Lesson 2 of 3 — Acids & Alkalis

The pH Scale

Edexcel IGCSE 4SS0 — Spec 2.29 & 2.30

🍋

→pH SCALE

0–14⚡ Universal Indicator ⚡

Today you become a
chemistry detective.Put a number on how acidic — and learn the 5 exact phrases that win marks.

🌍 Why this matters: Your stomach acid is pH 2 — strong enough to dissolve metal. Your blood is pH 7.4 — if it shifts by just 0.5, you die. Today you learn how scientists measure that difference.

Learning Objectives:

2.29 – Understand how the pH scale (0–14) classifies solutions as strongly acidic (0–3), weakly acidic (4–6), neutral (7), weakly alkaline (8–10) and strongly alkaline (11–14).
2.30 – Describe the use of universal indicator to measure the approximate pH value of an aqueous solution.

Navigation: Arrow Keys / Spacebar | 40 minutes

🖨️ Differentiated Pupil Packs

Each pack auto-prints the correct tier of arrival, starter, worksheet and exit ticket.

FAB 4 · Arrival Task

FAB 4 Arrival – Recall from Lesson 1 (Indicators)

Instructions: Answer all four in your book. These are recall from Lesson 1: Indicators. Q4 leads into today.

1. Litmus colour

What colour does litmus turn in an acid?

Red.

2. Indicator name

Name one indicator we used last lesson.

Litmus / phenolphthalein / methyl orange / universal indicator.

3. UI is special

Universal indicator is a mixture of dyes. What does it produce when added to a solution?

A range of colours (depending on whether the solution is acidic, neutral, or alkaline).

4. Lead-in 🔬

Lemon juice and stomach acid are both acids. Which one do you think is more acidic?

Stomach acid is more acidic. Today we will learn how to put a number on it (the pH scale).

1. Litmus in both

What colour does litmus turn in: (a) an acid? (b) an alkali?

(a) Red. (b) Blue.

2. Phenolphthalein

What colour does phenolphthalein turn in an alkaline solution?

Bright pink. (Colourless in acid.)

3. UI advantage

State one advantage of universal indicator over litmus.

UI shows a range of colours — you can estimate the pH, not just whether it's acid or alkali.

4. Lead-in 🔬

Lemon juice and stomach acid are both acids. Are they equally acidic? How could we measure the difference?

No — stomach acid is much stronger. We can use the pH scale (0–14) to measure how acidic or alkaline something is — that's today's lesson.

1. Compare litmus

State one advantage AND one limitation of using litmus paper.

Advantage: quick / simple to use. Limitation: only tells you acid or alkali — gives no information about strength.

2. Methyl orange

What colour does methyl orange turn in: (a) an acid? (b) an alkali?

(a) Red. (b) Yellow.

3. Why choose UI?

Why might a scientist choose universal indicator over litmus when testing an unknown solution?

UI gives an approximate pH via a range of colours, not just acid/alkali — this shows the strength of the acid or alkali too.

4. Lead-in 🔬

Lemon juice and stomach acid both turn UI red. Does that mean they have exactly the same pH? Explain your answer.

No. UI groups several pH values into "red" (pH 0–2). The actual pH could differ. We need a more precise scale (the pH scale, 0–14) — and ideally a pH meter — to tell them apart.

Starter – In Your Book

Starter: Using UI Colours to Read pH

Instructions: All tasks are recorded in your book. You only need what you learned in Lesson 1 about universal indicator. Tasks differ by tier — work on yours.

📗 Colour Detective

Four test tubes have been tested with universal indicator. Look at the colour of each one.

Tube A

Red

Tube B

Yellow

Tube C

Green

Tube D

Purple

💡 Reminder from Lesson 1: UI is red in acid, green when neutral, blue/purple in alkali.

📓 In your book — copy & complete:

Heading: "UI Colour Detective"

Tube A is red. It is an acid / alkali / neutral?
Tube B is yellow. It is an acid / alkali / neutral?
Tube C is green. It is an acid / alkali / neutral?
Tube D is purple. It is an acid / alkali / neutral?
Which tube is the strongest acid?

1. Acid 2. Acid (mildly) 3. Neutral 4. Alkali 5. Tube A — the redder the colour, the stronger the acid.

📙 Rank the Tubes

Five samples were tested with UI. The colour shown is what UI turned. Rank them in order from most acidic (1) to most alkaline (5).

Dark blue

Yellow

Red

Green

Orange

💡 Reminder from Lesson 1: UI runs red → orange → yellow → green → blue → purple as you go from acid to alkali.

📓 In your book:

Heading: "Ranking by UI Colour"

Write out the tubes in rank order: most acidic → most alkaline.
Underline the tube that is neutral.
Write a sentence: "I know Tube R is the most acidic because…"

Rank: R (most acidic, red) → T (orange) → Q (yellow) → S (neutral, green) → P (most alkaline, dark blue).
R is the most acidic because UI turns red in strong acids.

📘 Build the pH Scale

Five household substances were tested with UI. The result for each is shown below.

Substance	UI colour
Lemon juice 🍋	Red
Black coffee ☕	Light yellow
Pure water 💧	Green
Baking soda 🧁	Blue
Drain cleaner ⚠️	Purple

💡 Reminder from Lesson 1: UI gives an approximate pH. Each colour corresponds to a band on the 0–14 pH scale.

📓 In your book:

Heading: "Building the pH Scale"

Draw a horizontal number line from 0 to 14 across two-thirds of your page. Label every number.
For each of the 5 substances, place an arrow with the substance name on the line at where you think it sits, using the UI colour as your clue.
Underneath, write one sentence per substance explaining your placement (e.g. "Lemon juice is red in UI, so it must be a strong acid — I placed it near 2.").

Lemon juice ≈ pH 2 (red, strong acid). Coffee ≈ pH 5 (yellow, weak acid). Pure water = pH 7 (green, neutral). Baking soda ≈ pH 9 (blue, weak alkali). Drain cleaner ≈ pH 13 (purple, strong alkali). Reasoning should reference UI colour → acid/alkali strength.

I Do

I Do: The pH Scale (Spec 2.29)

🎯 Say this in the exam: "pH 0–3 is strongly acidic, pH 4–6 is weakly acidic, pH 7 is neutral, pH 8–10 is weakly alkaline, pH 11–14 is strongly alkaline." That one sentence is worth up to 3 marks.

👆 Tap any pH number above to reveal its classification, UI colour, and a real-world example.

📋 The 5 Classifications (learn these!)

• Strongly acidic: pH 0–3 e.g. battery acid, stomach acid, lemon juice

• Weakly acidic: pH 4–6 e.g. coffee, rainwater, milk

• Neutral: pH 7 e.g. pure water

• Weakly alkaline: pH 8–10 e.g. baking soda, toothpaste, sea water

• Strongly alkaline: pH 11–14 e.g. bleach, oven cleaner

🧠 Memory Hook + Examiner Trap

"Acids Attack — Alkalis Are Above"

• Acid → low numbers (0–6)

• 7 = neutral (the ceasefire 🤝)

• Alkali above 7 (8–14)

⚠️ Examiner trap: Never write "a bit acidic" — always write weakly acidic or strongly acidic. The five exact words win marks.

💡 Real world: Lower pH = more acidic = more dangerous to handle. Higher pH = more alkaline = also dangerous (bleach, oven cleaner can burn skin).

⚠️Strongly AcidicpH 0–3

🍋Weakly AcidicpH 4–6

💧NeutralpH 7

🪥Weakly AlkalinepH 8–10

🧴Strongly AlkalinepH 11–14

Don't write: "pH 7 is acid" or "higher pH means stronger acid"

Write: "pH 7 is neutral" and "lower pH means stronger acid"

The scale runs 0 → 14: low numbers are acidic, 7 is neutral, high numbers are alkaline. Memorise the direction!

The 5 exact phrases worth marks: strongly acidic · weakly acidic · neutral · weakly alkaline · strongly alkaline

Anything vaguer (e.g. "a bit acidic", "kind of basic") will lose marks. The examiner is looking for these 5 specific phrases.

We Do

We Do: pH Zone Sort

As a class: Click a substance card, then click the correct pH zone. Score: 0/8

🧪 Stomach acid
pH 1

☕ Coffee
pH 5

💧 Pure water
pH 7

🪥 Toothpaste
pH 9

🧴 Oven cleaner
pH 13

🍋 Lemon juice
pH 2

🧹 Bleach
pH 12

🥛 Milk
pH 6

Strongly
AcidicpH 0–3

Weakly
AcidicpH 4–6

Neutral
pH 7

Weakly
AlkalinepH 8–10

Strongly
AlkalinepH 11–14

I Do

I Do: Measuring pH with Universal Indicator (Spec 2.30)

Key Idea: Add UI → observe colour → compare to colour chart → read approximate pH → classify.

📝 The 5-Step Method

1. Add a few drops of UI to the solution.

2. Observe the colour that appears.

3. Compare to a pH colour chart.

4. Read the approximate pH.

5. Classify (strongly/weakly acidic, neutral, weakly/strongly alkaline).

💡 Key word: "Approximate" — UI gives an estimate, not an exact reading. Two main reasons: (1) colours between values are hard to read; (2) colour matching is subjective.

🌟 More precise alternative: A pH meter — an electronic probe — gives a precise numerical pH (e.g. 6.83). Used in industry, labs, and research.

Live demo: testing a sample

→

pH 8 = Weakly Alkaline

Watch the UI drop hit the sample. Yellow → blue tells you the solution went from weakly acidic to weakly alkaline.

035791114

pH 0–14

Watch UI cycle through every band — the colour spectrum is continuous, which is why values between bands are tricky to read.

We Do – Diagram Skill

We Do: Read the Tubes 🔬

As a class: Six test tubes have been tested with universal indicator. Click each tube — call out the pH range AND classification before you reveal. Score: 0/6

Red

—

Orange

—

Yellow

—

Green

—

Blue

—

Purple

—

👆 Click any tube to reveal its pH range and classification.

We Do – Find the Errors

Spot the Mistakes: Be the Examiner 🔍

Mark this student's answer. There are 3 mistakes hidden in the paragraph below — but not every underlined phrase is wrong! Read carefully and tap only the parts you think are mistakes.

"The pH scale runs from 0 to 14. A pH of 0 means there is no acid present. Lemon juice, which is strongly acidic at pH 5, is a good example. Pure water is special because it has a pH of 0 and turns UI red. On the pH scale, lower numbers mean more acidic and higher numbers mean more alkaline."

👆 Click a phrase you think is wrong. Find all 3 to win!

✅ Correct version:

"The pH scale runs from 0 to 14. A pH of 0 means the solution is strongly acidic. Lemon juice, which is weakly acidic at pH 5… wait — actually lemon juice is closer to pH 2, so it's strongly acidic! Pure water is special because it has a pH of 7 — neutral — and turns UI green. On the pH scale, lower numbers mean more acidic and higher numbers mean more alkaline."

🧠 Memory tip: Lower pH = stronger acid. Higher pH = stronger alkali. pH 7 = neutral middle. UI gives the colour clue.

We Do

We Do: Beat the Clock! 🏆

As a class: A UI colour will appear. Shout out the pH range and classification! Teacher reveals. Round: 1/7 | Score: 0

Red

pH 0–2 • Strongly Acidic

Independent Work – 8 mins

Independent Work

Answer independently. Use your scaffold sheet if needed. ⭐ = stretch question (one tier above).
❌ Never write: "a bit acidic" or "sort of alkaline" — use weakly/strongly.

Questions completed: 0/0

📗 Grades U–1 • Six question types • Q5 ⭐ stretch → Grade 2 • Q6 🌍 real-world synthesis

MCQ

1. A neutral solution has a pH of:

The middle number on the 0–14 scale.

0714

Short answer

2. What pH range is "weakly acidic"?

It is acidic but not strongly. Look for the middle acid range on the chart.

pH 4–6.

Long answer [3]

3. Describe what the pH scale tells us about a solution. Include in your answer: (a) the range of numbers, (b) what pH 7 means, (c) what low and high numbers mean.

Three things to mention: scale range / what 7 is / what low and high mean.

The pH scale runs from 0 to 14 [1]. pH 7 is neutral [1]. Low numbers (0–6) mean acidic and high numbers (8–14) mean alkaline [1].

Diagram / Experiment

4. A test tube of solution turns universal indicator purple.

(a) Estimate the pH of the solution.
(b) Classify the solution (acid / alkali / neutral, and weak or strong).

Purple is at the alkaline end of the UI chart, the very high numbers.

(a) pH 13–14. (b) Strongly alkaline.

⭐ Challenge GCSE [3]

5. A swimming pool is tested with universal indicator and turns green. The lifeguard says "the pool is acidic and not safe to swim in".
Is the lifeguard correct? Explain your answer fully.

What does green UI tell you about the pH? Is it acidic, alkaline or neutral?

No, the lifeguard is incorrect [1]. Universal indicator is green at pH 7, which is neutral [1]. The pool is therefore safe — neutral water is what swimming pools should be [1].

Synthesis [3]

6. A farmer tests their field's soil with universal indicator. The paper turns YELLOW. Their crops need neutral soil to grow well.
(a) Is the soil acidic, neutral, or alkaline? [1]
(b) To make the soil neutral, should the farmer add a substance that is acidic or alkaline? [1]
(c) Suggest one alkaline substance the farmer could add to the soil. [1]

Yellow on UI = pH 5–6 = weakly acidic. To cancel out an acid you add an alkali (a base). Farmers traditionally add lime.

(a) Acidic (weakly acidic, pH 5–6) [1]. (b) Alkaline — to neutralise an acid you need to add an alkali [1]. (c) Lime (calcium hydroxide / calcium carbonate) — farmers do this regularly. Other accept: ammonia, sodium hydroxide, baking soda [1].

📙 Grades 1–2 • Six question types • Q5 ⭐ stretch → Grade 3 • Q6 🌍 real-world synthesis

MCQ

1. Which solution is the most acidic?

Lower pH number = more acidic.

pH 1pH 5pH 9

Short answer

2. Name the five pH classifications in order from pH 0 to pH 14.

Start at the strongest acid and work upwards through neutral to alkali.

Strongly acidic, weakly acidic, neutral, weakly alkaline, strongly alkaline.

Long answer [4]

3. Explain the difference between a strong acid and a weak acid. Use the pH scale in your answer and give an example of each.

Mention the pH ranges (0–3 vs 4–6) and pick examples like stomach acid vs milk.

Strong acids have a low pH of 0–3 [1]; an example is stomach acid (pH 1) [1]. Weak acids have a higher pH of 4–6 [1]; an example is milk (pH ≈ 6) [1]. Strong acids are more acidic than weak acids.

Diagram / Experiment

4. A student adds a few drops of universal indicator to a sample of vinegar. The UI turns orange.

(a) Estimate the pH. [1]
(b) Classify the vinegar. [1]
(c) State one reason why this method only gives an approximate pH. [1]

Orange sits between red and yellow on the UI chart. What pH range is that?

(a) pH 3–4. (b) Weakly acidic / borderline strongly acidic. (c) UI gives only a colour match — the colour can be hard to read between two values, so the pH is not exact.

⭐ Challenge GCSE [4]

5. A factory checks the pH of waste water before releasing it into a river. The river normally has pH 7. The waste water is tested with UI and turns dark blue.
(a) Estimate the pH of the waste water. [1] (b) Classify the waste water. [1] (c) Explain why releasing the waste water directly into the river could be a problem. [2]

Dark blue UI = high pH = which classification? Then think about what would happen to fish and plants in pH 7 water.

(a) pH 9–10 [1]. (b) Weakly alkaline [1]. (c) The waste water is more alkaline than the river [1]; this could harm fish and plants that are adapted to pH 7 water [1].

Synthesis [4]

6. A swimming pool's water is tested before opening. The UI turns RED. The pool needs to be at pH 7 to be safe to swim in.
(a) Estimate the pH of the pool water. [1]
(b) Classify the water. [1]
(c) The lifeguard wants to make the water neutral. Should they add an acid or an alkali? [1]
(d) The pool is 50,000 litres. Suggest one reason why the lifeguard should add the substance slowly and in small amounts. [1]

Red UI is at the strongly acidic end. To cancel an acid you add an alkali. Adding too much, too fast could overshoot pH 7 and make the water alkaline instead.

(a) pH 1–2 [1]. (b) Strongly acidic [1]. (c) An alkali — to neutralise the acid and bring the pH up towards 7 [1]. (d) Adding all at once could overshoot pH 7 and make the pool alkaline (or unsafe). Adding slowly lets the lifeguard re-test as they go and stop at pH 7. Accept: safety / heat / mixing reasons [1].

📘 Grades 3–4 • Six question types • Q5 ⭐ stretch → Grade 5 • Q6 🌍 real-world synthesis

MCQ

1. A solution with pH 12 is best described as:

pH 11–14 is the highest band of alkali.

Weakly alkalineStrongly alkalineStrongly acidic

Short answer

2. Why is pH 7 described as neutral?

Pure water has pH 7. It is neither acidic nor alkaline.

Because the solution is neither acidic nor alkaline — pure water has pH 7.

Long answer [4]

3. Explain how universal indicator is used to measure the approximate pH of an unknown solution. Include two limitations of using UI.

Think method: add UI → see colour → match to chart. Then think why "approximate".

Add a few drops of UI to the unknown solution [1]. Observe the colour produced [1]. Compare it to a UI colour chart and read off the approximate pH [1]. Limitations: only approximate (colours between values are hard to read), colour matching is subjective, can't distinguish closely related pH values [1 for two limitations].

Diagram / Experiment

4. Three test tubes contain unknown solutions. Universal indicator is added to each.

Tube A

Red

Tube B

Green

Tube C

Violet

(a) Estimate the pH of each tube. [3]
(b) State which tube is the most acidic and which is the most alkaline. [2]
(c) Suggest one reason why a pH meter might give more reliable data than UI. [1]

Match each colour to the UI chart. pH meters give a number, not a colour to interpret.

(a) A: pH 0–2; B: pH 7; C: pH 13–14 [3]. (b) Most acidic: Tube A. Most alkaline: Tube C [2]. (c) A pH meter gives a precise numerical reading; UI is only approximate [1].

⭐ Challenge GCSE [5]

5. Two soap manufacturers test their products with UI. Soap X turns UI blue (pH ≈ 9). Soap Y turns UI dark blue (pH ≈ 10).
(a) State the pH classification of both soaps. [1] (b) Suggest why Soap Y might be more irritating to skin than Soap X. [2] (c) State one limitation of measuring pH with UI for this test, and what the manufacturer should do instead for a precise reading. [2]

Both are weakly alkaline but at different ends of that band. Higher pH = stronger alkali effect on skin. UI vs pH meter for accuracy.

(a) Both are weakly alkaline (pH 8–10) [1]. (b) Soap Y has a higher pH so is more strongly alkaline than X [1]; alkaline solutions damage skin proteins, so Y will irritate the skin more [1]. (c) UI only gives an approximate pH — two soaps both reading "blue" could actually have different pH values [1]. The manufacturer should use a pH meter for precise measurement [1].

Synthesis [6]

6. A chemistry technician is preparing four bottles for a class lesson. They drop UI into each: Bottle A turns red, Bottle B turns purple, Bottle C turns orange, Bottle D turns green.
(a) Identify which bottle is safest for pupils to handle. Justify your choice using pH. [2]
(b) The teacher asks the technician to use Bottles A and B together to make a neutral solution. Explain how this would work, and state one safety precaution they must take. [3]
(c) Justify whether UI was the best choice of indicator here, and what the technician should use if they need precise pH values. [1]

Match each colour to a pH range. Safest = closest to pH 7. Mixing strong acid + strong alkali = neutralisation. UI gives a colour, not a number — when do you need a number?

(a) Bottle D — green = pH 7 = neutral, the safest because it is neither acidic nor alkaline [1]. The other three are at extremes of the pH scale and could be corrosive to skin/eyes [1]. (b) Bottle A is strongly acidic (red, pH 1–2); Bottle B is strongly alkaline (purple, pH 13–14) [1]. Mixing equal amounts of A and B would neutralise each other, producing a solution close to pH 7 [1]. Safety: wear safety goggles and gloves; add slowly to avoid splashing or excess heat [1]. (c) UI was good for a quick classification (acid/alkali/neutral), but for precise pH values the technician should use a pH meter or data logger which gives a numerical reading [1].

Super Six – 3 mins

🌟 Super Six – Exit Ticket

No notes. From memory. Top row = recall from Lesson 1 (Indicators) | Bottom row = today's Lesson 2 (pH Scale).

L1 · Q1

What colour does litmus turn in an acid?

Red.

L1 · Q2

What colour does phenolphthalein turn in an alkali?

Pink.

L1 · Q3

What does universal indicator show that litmus cannot?

A range of colours / the approximate pH.

L2 · Q4

What pH is neutral?

pH 7.

L2 · Q5

What pH range is "strongly acidic"?

pH 0–3.

L2 · Q6

UI turns purple. Acid or alkali?

Strongly alkaline.

Knowledge Organiser – Lesson 2: The pH Scale

Spec: 2.29 (the pH scale and classifications) | 2.30 (using UI to measure approximate pH).

Term	Detail
pH Scale	Runs from 0 (most acidic) to 14 (most alkaline). Measures how acidic or alkaline a solution is. pH 7 is neutral.
Strongly acidic	pH 0–3. Examples: battery acid, stomach acid (pH 1), lemon juice (pH 2). Corrosive.
Weakly acidic	pH 4–6. Examples: tomato juice, black coffee, rainwater, milk (pH ~6).
Neutral	pH 7. Example: pure water. Neither acidic nor alkaline.
Weakly alkaline	pH 8–10. Examples: sea water, baking soda, toothpaste, milk of magnesia.
Strongly alkaline	pH 11–14. Examples: ammonia, bleach (pH 12), oven cleaner (pH 13), drain cleaner. Corrosive — burns skin.
Universal Indicator (UI)	A mixture of dyes that gives a range of colours to show the approximate pH of a solution.
pH meter	Electronic device that gives a precise numerical pH reading (e.g. 6.83). More accurate than UI.

UI Colour Chart:

Red
pH 0–2

Orange
pH 3–4

Yellow
pH 5–6

Green
pH 7

Blue
pH 8–9

Indigo
pH 10–11

Violet
pH 12–14

Method – using UI to find pH (Spec 2.30):

Add a few drops of universal indicator to the solution.
Observe the colour produced.
Compare to a UI colour chart.
Read off the approximate pH.
Classify as strongly/weakly acidic, neutral, or weakly/strongly alkaline.

Limitations of UI:

Only gives an approximate pH (not exact).
Colours between values can be hard to distinguish (e.g. yellow-green could be pH 6 or 7).
Colour matching is subjective — different people may read it differently.
For precision, use a pH meter instead.

Recall from Lesson 1 — Indicators:

Indicator	In acid	In alkali
Litmus	Red	Blue
Phenolphthalein	Colourless	Bright pink
Methyl orange	Red	Yellow

⚠️ Examiner Trap: Don't write "a bit acidic" — always use the five exact terms: strongly acidic, weakly acidic, neutral, weakly alkaline, strongly alkaline.

Arrival Task – Foundation (U–1)

Recall from Lesson 1: Indicators. Answer in your book.

1. What colour does litmus turn in an acid?

2. Name one indicator we used last lesson.

3. Universal indicator is a mixture of dyes. What does it produce when added to a solution?

4. Lead-in: Lemon juice and stomach acid are both acids. Which one do you think is more acidic?

Arrival Task – Middle (1–2)

Recall from Lesson 1: Indicators. Answer in your book.

1. What colour does litmus turn in: (a) an acid? (b) an alkali?

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

3. State one advantage of universal indicator over litmus.

4. Lead-in: Lemon juice and stomach acid are both acids. Are they equally acidic? How could we measure the difference?

Arrival Task – Higher (3–4)

Recall from Lesson 1: Indicators. Answer in your book.

1. State one advantage AND one limitation of using litmus paper.

2. What colour does methyl orange turn in: (a) an acid? (b) an alkali?

3. Why might a scientist choose UI over litmus when testing an unknown solution?

4. Lead-in: Lemon juice and stomach acid both turn UI red. Does that mean they have exactly the same pH? Explain.

Starter – Foundation: UI Colour Detective

Heading in your book: "UI Colour Detective". Answer in full sentences.

Four test tubes have been tested with universal indicator. Look at the colour of each.

Tube	UI Colour	Acid / Alkali / Neutral?
A	Red
B	Yellow
C	Green
D	Purple

5. Which tube is the strongest acid? Explain how you know.

💡 Reminder from Lesson 1: UI is red in acid, green when neutral, blue/purple in alkali.

Starter – Middle: Ranking by UI Colour

Heading in your book: "Ranking by UI Colour".

Five samples were tested with universal indicator. The colour each gave is shown below.

Sample	P	Q	R	S	T
UI colour	Dark blue	Yellow	Red	Green	Orange

1. Rank the tubes from most acidic to most alkaline. Write the letters in order.

2. Underline the tube that is neutral.

3. Complete this sentence: "I know Tube R is the most acidic because…"

💡 Reminder from Lesson 1: UI runs red → orange → yellow → green → blue → purple as you go from acid to alkali.

Starter – Higher: Building the pH Scale

Heading in your book: "Building the pH Scale".

Five household substances were tested with UI. The result for each is shown below.

Substance	UI colour
Lemon juice 🍋	Red
Black coffee ☕	Light yellow
Pure water 💧	Green
Baking soda 🧁	Blue
Drain cleaner ⚠️	Purple

1. Draw a horizontal number line from 0 to 14 across two-thirds of the page below. Label every number.

2. Place an arrow with each substance's name on the line where you think it sits, using the UI colour as your clue.

3. Below your number line, write one sentence per substance explaining your placement.

💡 Reminder from Lesson 1: UI gives an approximate pH. Each colour corresponds to a band on the 0–14 pH scale.

We Do: pH Classification

Sort each substance into the correct classification.

Substance	pH	Classification (strongly acidic / weakly acidic / neutral / weakly alkaline / strongly alkaline)
Stomach acid	1
Coffee	5
Pure water	7
Toothpaste	9
Bleach	12
Lemon juice	2
Milk	6
Oven cleaner	13

Spot the Mistake — Mark this student's answer

The student wrote: "The pH scale runs from 0 to 14. A pH of 0 means there is no acid present. Lemon juice, which is strongly acidic at pH 5, is a good example. Pure water is special because it has a pH of 0 with no ions in it."

Find the THREE mistakes — circle each error and write the correction below.

Mistake 1:

Mistake 2:

Mistake 3:

Scaffolding Sheet

pH Scale Classifications

pH Range	Classification	UI Colour	Examples
0–3	Strongly acidic	Red / Dark orange	Battery acid, stomach acid (pH 1), lemon juice (pH 2)
4–6	Weakly acidic	Orange / Yellow	Tomato juice, coffee (pH 5), rainwater, milk (pH ~6)
7	Neutral	Green	Pure water
8–10	Weakly alkaline	Blue	Sea water, baking soda, toothpaste (pH 9)
11–14	Strongly alkaline	Indigo / Violet	Ammonia, bleach (pH 12), oven cleaner (pH 13)

How to Measure pH with Universal Indicator (Method)

1. Add a few drops of UI to the solution → 2. Observe the colour → 3. Compare to a UI colour chart → 4. Read off the approximate pH → 5. Classify (strongly/weakly acidic, neutral, weakly/strongly alkaline).

Limitations of UI: Only gives an approximate pH. Colours between values can be hard to distinguish (e.g. yellow-green = pH 6 OR pH 7?). Subjective: different people may read it differently. For a precise reading: use a pH meter.

🎯 Examiner trap: Never write "a bit acidic" — always use the exact phrase: strongly acidic, weakly acidic, neutral, weakly alkaline, strongly alkaline.

🧠 Memory aid: "Acids Attack (low pH) — Alkalis Are Above (high pH)." pH is like a temperature gauge: 0 = dangerously acidic, 7 = safe and neutral, 14 = dangerously alkaline.

📝 Word Bank (Foundation)

Use these words to write full sentences: pH, acidic, alkaline, neutral, strongly, weakly, universal indicator, colour, red, orange, yellow, green, blue, purple, 0, 7, 14, range, scale.

✍️ Sentence Starters

• "The pH scale goes from ___ to ___."

• "A neutral solution has a pH of ___."

• "If UI turns ___, the solution is ___ (acid / neutral / alkali)."

• "A solution with pH ___ is classified as ___."

🛟 Quick Anchors for Q5 Challenge

• Green = pH 7 = neutral = safe. A pool that is green on UI is neutral, not acidic.

📝 Sentence Stems (Middle)

• "A pH of ___ is classified as ___ because it falls in the range ___."

• "Universal indicator turns ___ at this pH because…"

• "A strong acid has a low pH of 0–3; a weak acid has a higher pH of 4–6. An example of a strong acid is ___ and a weak acid is ___."

• "One limitation of UI is that…"

🛟 Quick Anchors for Q5 Challenge

• Dark blue UI ≈ pH 9–10 = weakly alkaline.

• Releasing solutions that are not pH 7 into a river can harm fish and plants that are adapted to neutral water.

📝 Exam Phrases (Higher)

• "Universal indicator gives the approximate pH because the colours between values overlap and are subjective to read."

• "A pH meter gives a precise numerical reading (e.g. 6.83) — more reliable than a colour match."

• "Both pH 9 and pH 10 are weakly alkaline, but the higher pH is more strongly alkaline, so it is more likely to irritate or damage skin."

• "Method for using UI: add UI → observe colour → compare to chart → read approximate pH → classify."

🛟 Quick Anchors for Q5 Challenge

• Blue (pH 9) and dark blue (pH 10) both classify as weakly alkaline — but the higher pH = more alkaline = more skin irritation.

• UI cannot tell two similar blues apart → use a pH meter for precise measurement.

Independent Work – Foundation (Grades U–1)

Six question types. Q5 = Challenge GCSE (stretch). Q6 = Real-World Synthesis (problem-solving).

Q1. Multiple Choice

A neutral solution has a pH of:

[ ] 0 [ ] 7 [ ] 14

Q2. Short Answer

What pH range is "weakly acidic"?

Q3. Long Answer [3 marks]

Describe what the pH scale tells us about a solution. Include in your answer:

(a) the range of numbers the scale uses
(b) what pH 7 means
(c) what low and high numbers mean

Q4. Diagram / Experiment

A test tube of solution turns universal indicator purple.

(a) Estimate the pH of the solution.

(b) Classify the solution (acid / alkali / neutral, and weak or strong).

⭐ Q5. Challenge GCSE Question (Grade 2) [3 marks]

A swimming pool is tested with universal indicator and turns green. The lifeguard says "the pool is acidic and not safe to swim in".

Is the lifeguard correct? Explain your answer fully.

🌍 Q6. Real-World Synthesis [3 marks]

A farmer tests their field's soil with universal indicator. The paper turns YELLOW. Their crops need neutral soil to grow well.

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

(b) To make the soil neutral, should the farmer add a substance that is acidic or alkaline? [1]

Independent Work – Middle (Grades 1–2)

Six question types. Q5 = Challenge GCSE (stretch). Q6 = Real-World Synthesis (problem-solving).

Q1. Multiple Choice

Which solution is the most acidic?

[ ] pH 1 [ ] pH 5 [ ] pH 9

Q2. Short Answer

Name the five pH classifications in order from pH 0 to pH 14.

Q3. Long Answer [4 marks]

Explain the difference between a strong acid and a weak acid. Use the pH scale in your answer and give an example of each.

Q4. Diagram / Experiment

A student adds a few drops of universal indicator to a sample of vinegar. The UI turns orange.

(a) Estimate the pH. [1]

(b) Classify the vinegar. [1]

⭐ Q5. Challenge GCSE Question (Grade 3) [4 marks]

A factory checks the pH of waste water before releasing it into a river. The river normally has pH 7. The waste water is tested with UI and turns dark blue.

(a) Estimate the pH of the waste water. [1 mark]

(b) Classify the waste water. [1 mark]

🌍 Q6. Real-World Synthesis [4 marks]

A swimming pool's water is tested before opening. The UI turns RED. The pool needs to be at pH 7 to be safe to swim in.

(a) Estimate the pH of the pool water. [1 mark]

(b) Classify the water. [1 mark]

(d) The pool is 50,000 litres. Suggest one reason why the lifeguard should add the substance slowly and in small amounts. [1 mark]

Independent Work – Higher (Grades 3–4)

Six question types. Q5 = Challenge GCSE (stretch). Q6 = Real-World Synthesis (problem-solving).

Q1. Multiple Choice

A solution with pH 12 is best described as:

[ ] Weakly alkaline [ ] Strongly alkaline [ ] Strongly acidic

Q2. Short Answer

Why is pH 7 described as neutral?

Q3. Long Answer [4 marks]

Explain how universal indicator is used to measure the approximate pH of an unknown solution. Include two limitations of using UI.

Q4. Diagram / Experiment

Three test tubes contain unknown solutions. Universal indicator is added to each.

Tube A: Red

Tube B: Green

Tube C: Violet

(a) Estimate the pH of each tube. [3]

(b) State which tube is the most acidic and which is the most alkaline. [2]

⭐ Q5. Challenge GCSE Question (Grade 5) [5 marks]

Two soap manufacturers test their products with UI. Soap X turns UI blue (pH ≈ 9). Soap Y turns UI dark blue (pH ≈ 10).

(a) State the pH classification of both soaps. [1 mark]

(b) Suggest why Soap Y might be more irritating to skin than Soap X. [2 marks]

(c) State one limitation of measuring pH with UI for this test, and what the manufacturer should do instead for a precise reading. [2 marks]

🌍 Q6. Real-World Synthesis [6 marks]

A chemistry technician is preparing four bottles for a class lesson. They drop UI into each:

Bottle A turns red | Bottle B turns purple | Bottle C turns orange | Bottle D turns green

(a) Identify which bottle is safest for pupils to handle. Justify your choice using pH. [2 marks]

(b) The teacher asks the technician to use Bottles A and B together to make a neutral solution. Explain how this would work, and state one safety precaution they must take. [3 marks]

(c) Justify whether UI was the best choice of indicator here, and what the technician should use if they need precise pH values. [1 mark]

🌟 Super Six – Exit Ticket (Foundation)

Top row: Lesson 1 (Indicators) | Bottom row: Lesson 2 (pH Scale)

L1·Q1 What colour does litmus turn in an acid?	L1·Q2 What colour does phenolphthalein turn in an alkali?	L1·Q3 What does universal indicator show that litmus cannot?
L2·Q4 What pH is neutral?	L2·Q5 What pH range is "strongly acidic"?	L2·Q6 UI turns purple. Acid or alkali?

🌟 Super Six – Exit Ticket (Middle)

Top row: Lesson 1 (Indicators) | Bottom row: Lesson 2 (pH Scale)

L1·Q1 State the colour of methyl orange in (a) acid and (b) alkali.	L1·Q2 Name an indicator used in titrations and say why it's suitable.	L1·Q3 In one sentence, compare litmus and UI.
L2·Q4 Classify a solution of pH 9.	L2·Q5 UI turns blue. Estimate pH and classify.	L2·Q6 Why does UI only give an "approximate" pH?

🌟 Super Six – Exit Ticket (Higher)

Top row: Lesson 1 (Indicators) | Bottom row: Lesson 2 (pH Scale)

L1·Q1 State one advantage AND one limitation of using litmus paper.	L1·Q2 Why is phenolphthalein useful in titrations?	L1·Q3 State two advantages of UI over litmus.
L2·Q4 UI reads "yellow-green". Estimate pH and explain the issue.	L2·Q5 Compare pH 1 and pH 13: classify each + UI colour.	L2·Q6 Suggest a more accurate alternative to UI and explain why.

Lesson Complete!

TA Focus

Cold Call

The pH Scale

🖨️ Differentiated Pupil Packs

FAB 4 Arrival – Recall from Lesson 1 (Indicators)

1. Litmus colour

2. Indicator name

3. UI is special

4. Lead-in 🔬

1. Litmus in both

2. Phenolphthalein

3. UI advantage

4. Lead-in 🔬

1. Compare litmus

2. Methyl orange

3. Why choose UI?

4. Lead-in 🔬

Starter: Using UI Colours to Read pH

📗 Colour Detective

📓 In your book — copy & complete:

📙 Rank the Tubes

📓 In your book:

📘 Build the pH Scale

📓 In your book:

I Do: The pH Scale (Spec 2.29)

📋 The 5 Classifications (learn these!)

🧠 Memory Hook + Examiner Trap

We Do: pH Zone Sort

I Do: Measuring pH with Universal Indicator (Spec 2.30)

📝 The 5-Step Method

Live demo: testing a sample

We Do: Read the Tubes 🔬

Spot the Mistakes: Be the Examiner 🔍

✅ Correct version:

We Do: Beat the Clock! 🏆

Independent Work

🏆 Finished? Add your name:

🌟 Super Six – Exit Ticket

Knowledge Organiser – Lesson 2: The pH Scale

Arrival Task – Foundation (U–1)

Arrival Task – Middle (1–2)

Arrival Task – Higher (3–4)

Starter – Foundation: UI Colour Detective

Starter – Middle: Ranking by UI Colour

Starter – Higher: Building the pH Scale

We Do: pH Classification

Spot the Mistake — Mark this student's answer

Scaffolding Sheet

pH Scale Classifications

How to Measure pH with Universal Indicator (Method)

📝 Word Bank (Foundation)

✍️ Sentence Starters

🛟 Quick Anchors for Q5 Challenge

📝 Sentence Stems (Middle)

🛟 Quick Anchors for Q5 Challenge

📝 Exam Phrases (Higher)

🛟 Quick Anchors for Q5 Challenge

Independent Work – Foundation (Grades U–1)

Q1. Multiple Choice

Q2. Short Answer

Q3. Long Answer [3 marks]

Q4. Diagram / Experiment

⭐ Q5. Challenge GCSE Question (Grade 2) [3 marks]

🌍 Q6. Real-World Synthesis [3 marks]

Independent Work – Middle (Grades 1–2)

Q1. Multiple Choice

Q2. Short Answer

Q3. Long Answer [4 marks]

Q4. Diagram / Experiment

⭐ Q5. Challenge GCSE Question (Grade 3) [4 marks]

🌍 Q6. Real-World Synthesis [4 marks]

Independent Work – Higher (Grades 3–4)

Q1. Multiple Choice

Q2. Short Answer

Q3. Long Answer [4 marks]

Q4. Diagram / Experiment

⭐ Q5. Challenge GCSE Question (Grade 5) [5 marks]

🌍 Q6. Real-World Synthesis [6 marks]

🌟 Super Six – Exit Ticket (Foundation)