The pH Scale and Indicators, Grade 9
pH is a number. The colour is only how you read it.
Grade 9 Natural Sciences asks a class to test ten household products and put them in order, from the most acidic to the most basic. That sounds like the same job as telling an acid from a base. It is not, and the difference is the whole lesson.
You can sort substances into two piles with almost any indicator. You can only rank them with one that gives a number. This page covers the pH scale, how each indicator actually behaves, the full prescribed practical task with its official 20 marks, and the two things that are commonly got wrong.
The pH scale, stated correctly
| pH | What it means |
|---|---|
| Below 7 | Acidic. The lower the number, the more acidic |
| Exactly 7 | Neutral |
| Above 7 | Basic, or alkaline if it dissolves in water. The higher the number, the more basic |
Worth being careful here, because a lot of classroom material is not. You will often read that acids run from 0 to 7, that bases run from 7 to 14, and that neutral substances are 7. That set of statements claims pH 7 three times over and it is the commonest printed error in the topic.
Below 7 is acidic. Exactly 7 is neutral. Above 7 is basic. Nothing overlaps.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 9 |
| Term | 2 |
| Strand | Matter and materials |
| Topic | Topic 8, Acids, bases and pH value |
| Status | Practical task 2 is formally assessed |
| Marks | 20, prescribed. Raw 25 scaled by four fifths |
The indicators, and what each one can actually tell you
This table is the page. Everything else follows from the last column.
| Indicator | In acid | Neutral | In base | Can it rank? |
|---|---|---|---|---|
| Red litmus paper | No change | Red | Blue | No |
| Blue litmus paper | Red | Blue | No change | No |
| Phenolphthalein | Colourless | Colourless | Pink | No |
| Bromothymol blue | Yellow | Green | Blue | Barely |
| Red cabbage water | Red, pink | Violet, purple | Blue, green, yellow | Roughly |
| Universal indicator | Red, orange, yellow | Green | Blue, violet, purple | Yes. It gives a number |
Litmus is two one-way tests, not one two-way test
Look at the first two rows again. Red litmus does nothing in an acid. Blue litmus does nothing in a base.
Each colour only reports one thing. Red litmus tells you "this is a base" and stays silent otherwise. Blue litmus tells you "this is an acid" and stays silent otherwise.
This matters because a very common school activity supplies red litmus only and then asks you to dip it in hydrochloric acid. Nothing happens, and a learner reasonably concludes the test failed or the acid was weak. It did not fail. You were handed half the test.
Buy both colours. They cost the same and each is useless without the other.
Why the practical task insists on two indicators
The assessed task asks for universal indicator plus one other, and then asks you to explain why. It is worth one mark and most learners lose it.
Here is the reasoning.
Step one of the task is sorting: put each substance into acidic, basic or neutral. Any indicator does that. Phenolphthalein is fine.
Step two is ranking: arrange the acids from most acidic to least acidic. Phenolphthalein cannot do this at all. Vinegar at pH 3, coffee at pH 5 and milk at pH 6,7 are all simply "colourless". Three different substances, one result, no order.
A single-colour indicator sorts. A graded indicator ranks. The task needs both jobs done, so it needs both kinds of indicator.
That is the answer to the question, and it is the most useful idea in the topic.
The practical task, step by step
What you need: universal indicator or universal pH paper · one other indicator · distilled water · at least ten household substances · test tubes · a rack · small beakers · safety goggles
Before you start
- Predict. Sort your substances into acidic, basic and neutral on paper, before testing anything
- Write a hypothesis naming which one you think will be most acidic and which most basic
- Write your method, and say what you will do about the solids
- Choose a control. Distilled water is the obvious one
The step everybody skips
Washing powder, bicarbonate of soda and salt are solids. A dry powder has no pH you can measure.
Dissolve one level teaspoon in 20 ml of distilled water, and use the same amount every time. If one group uses a heaped spoon in 10 ml and another uses a level spoon in 50 ml, the two results cannot be compared and the whole ranking falls apart.
Testing
- Sort first with your two-colour indicator. Three groups: acidic, basic, neutral
- Rank the acids with universal indicator, most acidic to least
- Rank the bases, least basic to most
- Confirm the neutrals
- Combine everything into one sequence, most acidic to most basic
Use distilled water, not tap water. Tap water is treated, carries dissolved minerals and has a pH of its own, so it is useless as a control and it shifts any solid you dissolve in it.
What the readings should roughly be
For the teacher, so you know when a result is wrong rather than merely surprising. Brands vary and so does dilution, so mark against the learner's own readings.
| Substance | Approximate pH |
|---|---|
| Lemon juice | 2 to 2,5 |
| Fizzy cola | 2,5 |
| Vinegar | 2,5 to 3 |
| Fruit juice, orange | 3,5 to 4 |
| Coffee | 5 |
| Tea | 5 to 6 |
| Milk, fresh | 6,5 to 6,8 |
| Distilled water | 7 |
| Salt water | 7 |
| Bicarbonate of soda solution | 8 to 9 |
| Soap | 9 to 10 |
| Washing powder | 10 to 11 |
The spread runs from about pH 2 to about pH 11. That is wide enough that a graded indicator earns its keep, and it is why the task can be marked on the order rather than on getting exact numbers.
Strong is not the same as concentrated
A learner who writes "vinegar is a strong acid because the paper went bright red" has made the classic mistake.
| Strong or weak | What kind of acid it is. How completely it comes apart in water. This does not change |
|---|---|
| Concentrated or dilute | How much of it is in the water. You can change this with a jug |
Vinegar is a weak acid in a fairly concentrated solution. Hydrochloric acid is a strong acid, and a very dilute solution of it can read a higher pH than neat vinegar. Strong describes the acid. Concentrated describes the bottle.
If it does not work
| Problem | Cause | Fix |
|---|---|---|
| A solid gives no reading at all | It is dry. pH is a property of a solution | Dissolve a level teaspoon in 20 ml of distilled water, the same every time |
| Everything reads 7 | Tap water was used instead of distilled | Distilled water only |
| The colour sits between two blocks on the chart | Normal | Record a range, "5 to 6". Do not invent a decimal |
| Milk reads differently from yesterday | It soured. Lactic acid formed | That is a real result, and a good one |
| Two substances tie | They genuinely have the same pH | A tie is a result. Say so rather than forcing an order |
| Red litmus does nothing in the acid | Correct behaviour | Use blue litmus for acids |
| Two groups disagree about the same product | Different brands, or different dilutions | Compare and discuss. This is the evaluation question answering itself |
| The strip stains fingers | Handled at the wet end | Tweezers, or hold the dry end |
Safety
- Goggles on. Not because vinegar is dangerous, but because the habit is built in Grade 9 and used in Grade 12
- Nothing from under the sink. Say this before learners are asked to bring things from home. Drain cleaner, oven cleaner, pool acid and bleach are strong enough to burn
- Nobody tastes anything, however familiar it looks
- Wash hands after handling washing powder and soap. Both are irritants as powders
- Dilute laboratory acids at this level are irritants rather than corrosives. They still go in eyes
- If the class also does the laboratory indicator activity, the sodium hydroxide is the hazard, not the acid
Disposal: everything on the household list goes down the sink with plenty of water.
One substitution worth making. A common activity in this topic burns a peanut to show that food contains energy, with no allergy warning printed anywhere. Burning a peanut aerosolises the protein, and a teacher cannot check a class list on the day. Use a cashew, a plain corn chip or a piece of dry pasta. They all burn and they all make the point.
How the marks work
This is one of the few practicals with an official mark allocation, so we have not invented one.
| Section | Marks |
|---|---|
| Write a hypothesis | 5 |
| Plan your investigation | 5 |
| Conduct your investigation | 8 |
| Analyse your data and draw conclusions | 2 |
| Evaluate your investigation | 5 |
| Raw total | 25 |
| Scaled, four fifths | 20 |
Our worksheet reproduces the prescribed task exactly and adds a separate 20-mark extension section, clearly labelled, so a teacher can hand in the official 20 and use the rest as they like.
The mark most often dropped is the one-mark question asking why two indicators were needed. The answer is in the section above.
If you have time
Test rainwater against tap water. Rainwater is naturally about pH 5,6, because carbon dioxide from the air dissolves in it to make carbonic acid. That is acid rain explained with one strip of paper.
Leave the milk out and test it again tomorrow. The pH falls as lactic acid forms. It is the cleanest demonstration in the topic that pH is a measurement of a substance right now, not a permanent label.
Test cola flat against cola fizzy. Let one glass stand overnight. The flat one reads slightly less acidic, because some of the carbonic acid has left as carbon dioxide.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 45 marks. Part A is the prescribed 20-mark practical task reproduced exactly, with the prediction table, the ranking tables and the full sequence. Part B is our own 20-mark extension
- Marking memorandum, with expected pH values, what to accept when a prediction was wrong, and the four answers that look right and score nothing
Related practicals
- Reactions of acids with bases, carbonates and metals, Grade 9. The next topic, where the indicators on this page are used to follow a neutralisation
- Make your own red cabbage indicator. The other indicator this task asks for, made from a supermarket cabbage
- Types of reaction in aqueous solution, Grade 10. Where acids and bases go next year
- Solubility and dissolving, Grade 10. Useful for the solids in this task
- Rate of reaction. Acids again, measured a different way
What you need to run it
We are putting together a pH and Indicators Kit for Grade 9 with the pH paper, both colours of litmus, an indicator solution, test tubes, a rack, beakers and goggles in one box, plus a printed teacher guide and the marking memo. Coming shortly.
The learner brings the ten household substances, and that is the point of the task, so there is nothing to ship there. Everything else is standard.
Buy both colours of litmus paper. Red and blue are the same price and each is useless without the other, for the reason set out above.
For the ranking step you need a graded indicator, and universal pH paper at R20 for 80 strips is the cheapest way to get a number rather than a colour. A class of 30 testing ten substances will use more than one pack, so buy two.
If you want a reading to one decimal place rather than a colour block, a digital pH tester does it, though it is not needed for this task. The rest is ordinary laboratory glassware.