Acids, Bases and Neutrals, Grade 7: The Litmus Test and Three Unsafe Claims
Three things a Grade 7 is commonly taught about acids and bases are not just wrong. They are unsafe.
- Identify a substance by tasting it. One of the eight substances on a widely used tasting list is milk of magnesia, which is a laxative medicine
- Acids feel rough on the skin. There is no such property, and the only way to check would be to touch an acid
- Neutral substances are not dangerous. Petrol, paraffin, methanol and mercury are all neutral
This is the page to send home. Everything else in the topic is ordinary chemistry. These three are habits, and habits outlast a term.
Never taste anything to identify it
The rule is simple and it has no exceptions: you taste food, and you never taste a chemical.
Tasting is how you tell sugar from salt in a kitchen. It is not a laboratory method, it never was, and every grade above this one forbids it.
The specific problem is milk of magnesia. It is magnesium hydroxide, sold as an antacid and a laxative, and asking thirty learners to put a drop of it on their tongue is dispensing medicine.
The same page usually carries a safety rule telling learners not to taste anything unless the teacher says it is safe, and then the activity overrides it. A page later, the very same topic often says tasting is not a good way to tell substances apart. The instruction and the warning are a page from each other.
What to do instead: use the indicator. Litmus is already in the topic, two units later, and it answers the same question in three seconds without anybody swallowing anything.
There is one version of the tasting activity that is defensible, and it is worth keeping: comparing ordinary drinks such as tea, rooibos, juice and cooldrink by taste, and then testing the same drinks with litmus to see how badly taste performs.
Food and drink only. Nothing from a cupboard, nothing from a bottle with a label. Say the rule out loud before the cups come out.
Acids do not feel like anything
| The claim | What is actually the case |
|---|---|
| "Acids feel rough on the skin" | There is no characteristic feel for an acid. The claim appears in key-word boxes, in property lists, and sometimes in the revision cloze as an answer to write in |
| "Bases feel slippery" | This one is true. A base reacts with the oils on your skin and turns them into soap, which is what you are feeling |
The danger is not the wrong fact. It is what testing it would require. A learner told acids feel rough has been given a reason to touch one.
And it gets examined. A revision exercise can ask a learner to complete "acids taste sour and feel ____ on the skin", with "rough" sitting in the word bank. A learner who knows better is marked wrong. Worth warning a class before the test rather than after.
Neutral does not mean safe
This is the worst of the three, because it is stated as a flat rule in a chemistry lesson.
Neutral means pH 7. It says nothing at all about whether something is poisonous, flammable or corrosive.
| Neutral substance | What it will do to you |
|---|---|
| Petrol | Highly flammable. The vapour ignites, not the liquid |
| Paraffin | Flammable, and swallowing it damages the lungs |
| Methanol | Blindness or death from a small amount swallowed |
| Mercury | A cumulative nerve poison, and the vapour is the dangerous part |
| Pure water | Genuinely harmless. Which is why the rule looks true until it is not |
The correct version is one sentence: pH tells you about acidity, and nothing else. Read the label for the rest.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 7 |
| Term | 2 |
| Strand | Matter and Materials |
| Topic | Acids, bases and neutrals |
| Status | Class activities. Nothing here is formally assessed |
| Marks | The topic revision is 20. Our worksheet is 40 |
The litmus practical, which is the one that works
You need both colours. That is not a shop talking: one colour alone cannot tell you the answer.
| Paper | In an acid | In a base | In a neutral |
|---|---|---|---|
| Blue litmus | Turns red | Stays blue | Stays blue |
| Red litmus | Stays red | Turns blue | Stays red |
Work out why you need both. If blue litmus stays blue, the substance is either a base or a neutral, and you cannot tell which. Only the red paper separates those two. One paper narrows it down. Two papers give you the answer.
The one instruction that is usually missing: dissolve the solids first.
Dry litmus on a dry aspirin tablet does nothing. Dry litmus on bicarbonate powder does nothing. Indicators work on solutions, so crush the solid, stir a little into water, and test that. Or at minimum wet the paper first.
This is the single commonest reason a class concludes that "the litmus does not work".
Good things to test: lemon juice, vinegar, cooldrink, tap water, distilled water, bicarbonate of soda solution, toothpaste stirred into water, soapy water, milk.
And a free demonstration hiding in the drinks list: tea is itself an indicator. Pour a cup, add a squeeze of lemon, and watch it go pale. That is the acid changing the pigment, and it costs nothing.
Strong and weak do not mean dangerous and safe
This is the mistake that follows a learner all the way to Grade 11, and it starts here.
Strong and weak describe how completely an acid splits up in water. A strong acid splits up completely. A weak acid only partly.
Concentrated and dilute describe how much of it is in the water. That is a different thing entirely.
| Example | Strong or weak | Why it does not match "dangerous" |
|---|---|---|
| Stomach acid | Strong | It is hydrochloric acid, and it is inside you right now. Dilute is what makes it survivable |
| Vinegar | Weak | You put it on chips |
| Concentrated ethanoic acid | Weak, and it will burn you | A weak acid at high concentration is genuinely hazardous |
Two dials, not one. Strength is about splitting up, concentration is about how much. Grade 11 needs both, and mixing them up in Grade 7 is a hard habit to break later.
The same confusion turns up on the base side. Magnesium hydroxide gets called a weak base because it is safe enough to swallow, but it is actually a strong base that barely dissolves. Hardly any of it goes into solution, which is exactly why it is gentle enough for an antacid. That is a solubility fact, not a strength one.
Four more things worth correcting
| What you will read | What is actually the case |
|---|---|
| "There are four tastes: salty, sweet, sour and bitter" | There are five. Umami, the savoury taste, was identified in 1908 and confirmed when its receptor was found in 2000 |
| "Oxygen from the air makes wine acidic" | Bacteria do it. Acetobacter turns the alcohol into ethanoic acid, and it needs oxygen to work. Oxygen is a requirement, not the agent. The same goes for milk going sour, where Lactobacillus is doing the work |
| "Acids are corrosive and bases are caustic" | Not a real distinction. Both are corrosive, both carry the same hazard symbol, and caustic is just an older word for the same behaviour |
| "An acid and a base cancel out to make a neutral substance" | Only if both are strong. Ammonia with hydrochloric acid gives ammonium chloride, which is acidic. Sodium hydroxide with vinegar gives sodium ethanoate, which is basic |
That last one is testable with the litmus already on the bench. Neutralise vinegar with bicarbonate of soda, let it dry, dissolve the solid and test it. It is not neutral, and a class that has just been told it should be will remember finding out.
Caffeine is also usually blamed for coffee's bitterness. It is a weak base, but most of the bitterness comes from compounds formed during roasting, not from the caffeine.
If it does not work
| Problem | Cause | Fix |
|---|---|---|
| Litmus does nothing on a powder or a tablet | Indicators need a solution | Dissolve it in a little water first, or wet the paper |
| Blue stays blue and the class cannot decide | Only one colour was used | Test with the red paper too. That is what it is for |
| Everything looks slightly purple | The neutral paper is being used on its own | Fine for a quick check. Confirm with red and blue |
| Bleach turns the paper white | It is destroying the dye, not showing a pH | Dilute it heavily, or use pH paper |
| Dishwashing liquid gives no change | It is close to neutral. That is the correct result | Nothing is broken. Report it |
| The colour fades while they write | Normal for wet paper | Read it immediately and record it then |
How the 40 marks are made up
| Part | Marks |
|---|---|
| Safety: tasting, touching and neutral substances | 10 |
| Properties of acids, bases and neutrals | 8 |
| The litmus test, and why two colours | 12 |
| Strong against weak, concentrated against dilute | 6 |
| Acids and bases in everyday life | 4 |
Nothing in this topic is formally assessed. The topic revision is 20 marks and it adds up correctly. Our worksheet is 40 and the split above is ours.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks, including the litmus results table and the three unsafe claims to correct
- Marking memorandum, with what to accept and the five answers that look right and score nothing
Related practicals
- The pH Scale and Indicators, Grade 9. Where the numbers come in, two years on
- Reactions of Acids, Grade 9. What acids do to metals and carbonates
- Natural Indicators for Acids and Bases. Red cabbage, turmeric and the rest, done properly
- The Periodic Table, Grade 7. The next topic in the same term
What you need to run it
| Item | Price | What it is for |
|---|---|---|
| Litmus paper, 100 pages, red | R55 | Turns blue in a base. One of the two you need |
| Litmus paper, 100 pages, blue | R55 | Turns red in an acid. The other one |
| Litmus paper, neutral purple | R59,69 | Does both in one strip, for a quick first look |
| pH test paper, 80 strips | R20 | When you want a number rather than a yes or no |
| Beaker, borosilicate, low form | R19 | For dissolving the solids, which is the step that gets skipped |
| Safety goggles | R30 | Habit, even on vinegar |
R110 buys red and blue for a hundred tests each, and that is genuinely the whole topic. It is also the purchase that replaces the tasting activity, which is the reason to make it.
A note on the colours, because it catches people out. Red litmus is for finding bases and blue litmus is for finding acids: the paper is named for the colour it starts as, not the colour it goes. Order both, and order the one you are most likely to run out of first, which is blue, because most of what a class brings from home is acidic.
Everything is in pH and litmus test papers.