---
title: "Strong and Weak Acids and the pH Scale"
description: "An acid in water releases hydrogen ions. A strong acid ionises fully while a weak one only partly does, so strength and concentration are separate ideas, and pH shifts by one for every tenfold change "
canonical: https://lightmysky.com/learn/science/strong-and-weak-acids-and-the-ph-scale-mt_Y4nSn5XH56
source: https://lightmysky.com/learn/science/strong-and-weak-acids-and-the-ph-scale-mt_Y4nSn5XH56.md
retrieved: 2026-09-12
---

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# Strong and Weak Acids and the pH Scale

An acid in water releases hydrogen ions. A strong acid ionises fully while a weak one only partly does, so strength and concentration are separate ideas, and pH shifts by one for every tenfold change in hydrogen ion concentration.

Subject: Science · Area: Matter & Materials · Ages 14 to 16
Page: https://lightmysky.com/learn/science/strong-and-weak-acids-and-the-ph-scale-mt_Y4nSn5XH56

## Ready when they can

- Separates strong from weak and concentrated from dilute, and gives an example of each pairing
- Predicts the pH change when an acid is diluted ten times
- Explains why a weak acid reacts more slowly with magnesium than a strong acid of the same concentration
- Writes what happens to hydrochloric acid and to ethanoic acid when each is added to water

## Lesson: Strong means split, not crowded

An acid releases hydrogen ions in water. A strong acid, like hydrochloric acid, splits fully and lets go of every ion. A weak acid, like ethanoic acid, splits only partly and holds most of itself back.

**Example.** Strength and concentration are separate dials. A concentrated weak acid can hold a higher pH than a dilute strong acid, because pH counts released ions, not bottles poured. Crowded is not the same as split.

The pH scale shifts by one for every tenfold change in hydrogen ions. Dilute an acid ten times and its pH rises by one. That is why a strong acid of the same concentration reacts faster with magnesium: it floods the metal with ions at once.

**Tip.** Write what each acid does in water. Hydrochloric acid gives all its hydrogen ions away, while ethanoic acid gives only a few and sets up a balance. Name one strong and one weak acid from memory.

**Recap.** Strong acids split fully, weak acids split partly, and each tenfold dilution lifts pH by one.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Acids, Alkalis & pH](https://lightmysky.com/learn/science/acids-alkalis-and-ph-mt_AN2kJE6I0s)
- [Redox: Oxidation and Reduction as Electron Transfer](https://lightmysky.com/learn/science/redox-oxidation-and-reduction-as-electron-transfer-mt_NMTikR0FTM)

## Opens up

- [Period 3 Oxides and the Acid-Base Divide](https://lightmysky.com/learn/science/period-3-oxides-and-the-acid-base-divide-mt_3kC5hTavIQ)
- [Titration: Finding an Unknown Concentration](https://lightmysky.com/learn/science/titration-finding-an-unknown-concentration-mt_7WLN35XxYx)
- [pH, Kw and Strong Acids and Bases](https://lightmysky.com/learn/science/ph-kw-and-strong-acids-and-bases-mt_KJ4yc25Z_l)
- [Weathering, Erosion and How Soil Forms](https://lightmysky.com/learn/science/weathering-erosion-and-how-soil-forms-mt_nQOPOt1VTH)