Acids, Alkalis and the pH Scale

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.28 — Using litmus, phenolphthalein and methyl orange to distinguish acids from alkalis
• 2.29 — Using the pH scale to classify solutions from strongly acidic to strongly alkaline
• 2.30 — Using universal indicator to measure approximate pH

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

An INDICATOR is a substance that changes colour depending on whether it is in an acid or an alkali. The three the specification names each give a clear answer to one question — acid or alkali — and nothing more than that.

LITMUS is red in acid and blue in alkali, and it is the one most people remember. PHENOLPHTHALEIN is colourless in acid and bright pink in alkali, which makes it excellent for spotting the moment an alkali appears. METHYL ORANGE is red in acid and yellow in alkali. All three sit somewhere in between in a neutral solution — litmus purple, phenolphthalein colourless, methyl orange orange.

The limitation of all three is worth naming immediately, because it is what the pH scale exists to fix. An indicator tells you WHICH side of neutral you are on and gives no idea HOW FAR. Litmus is exactly as red in a very dilute acid as in a concentrated one.

Think of it like a light switch against a dimmer

A switch tells you the light is on or off, which is genuinely useful and is all you need if that is the question. A dimmer tells you how bright, which is a different kind of information and needs a different kind of instrument. Litmus, phenolphthalein and methyl orange are switches — two states, one boundary, no gradations. The pH scale is the dimmer. And the reason the switches are not simply obsolete is that a switch gives a much sharper answer at the boundary itself, which is exactly what a titration needs.

The pH SCALE runs from 0 to 14 and measures how acidic or alkaline a solution is. Below 7 is ACIDIC, and the lower the number the more strongly acidic. Exactly 7 is NEUTRAL. Above 7 is ALKALINE, and the higher the number the more strongly alkaline.

Statement 2.29 — reading the scale

1. pH 0–3 — STRONGLY ACIDIC. Concentrated hydrochloric, sulfuric and nitric acids sit here.
2. pH 4–6 — WEAKLY ACIDIC. Vinegar, lemon juice, rainwater.
3. pH 7 — NEUTRAL. Pure water, and sodium chloride solution.
4. pH 8–10 — WEAKLY ALKALINE. Toothpaste, baking soda solution.
5. pH 11–14 — STRONGLY ALKALINE. Oven cleaner, concentrated sodium hydroxide.
6. The words matter in an answer: a solution at pH 5 is weakly acidic, and calling it 'a bit neutral' or 'nearly alkaline' earns nothing.

UNIVERSAL INDICATOR, statement 2.30, is how pH is measured in a school laboratory. It is a MIXTURE of several indicators, blended so that the combination changes colour gradually across the whole range rather than flipping at one point. Add a few drops to the solution and compare the colour against a printed chart.

The universal indicator colours, in order

1. RED — strongly acidic, around pH 1–2.
2. ORANGE then YELLOW — weakly acidic, around pH 3–6.
3. GREEN — neutral, pH 7.
4. BLUE — weakly alkaline, around pH 8–11.
5. PURPLE — strongly alkaline, around pH 12–14.
6. The order red-orange-yellow-green-blue-purple is the visible spectrum, which makes it easy to remember and hard to get backwards.

Universal indicator gives an APPROXIMATE pH and that word is in the specification deliberately. Matching a colour to a chart by eye is a judgement, and colours between the printed shades have to be estimated, so a reading of 'about 4' is the most that can honestly be claimed. A pH METER gives a numerical reading to a decimal place and is used where precision matters.

Choosing the right indicator for the job

(a) You need to know whether a colourless solution is acidic or alkaline. (b) You need to know roughly how acidic it is. (c) You need to detect the exact moment an alkali has been completely neutralised in a titration. Which would you use in each case?

1. (a) Any of litmus, phenolphthalein or methyl orange will do, since the question is only which side of neutral the solution is on.
2. (b) UNIVERSAL INDICATOR, because it is the only one of these that gives a value rather than a side.
3. (c) A SINGLE indicator such as phenolphthalein or methyl orange — NOT universal indicator.
4. The reason for (c) is that universal indicator changes gradually, so there is no sharp moment to see. A single indicator flips from one colour to another over a very small addition, which is exactly what marks an end point.
5. So the property that makes universal indicator good for (b) is precisely what disqualifies it for (c).

Answer: Any single indicator for (a), universal indicator for (b), and a single indicator again for (c) — for opposite reasons.

That last point is worth holding on to, because it is the one the titration lesson will need. A gradual colour change is informative when you want a value and useless when you want a moment. Universal indicator and single indicators are not better and worse; they answer different questions.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Statement 2.28 — the three indicators

The vocabulary of this page

Indicator

A substance that changes colour depending on whether it is in an acid or an alkali.

pH scale

A scale from 0 to 14 measuring how acidic or alkaline a solution is. Below 7 acidic, 7 neutral, above 7 alkaline.

Universal indicator

A mixture of indicators that changes colour gradually across the whole pH range, giving an approximate pH.

Neutral

At pH exactly 7 — neither acidic nor alkaline. Pure water is neutral.

Statement 2.29 — classifying by pH

• pH 0–3 — STRONGLY ACIDIC
• pH 4–6 — WEAKLY ACIDIC
• pH 7 — NEUTRAL
• pH 8–10 — WEAKLY ALKALINE
• pH 11–14 — STRONGLY ALKALINE
• Use these words in an answer; 'a bit neutral' or 'nearly alkaline' is not a classification

Statement 2.30 — measuring approximate pH

1. Add a few drops of universal indicator to a small sample of the solution.
2. Compare the colour produced against the printed pH colour chart.
3. Read off the approximate pH — red about 1–2, orange to yellow 3–6, green 7, blue 8–11, purple 12–14.
4. Report it as APPROXIMATE, because matching a colour by eye is a judgement.
5. Use a pH meter instead if a precise numerical value is needed.

A single indicator against universal indicator

A. A SINGLE indicator flips sharply between two colours at one point. It answers 'which side of neutral' and marks an end point precisely.

B. UNIVERSAL indicator changes gradually across the whole range. It answers 'how far from neutral' but gives no sharp moment.

Model answer [2 marks]

A colourless solution turns phenolphthalein pink. State whether it is acidic or alkaline, and give the colour litmus would turn in it. [2]

The solution is alkaline, because phenolphthalein is colourless in acid and pink in alkali. Litmus would turn blue in it, since litmus is blue in alkaline solutions.

Model answer [4 marks]

Explain why universal indicator is used to find the pH of a solution but is not used to detect the end point of a titration. [4]

Universal indicator is a mixture of indicators which changes colour gradually across the whole pH range, so the colour produced can be matched against a chart to give an approximate pH. This gradual change is exactly what makes it unsuitable for a titration, because at the end point there is no single sharp change to observe and it would be impossible to judge the exact moment neutralisation was complete. A single indicator such as phenolphthalein changes sharply from one colour to another over a very small addition of solution, which gives a clear and precise end point.

Not this: A solution that turns litmus deep red must be more acidic than one that turns it a paler red.

This: Litmus gives no information about strength at all. It is red in any acid, dilute or concentrated, and the depth of colour depends on how much indicator was added. Only the pH scale measures how acidic something is.

Mark-losing trap. Phenolphthalein is COLOURLESS in acid and PINK in alkali — not red.

Mark-losing trap. Universal indicator gives an APPROXIMATE pH. A pH meter is what gives a precise number.

Mark-losing trap. Use the classification words: strongly acidic, weakly acidic, neutral, weakly alkaline, strongly alkaline.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — What colour is litmus in an alkaline solution?
2. Grade 7 · Classify [2 marks] — A solution has a pH of 12. Classify it using the terms on the pH scale.
3. Grade 8 · Explain [3 marks] — A student says a solution that turns litmus a deep red must be a stronger acid than one that turns it a paler red. Explain what is wrong.
4. Grade 9 · Explain [6 marks] — Select every statement that belongs in a full-mark explanation of why universal indicator is suitable for measuring the pH of a solution but unsuitable for finding the end point of a titration.
5. 9+ · Analyse [6 marks] — A student tests two acids with universal indicator. Acid A gives pH 1 and acid B gives pH 5. They conclude that acid A must be more concentrated than acid B. Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one who invented the indicator, and the one who showed the accepted definition of an acid was wrong. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Robert Boyle — the one who invented the indicator

Indicators are Boyle's invention. He noticed that syrup of violets turned red with acids and green with alkalis, and rather than treating it as a curiosity he saw that it made ACIDITY into something testable rather than a matter of taste. He went on to soak paper in plant extracts, producing the first test papers, and to define acids and alkalis by what they DO — dissolve certain substances, react with each other, change these colours — rather than by any theory of what they are. That behavioural definition is essentially the one this page uses.

• “How did you notice that a flower extract could test for acids?”
• “Why is testing better than tasting?”
• “What did you think an acid actually was?”
• “How did you make the first test papers?”
• “Why define a substance by what it does rather than what it is?”

Humphry Davy — the one who showed the accepted definition of an acid was wrong

This page classifies acids without saying what makes something an acid, and the history of that question is instructive. Lavoisier had declared that every acid must contain oxygen — he named the element from the Greek for acid-former on exactly that belief. Davy showed that hydrochloric acid contains no oxygen whatever, only hydrogen and chlorine, which destroyed the definition and left the field without one for years. What acids actually had in common turned out to be hydrogen, which is the subject of the next page.

• “What did Lavoisier think every acid contained?”
• “How did you show hydrochloric acid had no oxygen in it?”
• “What did chemists do without a definition of an acid?”
• “What do all acids actually have in common?”
• “How did people react to you contradicting Lavoisier?”

Then defend it

On Incandio a lesson is not finished when the questions come out right. You teach the idea back to Ember, an AI apprentice who asks the awkward question, and then you argue it against Robert Boyle in a structured debate marked against descriptors you can read before you enter. Learn it, teach it, then defend it — all three happen on this page once the app loads.

Carry on through the course

• Previous lesson: Uses of Metals, and Alloys
• Next lesson: Hydrogen Ions and Neutralisation
• Group 1 — the Alkali Metals — Metals soft enough to cut with a knife, which set fire to water — and get worse as you go down.
• Group 7 — the Halogens — A green gas, a brown liquid and a grey solid — and a trend that runs the opposite way to Group 1.
• The Gases in the Air — Four gases, one of which is doing almost all the chemistry — and how to measure it without ever touching it.
• Combustion and Carbonates — Three things burning in oxygen, one carbonate breaking apart, and the 0.04% gas that changes the climate.
• The Reactivity Series — Putting eleven metals in order by watching what each one will and will not do.
• All of Chemistry · Incandio Science