Acids and alkalis · Process
Neutralisation
Mix the two most dangerous bottles in the lab in the right amounts and you can pour the result down the sink. Where did the danger go?
Start here
Hydrochloric acid in one beaker. Sodium hydroxide in the other. Both would burn you. Mixed carefully, the result is salty water.
The mixture warms up while it happens. Test it afterwards and universal indicator comes out green — pH 7. Boil the water off and there are white crystals in the bottom of the dish. Taste them, if it were a kitchen and not a laboratory, and they would taste of table salt.
What happened to the acid and the alkali?
They reacted, and made two new substances. Nothing vanished — the atoms are all still in the beaker, rearranged into water and a compound called a salt. The white crystals are the proof: they were not in either bottle at the start. This is a chemical reaction like any other, and it has a name: neutralisation.
Neutralisation is the reaction between an acid and a base. With an alkali the products are always the same two things, whichever acid and whichever alkali you started with.
The rule
An acid and an alkali always make the same two things.
acid + alkali
salt + water
the salt takes its metal from the alkali and its family name from the acid
“Salt” here does not only mean the stuff on chips. It means the whole family of compounds made when the hydrogen in an acid is swapped for a metal — sodium chloride is one of thousands. Which salt you get depends on which acid and which alkali you started with, and that is a lesson of its own.
Your turn · add the alkali drop by drop
25 cm³ of hydrochloric acid. Universal indicator already added.
Alkali goes in one drop at a time. Watch the number, not the colour — the colour is only the number in disguise.
pH 1pH 2pH 3pH 7pH 11pH 12pH 13
0 drops of alkali added
Still acidic
Still acidic
Still acidic
Neutral — exactly
Now alkaline
Now alkaline
Red, and strongly acidic. Every drop of alkali that goes in will be destroyed on contact by the acid already there.
The reading has barely moved. There is far more acid in the beaker than alkali added so far, so each drop is used up as fast as it arrives.
Orange, pH 3, and almost all the acid is gone. The next drop has almost nothing left to react with.
Green. Every particle of acid has met a particle of alkali and both are gone, replaced by salt and water. One drop either side of this and you would miss it.
Past it. There is no acid left to consume the alkali, so alkali is now simply accumulating in the beaker.
Deep purple. From here, adding more barely moves the reading either — the same flattening as at the start, in the other direction.
The reading, drop by drop
0 drops · · · 20 drops
Look at the shape of what you just did.
For the first nine drops almost nothing moves. Then one single drop takes the reading from 3 to 11 and the colour goes straight through green without stopping. After that it flattens off again.
That cliff is the moment the last of the acid is used up. Before it, every drop of alkali is consumed instantly by acid that is still there. After it, there is no acid left to consume anything, so the alkali just piles up. Neutral is not a region you drift into. It is a single point you cross.
Key fact
acid + alkali makes salt + water. Nothing is destroyed: the atoms are rearranged into two new substances, and the pH ends at 7 only if the amounts match exactly.
Four problems · one reaction
What would you add, and why that?
0 of 4 decided
Every one of these is neutralisation doing a job outside a laboratory. Commit before you read.
A field is too acidic for wheat. What does the farmer spread on it?
Lime — calcium hydroxide or calcium carbonate, an alkali. It neutralises the acid in the soil and brings the pH up towards 7, where most crops take up nutrients best. Vinegar would make it worse; fertiliser feeds the plant but does nothing about the pH.
Someone has indigestion: stomach acid where it should not be. What is in the tablet they take?
A base — usually magnesium hydroxide or calcium carbonate. It neutralises the excess acid on contact. It has to be a mild one: a strong alkali would do more damage than the acid it was sent to deal with.
Bacteria in your mouth make acid that dissolves tooth enamel below about pH 5.5. What does toothpaste do about it?
It is mildly alkaline, so it neutralises the acid the bacteria produce and lifts the mouth back above the level at which enamel dissolves. The scrubbing matters too — it removes the plaque the bacteria live in — but the chemistry is neutralisation.
A factory must dispose of a tank of dilute sulfuric acid. Pouring it into the river would kill the fish. What is done first?
Neutralise it, then measure the pH before anything is released. Dilution lowers the concentration but the water going into the river is still acidic, and a river has its own life that depends on staying near neutral. Boiling it concentrates the acid rather than removing it.
Think again
“Neutralising an acid destroys it; only water is left.”
The beaker does end up looking like water. Commit before you read on.
Boil the water off and the answer is sitting in the dish. A neutralised beaker contains a salt dissolved in water, and that salt is made of atoms that came out of the acid and the alkali. Weigh everything before and after: the mass is unchanged, because this is conservation of mass and no reaction escapes it.
What has gone is the behaviour. The thing that made the acid corrosive has been used up in making water, and so has the thing that made the alkali corrosive. Neutralisation does not delete the atoms. It puts them somewhere harmless.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
Complete the word equation: acid + alkali makes what?
Rung 2 · The one that catches people
A beaker of acid has alkali added drop by drop. After nine drops the pH is 3; after ten it is 11. What happened at the tenth drop?
Rung 3 · Explain
A student neutralises hydrochloric acid with sodium hydroxide and says “the acid has disappeared”. Explain what has actually happened to the atoms, and how you could prove it.
Rung 4 · Take it somewhere new
A lake has become acidic because of pollution and the fish are dying. Explain how you would decide how much lime to add, and why adding a very large amount all at once would be a bad idea.
Key note
Neutralisation is the reaction of an acid with a base: acid + alkali makes salt + water. The pH moves towards 7 as the acid is used up, and the change is sudden rather than gradual. Nothing is destroyed — the salt formed is still dissolved in the beaker, and boiling the water off leaves it behind as crystals.
Going further
The cliff you plotted is why a chemist measuring an unknown acid uses a single drop of a sharp indicator rather than universal indicator. Run alkali in from a burette, watch for the one drop that flips the colour, and read the volume off the scale: that volume tells you exactly how much acid was in the flask. The technique is called titration and it is how the strength of everything from vinegar to blood plasma is checked.
Neutralisation also runs on an industrial scale where nobody would call it chemistry. Power station chimneys are washed with a spray of calcium hydroxide to catch the sulfur dioxide that would otherwise fall as acid rain; the salt produced is calcium sulfate, which is plasterboard. A reaction that started as two beakers on a bench ends up as the walls of a house.
Before this lesson
Next in this unit
At GCSE this becomes
- Titration with a burette, the ionic equation H+ + OH− makes H2O, and calculating concentrations from titration results.
Where to next
Ask Mr Badmus AI
Still not sure why the pH jumps instead of climbing?
The mixture warms as it reacts, and a concentrated acid mixed with a concentrated alkali warms enough to boil and spit. School titrations use dilute solutions of both, a drop at a time, with eye protection on — which is also the only way to find the one drop that matters. Teacher demonstration. Eye protection on, and watch from your seat. The acid and alkali here are dilute, but both sting in the eyes.
Lesson content © MrBadmusAI.