC BY 4.0 — Textbook
At 25 °C acidic solutions have a pH below 7, neutral solutions a pH of 7 and basic solutions a pH above 7. (awaiting scientific review)
- 8.3: pH and pOH (Chemistry: Physical Principles), CC BY 4.0 — Textbook
In IAPWS's 2024 formulation for the ionisation constant of water, pKw of liquid water along the saturation line (Table 3, row psat) is 14.95 at 0 °C, 13.99 at 25 °C and 12.25 at 100 °C. (awaiting scientific review)
- R11-24: Revised Release on the Ionization Constant of H2O (Table 3, pKw from 0 °C to 1000 °C) — Government or standards body
Water's self-ionisation is endothermic, so the extent of ionisation and Kw increase with temperature: at 100 °C Kw is about 5.6 × 10⁻¹³, roughly 50 times its value at 25 °C. (awaiting scientific review)
- 14.2: Brønsted-Lowry Acids and Bases (Chemistry 2e, OpenStax, on LibreTexts), CC BY 4.0 — Textbook
Pure water at 80 °C has a hydronium ion concentration of 4.9 × 10⁻⁷ M, so at that temperature neutral solutions have pH = pOH = 6.31; pH values are taken to refer to 25 °C unless stated otherwise. (awaiting scientific review)
- 14.3: pH and pOH (Chemistry 2e, OpenStax, on LibreTexts), CC BY 4.0 — Textbook
What a pH measurement actually responds to is hydrogen ion activity, not hydrogen ion concentration. (awaiting scientific review)
- CADDIS Volume 2: pH — Government or standards body
Ocean acidification is a reduction in the pH of the ocean over an extended period, caused primarily by the ocean's uptake of carbon dioxide from the atmosphere; the ocean absorbs about 30 percent of the CO₂ released into the atmosphere. (awaiting scientific review)
- What is Ocean Acidification? — Government or standards body
The optimum pH for most aquatic organisms falls between 6.5 and 8. (awaiting scientific review)
- CADDIS Volume 2: pH — Government or standards body
Air-saturated water has a hydronium ion concentration of about 2.0 × 10⁻⁶ M, caused by dissolved carbon dioxide and about 20 times that of pure water. (awaiting scientific review)
- 14.3: pH and pOH (Chemistry 2e, OpenStax, on LibreTexts), CC BY 4.0 — Textbook

Claims marked “awaiting scientific review” cite the sources listed but have not yet been signed off by a scientific reviewer.

Content status: published 1 October 2026.

Scientific review: this version has not yet been signed off by a scientific reviewer.
The Advanced explanation has not yet been reviewed for age suitability.