Acids and alkalis · Process
Acids and carbonates
A cathedral has stood for eight hundred years and the faces on its statues have gone. Nobody touched them — the rain did it, one reaction at a time.
Start here
Drop a marble chip into acid and it fizzes like a tablet in water. Marble is rock.
The chip shrinks and streams bubbles from every surface. The gas is not hydrogen — a lit splint held to it goes out instead of squeaking. Bubble the gas through clear limewater and the limewater turns milky white.
What is the gas?
Carbon dioxide. Turning limewater milky is the test for it, and nothing else you will meet does that. Marble is calcium carbonate — the carbon and the oxygen were locked into the rock, and the acid has taken them out and let them go as gas. Chalk, limestone and eggshell are the same compound and all do the same thing.
A carbonate is any compound containing a carbonate group — calcium carbonate, sodium carbonate, copper carbonate. Every one of them reacts with acid the same way, and there are three products, not two.
The rule
An acid and a carbonate always make the same three things.
acid + carbonate
salt + water + carbon dioxide
the carbon dioxide comes out of the carbonate, not the acid
The carbon dioxide is the giveaway. It is the fizz, and it is what limewater detects.
Watched first · five steps
The limewater test
0 of 5 revealed
Each step has a reason, and each reason is a way this test can be got wrong. Reveal them one at a time.
Put the carbonate in a boiling tube and add the acid.
A boiling tube, not a test tube — the fizzing throws liquid up the sides, and a tube that is too small delivers acid spray to whoever is holding it.
Fit a bung with a delivery tube straight away.
Carbon dioxide is produced from the first second. Anything that escapes before the bung goes in is gas you cannot test, and the fastest fizzing happens at the start.
Run the delivery tube into a second tube of clear limewater.
The end of the tube must be under the surface of the limewater. Above it, the gas bubbles into the air and the limewater never meets it.
Watch the limewater, not the reaction.
The interesting tube is the second one. Clear limewater turning milky white is the positive result — a fine white solid forming in what was a clear liquid.
Keep bubbling and it goes clear again.
Not a mistake, and not a failed test. Excess carbon dioxide dissolves the white solid back into something colourless. The test is the change to milky, so read it when it happens rather than at the end.
Your turn · four white solids
Which of these are carbonates? Only the acid can tell you.
0 of 4 decided
Three of these look almost identical in the bottle, which is the point.
Marble chips
hard white lumps
It fizzes — this one is a carbonate.
Calcium carbonate. Steady fizzing from every surface, and the limewater goes milky within seconds.
hydrochloric acid + calcium carbonatemakescalcium chloride + water + carbon dioxide
Baking soda
fine white powder
It fizzes — this one is a carbonate.
Sodium hydrogencarbonate — a carbonate with hydrogen in it, and it behaves the same way. Violent fizzing because the powder has an enormous surface area.
hydrochloric acid + sodium hydrogencarbonatemakessodium chloride + water + carbon dioxide
Table salt
white crystals
No fizzing. Not a carbonate.
Sodium chloride. Nothing happens at all: it is already a salt, and there is no carbonate in it for the acid to break open. It just dissolves.
Copper carbonate
green powder
It fizzes — this one is a carbonate.
The only one on the bench that is not white — and it fizzes hard, giving a blue-green solution of copper chloride. Carbonates are a family, not a single substance.
hydrochloric acid + copper carbonatemakescopper chloride + water + carbon dioxide
Key fact
acid + carbonate makes salt + water + carbon dioxide. Limewater turning milky is the test for that gas — and nothing else turns it milky.
Three consequences
One reaction, three places it decides the outcome
0 of 3 decided
Commit to each before reading. None of the three is about a laboratory.
Why do old gravestones made of limestone go unreadable while granite ones stay sharp?
Because rain is slightly acidic and limestone is calcium carbonate, so the stone is slowly reacted away — letters first, since they are the thinnest part. Granite contains no carbonate, so the same rain runs off it and does nothing. Hardness is a side issue; this is a chemical difference, not a physical one.
A fizzy indigestion tablet contains a carbonate and a solid acid. Why does it only fizz when dropped in water?
Both ingredients sit dry in the tablet doing nothing, because their particles cannot move past each other. Water dissolves them, the particles mix, and the reaction starts — giving carbon dioxide, which is the fizz. The carbonate then goes on to neutralise the acid in the stomach.
A farmer wants to know whether a field of soil contains chalk. What is the two-minute test?
Drip dilute acid on it. Fizzing means a carbonate is present, and in soil that means chalk or limestone. Collecting the gas and turning limewater milky would confirm it. Geologists carry a small bottle of acid in the field for exactly this.
Think again
“A gas that puts out a splint is carbon dioxide.”
The splint did go out. Commit before you read on.
Almost every gas puts a splint out. Nitrogen does, argon does, and so does a tube of nothing much at all — a flame needs oxygen, and anything that is not oxygen will smother it. “The splint went out” narrows the answer to not oxygen, which is barely a clue.
Limewater is different: it goes milky for carbon dioxide and for nothing else on your bench. A good test has one answer. A test that dozens of substances could pass is not evidence.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
Complete: acid + carbonate makes what?
Rung 2 · The one that catches people
A gas is bubbled through limewater and the limewater stays clear. What can you conclude?
Rung 3 · Explain
Marble chips are dropped into dilute hydrochloric acid. Describe what you would see, name all three products, and explain how you would prove the gas is carbon dioxide.
Rung 4 · Take it somewhere new
Two white powders are on the bench: one is a carbonate and one is not, and you may not taste or touch either. Design a test that would tell them apart, and explain what result would settle it.
Key note
Acids react with carbonates to give a salt, water and carbon dioxide. The fizzing is the carbon dioxide leaving, and it is identified by bubbling it through limewater, which turns milky. Calcium carbonate is chalk, limestone and marble — which is why acid rain damages buildings and statues made of them.
Going further
Whole landscapes are made by this reaction. Rain is slightly acidic before it picks up any pollution at all, and where it falls on limestone it dissolves it — slowly, along cracks, for thousands of years. The result is limestone pavement above ground and cave systems below it, and every stalactite is the same reaction running backwards as the water dries and leaves the calcium carbonate behind.
The same chemistry sits in the ocean. Carbon dioxide dissolving into seawater makes it very slightly more acidic, and shells and coral skeletons are calcium carbonate. A change of a few tenths of a pH unit across an ocean is not a laboratory abstraction — it is the difference between a shell that thickens and one that thins.
Before this lesson
At GCSE this becomes
Where to next
Ask Mr Badmus AI
Still not sure why limewater goes cloudy?
Eye protection throughout. Dilute acid on a carbonate fizzes hard enough to throw liquid up the tube, which is why the reaction goes in a boiling tube rather than a test tube and why the bung is fitted from the side rather than over the mouth. Limewater is an alkali: wash it off skin, and off the bench.
Lesson content © MrBadmusAI.