Acids and alkalis · Classify
Acids and alkalis
Two clear, colourless liquids. One will strip the skin off your hand and the other is safe to drink — and nothing you can see tells you which is which.
Start here
Two beakers, both clear, both colourless. One is hydrochloric acid. One is sodium hydroxide.
The labels have come off. Both are dangerous, so tasting is out and so is touching. They are the same temperature, they pour the same, they smell of nothing. Everything your eyes can do has already been done.
How do you find out which is which?
Add something that changes colour. Acid and alkali are not things you can see, weigh or smell — they are things a substance does, and the only way to read them is to make them act on something. A dye that turns one colour in acid and another in alkali answers the question in a second. That dye is called an indicator, and it is the whole of the next lesson.
An acid is a substance that releases hydrogen into solution when it dissolves in water. That is what makes vinegar sharp, what makes your stomach able to break down meat, and what makes battery acid eat through cloth. Acids have a pH below 7.
An alkali is the opposite kind of substance: it cancels acids out. Alkalis are the ones that dissolve; the wider family they belong to is called bases, and a base that dissolves in water is an alkali. Alkalis have a pH above 7.
Everything else is neutral — pH exactly 7, neither one nor the other. Pure water is the obvious example; most of what you drink sits just on the acid side of it, which is the next lesson's scale.
Your turn · eight bottles on the bench
Acid, alkali or neutral? Commit on all eight.
0 of 8 decided
Guessing is allowed — the ones you get wrong are the ones worth reading.
Lemon juice
kitchen
Lemon juice is an acid.
Citric acid, pH about 2. Sharp enough to taste and weak enough to drink — which is the first sign that acid and danger are separate questions.
Oven cleaner
cleaning cupboard
Oven cleaner is an alkali.
Sodium hydroxide, pH about 13. It dissolves baked-on fat by taking it apart chemically, and it will do the same to skin. The most dangerous bottle on this bench is not an acid.
Pure water
wash bottle
Pure water is neutral.
pH exactly 7 — the definition of neutral. Tap water is usually a shade either side of it depending on what the rock it travelled through was made of.
Vinegar
kitchen
Vinegar is an acid.
Ethanoic acid, pH about 3. The same acid that will strip limescale off a kettle goes on chips.
Indigestion tablet in water
chemist
An indigestion tablet in water is an alkali.
A mild alkali, pH about 9 or 10 — sold precisely because it cancels out stomach acid. Mild enough to swallow, which is the whole point of it.
Battery acid
garage
Battery acid is an acid.
Sulfuric acid, pH close to 0. This is the one that matches the mental picture: it burns cloth, skin and metal on contact.
Sodium chloride solution
salt in water
Sodium chloride solution is neutral.
pH 7. It is made from an acid and an alkali reacting together, and what is left over sits exactly in the middle — which is the next lesson but one.
Garden lime
shed
Garden lime is an alkali.
Calcium hydroxide, pH about 12. It is spread on soil that has turned too acidic for crops — neutralisation used on a field-sized scale.
Look at where they turn up.
The acids are in the food cupboard and in your own stomach. The alkalis are in the cleaning cupboard, and one of them is in the soil bag. Neither list is a list of poisons — you eat acids every day, and the alkali in indigestion tablets is sold in a chemist. Dangerous and acidic are two different questions, and the bench proves it: the most dangerous thing here is an alkali.
Key fact
Acids have a pH below 7 and alkalis have a pH above 7. An alkali is a base that dissolves in water, and it cancels an acid out. Neutral is exactly 7.
Three judgements
How dangerous, and why
0 of 3 decided
Commit to each before you read what happens. Two of the three catch almost everybody.
A bottle of concentrated acid is diluted with a lot of water. Is it still an acid?
Yes. Diluting adds water, so there is less acid in each cm³ and it does far less damage — but every drop of it is still acid and the pH is still below 7. Weaker in effect, unchanged in kind. That difference between how much acid and how fierce the acid is has caught out generations of students.
Someone spills sodium hydroxide on their hand and feels nothing much. Should they wash it off?
Immediately, under running water, for a long time. Alkalis do not sting on contact the way acids do — they feel soapy, because they are turning the fat in your skin into soap. By the time it hurts, the damage is done. The lack of pain is the hazard.
Is the acid in your stomach stronger than the acid in vinegar?
Considerably. Stomach acid is hydrochloric acid at roughly pH 2; vinegar is around pH 3, and the scale is not evenly spaced — each step is a factor of ten. Your own stomach is more corrosive than a bottle of malt vinegar, and it is kept in by a lining that replaces itself every few days.
Think again
“Acids are the dangerous ones. Alkalis are what you use to make things safe.”
Every hazard film you have seen involves an acid. Commit before you read on.
Strong alkalis are, if anything, worse. Sodium hydroxide — the alkali in oven cleaner and drain unblocker — dissolves fat and skin, and eye damage from it can be permanent. It does not sting straight away, which is exactly why people leave it on their hands. A strong acid announces itself; a strong alkali feels soapy.
Meanwhile the acid in an orange is safe enough to drink a litre of. Being an acid or an alkali says which side of 7 it sits on. It does not say how hard it hits. That is a separate question, and it has its own scale — coming next lesson.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What is an alkali?
Rung 2 · The one that catches people
A student says: “Orange juice is an acid, so it must be dangerous.” What is wrong with the reasoning?
Rung 3 · Explain
Two unlabelled colourless liquids are on the bench: one is an acid and one is an alkali. Explain how you would find out which is which, and why looking at them cannot answer it.
Rung 4 · Take it somewhere new
A gardener is told their soil is too acidic for the crop they want to grow. Explain what they should add and why it works — and say what could go wrong if they add far too much.
Key note
Acids have a pH below 7, alkalis above 7, and neutral substances exactly 7. An alkali is a base that dissolves in water, and the two cancel each other out. Whether something is acidic tells you nothing about whether it is safe: lemon juice is an acid you can drink and oven cleaner is an alkali that will burn you.
Going further
Not every base is an alkali. Copper oxide, magnesium oxide and calcium carbonate all neutralise acids perfectly well, but they barely dissolve in water — so they are bases and not alkalis. The distinction matters in the lab: to neutralise an acid with a base that will not dissolve, you warm the acid and stir the solid in until no more will react, then filter off what is left. That is a method you will meet again when you make a salt on purpose.
Your stomach runs at about pH 2, which is strong enough to dissolve iron filings and does dissolve the meat you eat. The lining survives because it is replaced constantly and coated in mucus; where that coating fails, the acid attacks the stomach itself and the result is an ulcer. For a long time ulcers were treated as a stress problem. In 1984 an Australian doctor drank a flask of bacteria to prove they were an infection, gave himself gastritis, and eventually won a Nobel Prize for it.
Before this lesson
At GCSE this becomes
- Acids as sources of hydrogen ions (H+) in solution, alkalis as sources of hydroxide ions (OH−), and the difference between a strong acid and a concentrated one.
Where to next
- Next: The pH scale and indicators
- Previous: Which reaction is this?
Types of reaction
Ask Mr Badmus AI
Still unsure why a base is not always an alkali?
Oven cleaner and drain unblocker are strong alkalis and battery acid is a strong acid. None of them is a thing to test at home: a strong alkali does not hurt on contact, so there is no signal telling you to wash it off, and by the time there is one the damage is done. Eye protection on before you start. Even dilute acid and alkali sting badly if they get in your eyes. If anything splashes on your skin, wash it off with plenty of cold water and tell your teacher straight away.
Lesson content © MrBadmusAI.