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K324 / 2027
Acid-Base Chemistry

Lesson 1 of 7

Acids, alkalis and pH

Read acidity as an ion balance, not just a colour.

O-Level 6092 (2026) / SEC G3 K324 (2027)

  • I can define acids and alkalis.Syllabus

    Syllabus K324, 5.1(a). Define acids and alkalis

  • I can compare acidity and alkalinity.Syllabus

    Syllabus K324, 5.1(b). Compare acidity and alkalinity

  • I can explain control of soil acidity.Syllabus

    Syllabus K324, 5.1(f). Explain control of soil acidity

Acids, alkalis and pH

Read acidity as an ion balance, not just a colour.

Make a guess. You are not marked.

Hydrochloric acid of pH 1 is diluted ten times with water. What is the pH now?

  1. Exactly pH 7, because water is neutral
  2. Still pH 1, as the acid is the same
  3. Higher than 1 but still below 7
Show the answer

Higher than 1 but still below 7

There are fewer hydrogen ions in each cm3, so the solution is less acidic. Adding water never makes an acid neutral or alkaline.

An acid produces H+ ions in aqueous solution. An alkali is a soluble base that produces OH- ions in water. A base neutralises an acid; insoluble copper(II) oxide is a base but is not an alkali. Acidic and alkaline solutions both contain H+ and OH-; their relative concentrations differ.

Universal Indicator and the pH scale
Solution at room temperatureRelative ionspH and typical indicator colour
AcidicMore H+ than OH-Below 7; red through orange/yellow as pH increases
NeutralEqual H+ and OH-7; green
AlkalineMore OH- than H+Above 7; blue through purple

Compare colours with the supplied indicator chart. Universal Indicator estimates a pH range; a suitable calibrated pH probe gives a numerical reading. We use qualitative comparisons here, not calculations of pH from hydrogen-ion concentration.

Soil pH affects nutrient availability and plant growth. Calcium hydroxide can treat excessively acidic soil because it neutralises acid. Adding too much creates a different problem: overly alkaline soil. Determine the starting pH and add an appropriate amount rather than assuming more is better.

Check your understanding

Does a neutral solution contain no H+ ions?

  1. It has no H+ ions, because H+ ions are what make a solution acidic.
  2. It has H+ ions, balanced by an equal concentration of OH- ions.
  3. It has H+ ions if a little acid was added to it beforehand, and none otherwise.
Show the answer

It has H+ ions, balanced by an equal concentration of OH- ions.

Water itself ionises slightly, so any aqueous solution has some H+ and OH-. Neutral means the two concentrations are equal, not that the ions are absent.