Lesson 1 of 3 — Acids & Alkalis

Indicators & Universal Indicator

Spec: 2.28 & 2.30

🌍 Real World: Doctors test urine with indicators. Farmers test soil pH. Pool technicians test swimming pool water. Today you learn the science behind ALL of these.

Learning Objectives:

2.28 – Describe the use of litmus, phenolphthalein and methyl orange to distinguish between acidic and alkaline solutions.
2.30 – Describe the use of universal indicator to measure the approximate pH value of an aqueous solution.

Navigation: Arrow Keys or Spacebar | Lesson time: 40 minutes

🖨️ Pupil Packs (One Click = All Pages)

Arrival Task

Arrival Task – Recall from Last Lesson

Instructions: Answer from memory in your books. Q1–3 recall our Atmosphere topic. Q4 leads into today.

1. Atmosphere Recall

Name the two most abundant gases in the Earth's atmosphere and give their approximate percentages.

Nitrogen (~78%) and Oxygen (~21%).

2. Atmosphere Recall

Describe one way the early Earth's atmosphere was different from today's.

Much more CO₂, little or no oxygen. Volcanoes released CO₂, water vapour and nitrogen.

3. Atmosphere Recall

State one human activity that is increasing the amount of CO₂ in the atmosphere.

Burning fossil fuels (coal, oil, gas). Also accept: deforestation, cement manufacture.

4. Lead-in 🔬

CO₂ dissolves in rainwater to form a weak acid. How could a scientist tell if an unknown liquid is an acid without tasting it?

Use an indicator – a substance that changes colour in the presence of acids or alkalis.

Starter – Record in Books

Starter: Copy & Complete (5 mins)

Instructions: Copy the table into your book. Predict whether each substance is an acid, alkali, or neutral.

📝 Copy this table:

Substance	Acid, Alkali or Neutral?	Clue / How do you know?
Lemon juice	Acid ✓
Soap
Pure water
Vinegar
Bleach
Stomach acid

🧪 Everyday Clues

💡 Hint: Does it sting / taste sour? → Probably acidic. Does it feel slippery / soapy? → Probably alkaline.

🌟 Extension: If you couldn't taste or touch the substance, what tool could you use to test it safely?

I Do

I Do: Three Key Indicators (2.28)

Key Idea: An indicator is a dye that changes colour to show whether a solution is acidic or alkaline. 🎯 Examiners want: the word "dye" or "substance", the phrase "changes colour", and "acidic or alkaline". Miss any of those = lose the mark.

🧪 Litmus

Red in acid

Blue in alkali

Comes as paper or liquid. The oldest indicator — used for over 700 years! 🧠 Memory aid: "Litmus = Literally just Red or Blue"

🧪 Phenolphthalein

Colourless in acid

Pink in alkali

🧠 Memory aid: "Phenolphthalein = Phantom in acid (invisible!), Pink in alkali." Used in titrations because the colour snaps from nothing to bright pink — impossible to miss.

🧪 Methyl Orange

Red in acid

Yellow in alkali

🧠 Memory aid: "Methyl orange = Mars bar Red in acid, Yellow sunshine in alkali." 🚦 Traffic-light colours make it easy to remember.

How an Indicator Works

▶ CLICK TO WATCH: Litmus in Acid vs Alkali

ACID

ALKALI

💡 Memory Tricks:
• Litmus: Red in acid, Blue in base
• Phenolphthalein: Pink in alkali
• Methyl Orange: like traffic lights – Red 🔴 → Yellow 🟡

We Do

We Do: Indicator Match Game

As a class: Select an indicator + solution → tap the matching colour.

🔴 Turns Red

🔵 Turns Blue

🫧 Stays Colourless

🩷 Turns Bright Pink

🟡 Turns Yellow

🟠 Turns Orange (decoy)

I Do

I Do: Universal Indicator (2.30)

Key Idea: Universal indicator is a mixture of dyes that gives a range of colours to show the approximate pH of a solution.

🌈 What Makes It Special?

Litmus, phenolphthalein and methyl orange each only tell you acid or alkali (two colours).

Universal indicator gives a range of colours – from red through yellow, green, blue to purple – so you can estimate the pH (how acidic or alkaline).

📏 What Is pH?

pH is a scale from 0 to 14 that measures how acidic or alkaline a solution is. You will learn the full scale next lesson – for now, just know:

• Low pH (0–6) = acidic | pH 7 = neutral | High pH (8–14) = alkaline

⚠️ Limitation

Universal indicator only gives an approximate pH – the colours can be tricky to read between values. For precise measurements, a pH meter is better.

Universal Indicator Colour Chart

0–1

3–4

5–6

8–9

10–11

12–14

← ACIDIC NEUTRAL ALKALINE →

Litmus vs Universal Indicator

Litmus

Only 2 colours

Red or Blue

Tells: acid or alkali

Universal

Many colours

Full rainbow

Tells: approximate pH

🔍 Spot the Mistake

Spot the Mistake: Indicators

A student wrote the answer below. It contains THREE mistakes.
Discuss with your partner first. Can you find all three before we reveal?

❌ WRONG ANSWER

"Litmus paper turns pink in acid and stays colourless in alkali. Phenolphthalein turns red in acid and blue in alkali."

👥 PEER TEACH: Tell your partner what's wrong and what the correct answer should be.

We Do

We Do: Lab Bench – Predict the UI Colour

As a class: Click a beaker, then click the colour you think universal indicator would show. Score: 0/5

🍋 Lemon juice

💧 Pure water

🧴 Oven cleaner

🥛 Milk

🪥 Toothpaste

👆 Select a beaker above, then click the UI colour below:

Independent Work – 8 mins

Independent Work

Answer these questions independently. Use your scaffolding sheet if needed.

Questions completed: 0/0

📗 Grades U–1 • Grade 2 challenge marked with ⭐

MCQ

1. What colour does litmus turn in acid?

Think: R for Red, R for... what starts with A?

RedBlueGreen

MCQ

2. Phenolphthalein in alkali turns:

Pink is the colour. Think flamingos...

RedColourlessPink

Short

3. What is an indicator?

It changes colour to show if something is acidic or...

A dye that changes colour to show whether a solution is acidic or alkaline.

Short

4. Name the indicator that turns yellow in alkali.

This indicator has 'orange' in its name but turns a different colour in alkali

Methyl orange.

⭐ Exam [3 marks]

5. A student tests a solution. Litmus turns red, phenolphthalein stays colourless. (a) Is it acidic or alkaline? [1] (b) Give evidence from both indicators. [2]

Both indicators agree: it's not alkaline. What's left?

(a) Acidic. (b) Litmus turns red in acid. Phenolphthalein stays colourless in acid (it only turns pink in alkali).

📙 Grades 1–2 • Grade 3 challenge marked with ⭐

MCQ

1. Universal indicator is a mixture of dyes. True or false?

UI is a MIXTURE of indicators, not just one

TrueFalse

MCQ

2. Methyl orange turns yellow. The solution is:

It turns yellow in alkali. The solution can't be acidic because in acid it would be red

AcidicAlkalineNeutral

Short

3. State one advantage of universal indicator over litmus.

Think: UI gives you MORE information than litmus. What extra info?

UI shows a range of colours giving the approximate pH, not just acid or alkali.

Short

4. UI turns green. What does this tell you?

Green = neutral. What pH is that?

The solution is neutral (approximately pH 7).

⭐ Exam [4 marks]

5. Compare litmus and universal indicator. State what each shows, and give one advantage and one disadvantage of UI.

One gives a pH range, the other is better for titrations with a sharp change

Litmus shows only acid (red) or alkali (blue) – two colours. UI shows a range of colours giving approximate pH. Advantage: UI tells you how acidic/alkaline. Disadvantage: colours can be hard to read precisely between values.

📘 Grades 3–4 • Grade 5 challenge marked with ⭐

MCQ

1. Phenolphthalein is better than UI for titrations because:

Titrations need a SHARP colour change at the endpoint

It's cheaperSharp two-colour changeGives exact pH

Short

2. State the colours of all three named indicators in acid and alkali.

There are 3 named indicators. List each with both colours

Litmus: red/blue. Phenolphthalein: colourless/pink. Methyl orange: red/yellow.

Short

3. Explain why UI only gives an "approximate" pH.

Think about what happens when the colour is BETWEEN two pH values

The colours can be hard to distinguish between adjacent pH values – e.g. is it yellow-green (pH 6-7) or green (pH 7)? A pH meter would be more precise.

📝 Say this in the exam: "Universal indicator shows a range of colours to estimate the approximate pH of a solution." That sentence alone is worth 2 marks.

⭐ Exam [5 marks]

4. A technician has litmus, phenolphthalein, methyl orange, and UI. She needs to find the pH of 5 unknown solutions. (a) Which should she use? [1] (b) Why not litmus? [2] (c) State one limitation and suggest a better alternative. [2]

Which method gives a NUMBER not just a COLOUR?

(a) Universal indicator. (b) Litmus only shows acid/alkali (2 colours) – can't estimate pH or tell strongly vs weakly acidic. (c) UI only approximate; a pH meter gives precise numerical readings.

Exit Ticket – 3 mins

Exit Ticket

No notes. Answer from memory to check what you've learned today.

1) What colour does litmus turn in an acidic solution?

Red.

2) What colour does phenolphthalein turn in an alkaline solution?

Bright pink.

3) Name one indicator we learned about today.

Litmus / Phenolphthalein / Methyl orange / Universal indicator.

Knowledge Organiser – Lesson 1: Indicators

Term	Definition / Detail
Indicator	A dye that changes colour to show whether a solution is acidic or alkaline.
Litmus	Red in acid · Blue in alkali. Available as paper or liquid.
Phenolphthalein	Colourless in acid · Bright pink in alkali. Sharp colour change – used in titrations.
Methyl Orange	Red in acid · Yellow in alkali. Also used in titrations.
Universal Indicator	A mixture of dyes that shows a range of colours to give the approximate pH of a solution.
pH	A scale from 0–14 measuring how acidic or alkaline a solution is. Low = acidic, 7 = neutral, high = alkaline.

Universal Indicator Colour Chart:

pH 0–2
Red

pH 3–4
Orange

pH 5–6
Yellow

pH 7
Green

pH 8–9
Blue

pH 10–12
Indigo

pH 13–14
Violet

Arrival Task – Recall from Last Lesson

Answer from memory.

1. Name the two most abundant gases in the atmosphere and their approximate percentages.

2. Describe one way the early atmosphere was different from today's.

3. State one human activity increasing CO₂ in the atmosphere.

4. Lead-in: CO₂ dissolves in rainwater to form a weak acid. How could a scientist test an unknown liquid without tasting it?

Starter: Acid, Alkali or Neutral?

Copy and complete. Predict whether each substance is an acid, alkali, or neutral, and explain your reasoning.

Substance	Acid, Alkali or Neutral?	Clue / How do you know?
Lemon juice
Soap
Pure water
Vinegar
Bleach
Stomach acid

We Do: Indicator Colour Table

Complete the table using the colour changes for each indicator.

Indicator	Colour in Acid	Colour in Alkali
Litmus paper
Phenolphthalein
Methyl orange

Universal Indicator: For each substance, write the colour UI would show and whether it is acidic, neutral or alkaline.

Substance	UI Colour	Acidic / Neutral / Alkaline?
Lemon juice
Pure water
Oven cleaner
Milk
Toothpaste

Scaffolding & Help Sheet

Indicator Colour Summary

Indicator	Colour in ACID	Colour in ALKALI
Litmus	Red	Blue
Phenolphthalein	Colourless	Bright pink
Methyl orange	Red	Yellow

Universal Indicator

Universal indicator is a mixture of dyes that produces different colours at different pH values.

It tells you how acidic or alkaline a solution is (approximate pH), unlike litmus which only tells you acid or alkali.

Red
Strong acid

Orange
Acid

Yellow
Weak acid

Green
Neutral

Blue
Weak alkali

Purple
Strong alkali

📝 Word Bank (Foundation)

Use these words in your answers: indicator, acid, alkali, neutral, red, blue, pink, colourless, yellow, universal indicator, pH, colour change, litmus, phenolphthalein, methyl orange

✍️ Sentence Starters

• An indicator is a substance that...

• In acid, litmus turns...

• Universal indicator is more useful than litmus because...

• The colour tells us whether the solution is...

Why Use Different Indicators? (Higher)

• Phenolphthalein is preferred in titrations because it has a sharp, clear two-colour change (colourless → pink).

• Universal indicator shows a range of colours, making the exact end point harder to spot.

• Limitation: UI only gives an approximate pH. A pH meter gives a precise numerical reading.

Independent Work – Foundation (Grades U–1)

Multiple Choice & Short Answer

1. What colour does litmus paper turn in an acidic solution?

[ ] Blue [ ] Red [ ] Green

2. What colour does phenolphthalein turn in an alkaline solution?

[ ] Red [ ] Colourless [ ] Pink

3. What is an indicator?

4. Name the indicator that turns yellow in an alkaline solution.

Long Answer

5. Complete the indicator colour table:

Indicator	Colour in Acid	Colour in Alkali
Litmus
Phenolphthalein
Methyl orange

6. A student adds universal indicator to a solution and it turns green. What does this tell the student?

Exam-Style Questions

7. [3 marks] A student tests an unknown solution. Litmus turns red. Phenolphthalein stays colourless. Methyl orange turns red.

(a) Is the solution acidic, alkaline, or neutral? [1 mark]

(b) State the evidence from two indicators that support your answer. [2 marks]

8. [3 marks] Explain what universal indicator is and state one way it is more useful than litmus paper.

Independent Work – Middle (Grades 1–2)

Multiple Choice & Short Answer

1. Methyl orange turns yellow in a solution. The solution is:

[ ] Acidic [ ] Alkaline [ ] Neutral

2. Universal indicator is a mixture of dyes. True or False?

[ ] True [ ] False

3. What colour would universal indicator turn in a neutral solution?

4. Explain one advantage of universal indicator over litmus.

Long Answer

5. A student tests three solutions. Use the indicator results to identify each as acidic, alkaline or neutral.

Solution	Litmus	Phenolphthalein	Methyl Orange
X	Red	Colourless	Red
Y	Blue	Pink	Yellow
Z	Purple	Colourless	Orange

6. Universal indicator is added to a solution and turns blue. Estimate the pH range and classify the solution.

Exam-Style Questions

7. [4 marks] A student adds universal indicator to four solutions. Describe what each colour tells them.

UI Colour	Acid / Neutral / Alkali?	Approximate pH
Red
Green
Blue
Violet

8. [4 marks] Compare litmus paper and universal indicator. In your answer, state what each shows and give one advantage and one disadvantage of universal indicator.

Independent Work – Higher (Grades 3–4)

Multiple Choice & Short Answer

1. Phenolphthalein is preferred over universal indicator in a titration because:

[ ] It is cheaper [ ] It has a sharp two-colour change [ ] It gives exact pH

2. State the colours of all three named indicators in acidic and alkaline solutions.

3. Explain why universal indicator only gives an approximate pH.

Long Answer

4. A student argues that litmus is the best indicator because it's the simplest. Evaluate this claim, using your knowledge of different indicators. [3 marks]

5. Design a method a student could use to determine whether an unknown solution is a strong acid, weak acid, neutral, weak alkali or strong alkali. State which indicator(s) you would use and explain your choice. [3 marks]

Exam-Style Questions

6. [5 marks] A technician is asked to determine the pH of five unknown solutions. She has litmus paper, phenolphthalein, methyl orange and universal indicator available.

(a) Explain why litmus paper alone would not be sufficient for this task. [2 marks]

(b) Explain why universal indicator is the most appropriate choice. [2 marks]

7. [6 marks] A student carries out a titration using phenolphthalein as the indicator. The alkali (NaOH) is in the conical flask and acid (HCl) is in the burette.

(a) What colour is the solution in the flask at the start? [1 mark]

(b) Describe the colour change observed at the end point. [1 mark]

(d) Another student repeats the experiment using methyl orange instead. Describe the expected colour change at the end point. [2 marks]

Exit Ticket – Foundation (U–1)

1) What colour does litmus turn in an acidic solution?

2) What colour does phenolphthalein turn in an alkaline solution?

3) Name one indicator we learned about today.

Exit Ticket – Middle (1–2)

1) What is the difference between litmus and universal indicator?

2) Methyl orange turns yellow in a solution. What type is it?

3) What colour would universal indicator be in a strong acid?

Exit Ticket – Higher (3–4)

1) Explain why phenolphthalein might be preferred over UI in a titration.

2) A student adds UI to a solution and it turns blue. What can they conclude?

3) State one limitation of using universal indicator.

Indicators & Universal Indicator

🖨️ Pupil Packs (One Click = All Pages)

Arrival Task – Recall from Last Lesson

1. Atmosphere Recall

2. Atmosphere Recall

3. Atmosphere Recall

4. Lead-in 🔬

Starter: Copy & Complete (5 mins)

📝 Copy this table:

🧪 Everyday Clues

I Do: Three Key Indicators (2.28)

🧪 Litmus

🧪 Phenolphthalein

🧪 Methyl Orange

How an Indicator Works

We Do: Indicator Match Game

I Do: Universal Indicator (2.30)

🌈 What Makes It Special?

📏 What Is pH?

⚠️ Limitation

Universal Indicator Colour Chart

Litmus vs Universal Indicator

Spot the Mistake: Indicators

❌ The 3 Mistakes:

We Do: Lab Bench – Predict the UI Colour

Independent Work

🏆 Finished? Add your name:

Exit Ticket

Knowledge Organiser – Lesson 1: Indicators

Arrival Task – Recall from Last Lesson

Starter: Acid, Alkali or Neutral?

We Do: Indicator Colour Table

Scaffolding & Help Sheet

Indicator Colour Summary

Universal Indicator

📝 Word Bank (Foundation)

✍️ Sentence Starters

Why Use Different Indicators? (Higher)

Independent Work – Foundation (Grades U–1)

Multiple Choice & Short Answer

Long Answer

Exam-Style Questions

Independent Work – Middle (Grades 1–2)

Multiple Choice & Short Answer

Long Answer

Exam-Style Questions

Independent Work – Higher (Grades 3–4)

Multiple Choice & Short Answer

Long Answer

Exam-Style Questions

Exit Ticket – Foundation (U–1)

Exit Ticket – Middle (1–2)

Exit Ticket – Higher (3–4)

We Do – Class Evidence