S3 Science — Acids and Alkalis: pH and Neutralisation Lesson Prep Guide

2026-11-15

← Back to the Secondary Science chapter index

Tutor lesson-prep guides · Secondary Science

Students know acid means pH below 7, then assume dilution changes nothing and treat sulfuric acid like hydrochloric acid. This lesson links pH, dilution and mole ratio through the hydrogen ion count.

Objectives — Students can: (1) explain pH in terms of hydrogen ion concentration and state that diluting ten times raises pH by one; (2) write acid plus alkali giving salt and water, choose a suitable indicator and give its colour change; (3) compute the volume needed for neutralisation from moles equals concentration times volume, including the doubled ratio for a diprotic acid.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
The pH range	What does pH 7 mean?	Neutral, as in pure water
Concentration	What does 0.1 M mean?	0.1 mole of solute per litre of solution
Volume conversion	How many litres is 25 cm3?	0.025 litre, divide by 1,000

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Three beakers	Show hydrochloric acid, water and sodium hydroxide with universal indicator, ask which is the acid	Match colours to pH
5-15 min	pH and dilution	Show 1e-3 M hydrogen ions giving pH 3, then ask for pH after a hundredfold dilution	Say pH rises by 2
15-28 min	Neutralisation and ratios	Show 25 cm3 of 0.1 M alkali needing 25 cm3 of 0.1 M acid, then switch to sulfuric acid	Explain why sulfuric acid needs double the volume
28-37 min	Choosing an indicator	Compare phenolphthalein and methyl orange ranges and ask which suits strong acid into strong alkali	Choose by the pH at the end point
37-42 min	Independent practice	Three questions on pH, neutralisation volume and dilution; tour asking for the mole count	Moles first, volume second
42-45 min	Exit check	Collect answers and ask how much alkali sulfuric acid needs	Answer double

03 Teaching script (read it out)

Scene 1: pH is another way to write hydrogen ion concentration

pH is the negative logarithm of the hydrogen ion concentration. 1e-3 M gives pH 3, and pH 2 means 0.01 M. So each step down in pH makes the acid ten times stronger, not twice. Diluting a hundredfold makes the concentration a hundred times smaller, so pH rises by 2. Students often say dilution leaves pH unchanged because it is still an acid. Overturn it with numbers: pH 3 diluted a hundredfold becomes pH 5, still an acid, but a different number.

Students give the pH after a hundredfold dilution of pH 3

Scene 2: Neutralisation: acid plus alkali gives salt and water

Neutralisation is hydrogen ions plus hydroxide ions giving water. Hydrochloric acid with sodium hydroxide gives sodium chloride and water; sulfuric acid gives sodium sulfate and water. The ratio matters: one hydrochloric acid gives one hydrogen ion, so it reacts one to one; sulfuric acid gives two, so the same number of moles needs twice the alkali. In the lab choose the indicator by the end point. Strong acid into strong alkali usually uses phenolphthalein, whose pink fades near pH 8 to 9 and is easy to see.

Students explain why sulfuric acid needs twice the alkali

Scene 3: Moles give the volume

Moles equal concentration times volume, and volume must be in litres. 25 cm3 of a 0.1 M solution is 0.1 times 0.025, which is 0.0025 mole. A one to one neutralisation needs the same 0.0025 mole of acid, which at 0.1 M is 0.0025 divided by 0.1, so 0.025 litre or 25 cm3. With sulfuric acid: 20 cm3 of 0.1 M holds 0.002 mole, so 0.004 mole of hydrogen ions, needing 0.004 mole of alkali, which is 40 cm3. Write the unit at every step and convert back to cm3 only at the end, and litres will not be mixed with cubic centimetres.

Students give the mole equation and the volume unit it needs

04 Question types and worked steps

Type	Example	Steps	Answer
pH from concentration	A solution has 1e-3 M hydrogen ions. What is its pH?	1) Take the negative logarithm. 2) The logarithm is 3	pH 3
pH after dilution	pH 3 acid diluted a hundredfold. New pH?	1) A hundred is ten squared. 2) Add 2 to the pH	pH 5, still an acid
Neutralisation volume	25 cm3 of 0.1 M sodium hydroxide. Volume of 0.1 M hydrochloric acid needed?	1) Moles are 0.1 times 0.025, so 0.0025. 2) One to one. 3) Divide by 0.1 for 0.025 litre	25 cm3
Diprotic ratio	20 cm3 of 0.1 M sulfuric acid. Volume of 0.1 M sodium hydroxide needed?	1) 0.002 mole of acid. 2) 0.004 mole of hydrogen ions. 3) Divide by 0.1 for 0.04 litre	40 cm3

Sanity check (teach this): Three checks. Volume unit: the mole equation needs litres, so 25 cm3 must be divided by 1,000 first. Ratio: acids with more than one hydrogen ion, such as sulfuric and phosphoric, need the alkali moles multiplied accordingly. Direction: dilution only moves pH towards 7, so an acid never becomes pH above 7 and an alkali never below 7, and an answer outside that range means stop and recheck. Numerically 0.0025 mole of sodium hydroxide is 0.1 g at a molar mass of 40, which cross-checks the mole count.

05 Independent practice (3 questions)

Question	Answer	What to watch
A solution of pH 2 has what hydrogen ion concentration?	0.01 M	Whether pH and concentration were mapped in the wrong direction
10 cm3 of 0.2 M hydrochloric acid needs how much 0.1 M sodium hydroxide?	20 cm3	Whether the mole count of 0.002 came first
What colour is phenolphthalein at pH 5?	Colourless	Whether the pH 8 to 10 range was remembered

06 Common mistakes and what to say

Mistake	Student writes	Say this
Dilution leaves pH alone	It is still an acid, so the pH is the same	A hundredfold dilution raises pH by 2, work out the concentration once
Sulfuric treated as monoprotic	Sulfuric acid needs the same volume as hydrochloric acid	Sulfuric acid has two hydrogen ions, so it needs twice the alkali
Volume not converted	0.1 times 25 is 2.5 mole	Moles need litres, and 25 cm3 is 0.025 litre
Any indicator anywhere	Use methyl orange for every neutralisation	Check the end point pH; for strong acid and alkali phenolphthalein is clearest

07 Homework (paper, section and question numbers)

Solubility and crystallisation, question 1 — Both scale from a stated per-unit figure
Sound and waves, question 3 — Both write the relationship before substituting
The rock cycle and Earth resources, question 2 — Matching processes to conditions uses the same answer pattern

08 Exit check (last 5 minutes)

Question	Answer	Decision
What is the pH after diluting pH 3 a hundredfold?	pH 5	Answering pH 3 means dilution is assumed not to matter
How much alkali does the same number of moles of sulfuric acid need?	Double	Answering the same means the diprotic ratio was missed
How many moles in 25 cm3 of 0.1 M solution?	0.0025 mole	Answering 2.5 means cubic centimetres were not converted

09 Tutor FAQ

Must students memorise strong acid and alkali lists?

Only the common ones: hydrochloric, sulfuric and nitric acids are strong; sodium and potassium hydroxide are strong. Questions state the rest, so no long memorising is needed.

Do S3 students need titration curves?

No. S3 covers single-point neutralisation and indicator choice only. Curves and pH meters belong to S4 Chemistry.

What if the mole concept has not been taught?

Explain concentration as moles per litre, then moles as concentration times volume. Skip the Avogadro constant entirely and the arithmetic still works.

Find a tutor — free Tutor Registration →

A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.