S2 Science | Acids, Alkalis and Neutralisation: Lesson-Prep Guide for Tutors (indicators, pH log scale, equations, titration amounts)

2026-11-12

Tutor lesson-prep guides · Junior Science

Students describe pH 2 as very sour, judge strength by colour depth, and write neutralisation as acid plus alkali makes water. This lesson nails three things: pH is a log scale, neutralisation always makes a salt and water, and amounts are counted in moles.

Objectives — Students can: (1) state that litmus turns red in acid and blue in alkali while phenolphthalein is colourless in acid and neutral and pink in alkali, and explain why two indicators are used; (2) state that pH 2 and pH 5 differ a thousandfold in hydrogen ion concentration because each pH step is a factor of ten; (3) write hydrochloric acid plus sodium hydroxide giving sodium chloride and water, and compute that 25.0 cubic centimetres of 0.20 molar acid needs 50.0 cubic centimetres of 0.10 molar alkali and yields 0.2922 grams of salt; (4) give three everyday examples (antacid, lime on acid soil, alkali treatment of acid waste water) with the reason each dose must be controlled.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Moles and concentration	How many moles are in 25 millilitres of 0.20 molar solution?	0.005 moles; reteach dividing millilitres by one thousand
Formulas	Give the formulas of hydrochloric acid, sodium hydroxide and sodium chloride	HCl, NaOH and NaCl
Relative masses	What are the relative atomic masses of sodium and chlorine?	22.99 and 35.45, which add to 58.44

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-6 min	Classify before explaining	Four unknown solutions and indicators only; students give the reason	Write the colour and the decision
6-14 min	Indicator colour table	Show litmus and phenolphthalein on the same solution and explain the pair	Fill the table, no mnemonics
14-25 min	pH is a log scale	Use ten to the minus two and minus five for the thousandfold step, then pH 7	Compute one two-step comparison
25-35 min	Equation and amounts	Write HCl plus NaOH, then turn 0.005 moles into 50 millilitres and 0.2922 grams	Work one amount question
35-41 min	Three everyday cases	Antacid, lime on soil, waste water, each with the cost of over-dosing	State one risk for any case
41-45 min	Exit check	Collect three answers, mark live, name the act to reteach	Hand in three answers

03 Teaching script (read it out)

Scene 1: Act one: indicators answer one question, not two

“Both indicators answer the same question: does this liquid have more hydrogen ions than water. Litmus is red in acid and blue in alkali, but it is equally red at pH 5 and pH 6, so it cannot rank strength. Phenolphthalein turns pink only on the alkaline side and stays colourless in acid and neutral. Use both: pink from phenolphthalein proves you have crossed neutral, and litmus gives the direction. That is why titrations use phenolphthalein, because the endpoint is the first faint pink, which the eye catches easily. With litmus the red-to-purple change is muddy and you will always overshoot.”

Indicators say which side of neutral, never how strong.

Scene 2: Act two: one pH step is ten times

“pH 2 means hydrogen ion concentration of ten to the minus two, which is 0.01 moles per litre. pH 5 is ten to the minus five, which is 0.00001. Dividing gives a thousand. So pH 2 is not slightly more acidic than pH 5, it is a thousand times more acidic. The usual trap is saying pH 2 is three times as acidic as pH 5, which treats a log scale as subtraction. pH 7 is neutral, below 7 is acidic and above 7 is alkaline, and the direction must be said out loud because students reverse it. A sanity check: pH runs from about 0 to 14, never negative and never 20.”

pH is a log scale: one step, ten times.

Scene 3: Act three: neutralisation makes a salt and water, and amounts are moles

“Neutralisation does not make acid disappear. Hydrogen ions join hydroxide ions to form water, and the remaining ions form a salt. Hydrochloric acid plus sodium hydroxide gives sodium chloride plus water. They react one to one, so 25.0 cubic centimetres of 0.20 molar acid is 0.005 moles and needs 0.005 moles of alkali. At 0.10 molar that is 0.005 divided by 0.1, which is 0.05 litres, or 50.0 cubic centimetres. The salt is 0.005 times 58.44, which is 0.2922 grams. Volume alone never settles it, because equal volumes of different concentrations carry different amounts. The everyday cases follow the same logic: an antacid taken without limit over-neutralises the stomach, and lime spread without a soil test pushes soil alkaline, locking up iron and manganese so crops yellow.”

Salt and water, and amounts counted in moles.

04 Question types and worked steps

Type	Example	Steps	Answer
Indicator reading	A solution turns litmus blue and phenolphthalein pink. Is it acidic, neutral or alkaline?	(1) Litmus blue means alkaline (2) phenolphthalein pink confirms it (3) agree both results before concluding (4) write the reason, not just the label	Alkaline, with both indicators agreeing
pH comparison	How many times more hydrogen ions does pH 2 have than pH 5?	(1) Write ten to the minus two and ten to the minus five (2) divide (3) get one thousand (4) say concentration, not a sourness score	One thousand times
Neutralisation equation	Write the full equation for hydrochloric acid with sodium hydroxide.	(1) Write HCl and NaOH (2) swap the partners to NaCl and water (3) count atoms (4) add state symbols	HCl plus NaOH gives NaCl plus H2O
Titration amount	What volume of 0.10 molar sodium hydroxide neutralises 25.0 cubic centimetres of 0.20 molar acid?	(1) Convert to litres and multiply by concentration for 0.005 moles (2) one to one, so the same moles of alkali (3) divide by 0.10 for 0.05 litres (4) convert to 50.0 cubic centimetres	50.0 cubic centimetres, giving 0.2922 grams of salt

Sanity check (teach this): Consistency check: (1) below pH 7 is acidic and above is alkaline, and the direction is confirmed aloud. (2) Each pH step is ten times, two steps a hundred, three steps a thousand, so never speak of times more acidic per unit. (3) Neutralisation always gives a salt and water, and writing only water loses the salt. (4) Moles equal volume in litres times concentration, so millilitres must be converted. (5) Salt mass is moles times relative formula mass: 0.005 moles of sodium chloride is 0.2922 grams, and an answer of 2.922 grams is a decimal slip.

05 Independent practice (3 questions)

Question	Answer	What to watch
10.0 cubic centimetres of 0.20 molar acid is diluted to 100.0 cubic centimetres. Find the new concentration.	Moles stay at 0.002, volume is 0.1 litres, so 0.020 molar and pH about 1.699	Whether moles were conserved rather than dividing the concentration by ten
How many times more hydrogen ions does pH 3 have than pH 6?	Ten to the minus three divided by ten to the minus six is one thousand	Whether the difference of three was mistaken for three times
How many moles of sodium hydroxide neutralise 0.005 moles of sulphuric acid?	Sulphuric acid has two hydrogen ions, so 0.010 moles	Whether the number of acidic hydrogens was checked instead of assuming one to one

06 Common mistakes and what to say

Mistake	Student writes	Say this
pH treated as arithmetic	pH 2 is three times as acidic as pH 5	pH is a log scale, so one step is ten times. Three steps is a thousand times, not three.
Water only	HCl plus NaOH gives water	There is a salt too. Hydrogen and hydroxide make water, and the sodium and chloride left behind are sodium chloride.
Volumes treated as moles	25 millilitres of acid means 25 millilitres of alkali	Concentrations differ, so equal volumes are not equal moles. Convert to litres, multiply by concentration, then divide by the alkali concentration.
Wrong indicator for a titration	Use litmus and stop when the colour changes	Litmus red to purple is too muddy and you will overshoot. Use phenolphthalein and stop at the first pink.

07 Homework (paper, section and question numbers)

Junior Science chapter index — All chapters for the subject; compare with the electricity chapter
Lesson-prep master index — Other subjects show the same four-step amount calculation
Redo this lesson — Test three household liquids with an indicator, write the acid or alkali judgement, then solve one neutralisation amount

08 Exit check (last 5 minutes)

Question	Answer	Decision
How many times more hydrogen ions does pH 5 have than pH 6?	Ten times	Answering double means reteach act two
Which two products does neutralisation always give?	A salt and water	Water alone means rewrite the equation
Which indicator suits a titration, and why?	Phenolphthalein, because colourless to pink is easy to catch	Answering litmus means return to act one

09 Tutor FAQ

Should S2 students learn moles?

As far as moles equal volume times concentration, yes. The derivation can wait; the calculation is drilled until it is automatic.

What if the colour table will not stick?

Do not memorise it. Run the two tubes each lesson: litmus red and blue, phenolphthalein pink. The memory comes from the action.

Are strength and concentration the same?

No. Strength is how fully an acid ionises and concentration is how much is dissolved. Say it once at S2 and leave the detail for S5.

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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.