The pH Scale Revisited

Introduction

You first met the pH scale as a colour chart: red for acids, green for neutral, purple for alkalis. That was a useful start, but chemists need more than colours. They need a number that tells them precisely how acidic a solution is, so they can compare lemon juice with vinegar, check that swimming-pool water is safe or control a factory reaction. This page revisits the pH scale with the ionic model, explaining what the numbers actually measure.

Core explanation

What pH measures. pH is a measure of the concentration of hydrogen ions, H⁺(aq), in a solution. The lower the pH, the higher the concentration of H⁺ ions. Most solutions met in school fall between pH 0 and pH 14.

Water contains both ions. Even pure water ionises very slightly:

H₂O(l) ⇌ H⁺(aq) + OH⁻(aq)

Only about two molecules in every thousand million are ionised at any moment, but it means every aqueous solution contains both H⁺ and OH⁻ ions. What changes is their balance:

- Acidic (pH below 7): more H⁺ than OH⁻. - Neutral (pH 7 at 25 °C): equal amounts of H⁺ and OH⁻. - Alkaline (pH above 7): more OH⁻ than H⁺.

Adding an acid increases H⁺ and lowers pH; adding an alkali increases OH⁻, which removes H⁺ by forming water, so pH rises.

Typical pH values:

Solution Approximate pH --- --- Hydrochloric acid, 1 mol/dm³ 0 Stomach acid 1.5–3.5 Lemon juice 2–3 Vinegar about 3 Black coffee about 5 Pure water (25 °C) 7 Blood 7.35–7.45 Sea water about 8.1 Baking soda solution about 8.5 Household ammonia 11–12 Sodium hydroxide, 1 mol/dm³ 14

Describing strength of acidity. Solutions of pH 0–3 are usually called strongly acidic, pH 4–6 weakly acidic, pH 8–10 weakly alkaline and pH 11–14 strongly alkaline. These descriptions refer to the pH value, not to whether the acid is "strong" in the ionisation sense, which is explained later in the unit.

Beyond 0 and 14. The scale is not strictly limited. Very concentrated acids can have a pH below 0, and very concentrated alkalis a pH above 14. The 0–14 range simply covers the solutions met most often.

Measuring quantitatively. Universal indicator gives pH to about the nearest whole number. A pH meter or data-logging probe can give readings to 0.01 of a unit, which is essential in medicine, food science and environmental monitoring.

Step-by-step reasoning

To interpret a pH value:

1. Compare it with 7. 2. Below 7: acidic, with H⁺ in excess. Above 7: alkaline, with OH⁻ in excess. Equal to 7: neutral. 3. Judge how far it is from 7: the further away, the more strongly acidic or alkaline. 4. Predict the universal indicator colour from the value.

Visual explanation

Draw a horizontal bar from 0 to 14, coloured red, orange, yellow, green, blue, then purple. Above it, draw a set of H⁺ symbols that thin out from left to right; below it, OH⁻ symbols that grow denser from left to right. The two sets cross exactly at pH 7, the neutral point. In the indicator simulation, move the slider along this bar and watch the colours change.

Real-world analogy

pH is like a thermometer for acidity. "Hot" and "cold" are useful words, but a number such as 38 °C tells a doctor much more. In the same way, "acidic" is helpful, but pH 2 and pH 6 describe very different solutions, even though both are acidic.

Real-world example

Swimming-pool water is kept at about pH 7.2 to 7.6. If the pH drifts too low, the water irritates eyes and corrodes metal fittings; if it drifts too high, the chlorine disinfectant works less effectively and scale forms. Pool operators test the pH regularly and add small amounts of acid or alkali to correct it.

Why?

Why is 7 the neutral value? At 25 °C, pure water ionises to give equal concentrations of H⁺ and OH⁻, each 1 × 10⁻⁷ mol/dm³. The pH scale is defined so that this concentration of H⁺ corresponds to 7. Neutral simply means the two ions are balanced.

Common misconception

"An alkaline solution contains no H⁺ ions." Every aqueous solution contains both H⁺ and OH⁻ ions. In an alkaline solution the H⁺ concentration is very small but never zero; OH⁻ ions are simply in excess.

Worked example

Question: Samples have pH values: A = 2, B = 7, C = 9, D = 13. Classify each and state which has the highest concentration of H⁺ ions.

Reasoning: Below 7 is acidic, 7 neutral, above 7 alkaline. The lowest pH has the highest H⁺ concentration.

Answer: A strongly acidic, B neutral, C weakly alkaline, D strongly alkaline. A has the highest H⁺ concentration.

Quick check

1. A solution has pH 11. Are H⁺ or OH⁻ ions in excess? Answer: OH⁻ ions are in excess, because the pH is above 7.

Exam focus

Remember that a lower pH means more H⁺ ions. Examiners frequently ask you to put solutions in order of acidity or to link pH to indicator colour. Say "neutral at pH 7" and, for higher marks, explain it as equal H⁺ and OH⁻ concentrations.

Advanced insight

The neutral point is only exactly 7 at 25 °C. The ionisation of water is endothermic, so at higher temperatures more water ionises and both H⁺ and OH⁻ concentrations rise. At 50 °C, pure water has a pH of about 6.6, yet it is still neutral, because H⁺ and OH⁻ remain equal.

Summary

pH measures hydrogen ion concentration: the lower the pH, the more H⁺ ions. All aqueous solutions contain both H⁺ and OH⁻. Acidic solutions have an excess of H⁺ (pH below 7), alkaline solutions an excess of OH⁻ (pH above 7), and neutral solutions equal amounts (pH 7 at 25 °C). pH meters measure pH more precisely than indicators, and values can fall slightly outside 0–14.

Practice questions

1. Which ion is in excess in a solution of pH 4? Answer: The hydrogen ion, H⁺. 2. Arrange lemon juice, pure water and household ammonia in order of increasing pH. Answer: Lemon juice (about 2–3), pure water (7), household ammonia (11–12). 3. Explain what happens to the pH of an acid when an alkali is added gradually. Answer: The pH rises, because OH⁻ ions react with H⁺ ions to form water, lowering the H⁺ concentration. 4. Why is a pH meter preferred over universal indicator for monitoring blood samples? Answer: Blood pH must be known to within about 0.05 of a unit; a meter reads to 0.01, whereas indicator only gives the nearest whole number.