Acids and alkalis · Process
Acid + metal
Drop magnesium into acid and the tube fizzes hard enough to warm your hand. Drop copper in and nothing happens at all — for a week.
Start here
A strip of magnesium goes into dilute hydrochloric acid. The tube fills with bubbles and the metal disappears.
The tube gets warm. Within a minute the magnesium is gone and the liquid is clear. Collect the gas coming off, hold a lit splint to it, and it goes off with a squeak.
Where did the gas come from?
From the acid. Every acid contains hydrogen — that is what makes it an acid — and a reactive metal takes the acid apart and lets the hydrogen go. The magnesium has not turned into gas; it has gone into the solution as a salt, invisible and dissolved. Boil the liquid dry and it is waiting for you as white crystals of magnesium chloride.
A reactive metal reacts with an acid, and there are always two products.
The rule
A reactive metal and an acid always make the same two things.
acid + metal
salt + hydrogen
the hydrogen comes out of the acid, not the metal
Which salt depends on the acid. Hydrochloric acid makes chlorides, sulfuric acid makes sulfates, nitric acid makes nitrates. The first word of the salt's name is the metal; the second comes from the acid.
And it only happens if the metal is reactive enough. The reactivity series decides which tubes fizz and which sit there doing nothing.
Your turn · eight tubes
Four metals, two acids. Pick a tube, predict, then run it.
0 of 8 run
The copper row is worth running as well — a tube that does nothing is telling you something precise.
| Metal added to | hydrochloric acid | sulfuric acid |
|---|---|---|
| Magnesium | ||
| Zinc | ||
| Iron | ||
| Copper |
Magnesium in hydrochloric acid
Magnesium in sulfuric acid
Zinc in hydrochloric acid
Zinc in sulfuric acid
Iron in hydrochloric acid
Iron in sulfuric acid
Copper in hydrochloric acid
Copper in sulfuric acid
A measured strip of magnesium dropped into 10 cm³ of dilute hydrochloric acid at room temperature.
A measured strip of magnesium dropped into 10 cm³ of dilute sulfuric acid at room temperature.
Measured zinc granules dropped into 10 cm³ of dilute hydrochloric acid at room temperature.
Measured zinc granules dropped into 10 cm³ of dilute sulfuric acid at room temperature.
Measured iron filings dropped into 10 cm³ of dilute hydrochloric acid at room temperature.
Measured iron filings dropped into 10 cm³ of dilute sulfuric acid at room temperature.
Measured copper turnings dropped into 10 cm³ of dilute hydrochloric acid at room temperature.
Measured copper turnings dropped into 10 cm³ of dilute sulfuric acid at room temperature.
Predict before you run it.
Vigorous. Bubbles stream off and the tube becomes hot to hold. Over in under a minute.
Magnesium is high above hydrogen in the reactivity series, so it takes the hydrogen's place in the acid without hesitating. The hydrogen is pushed out as a gas and the magnesium ends up dissolved as magnesium chloride — which is why the strip disappears and the liquid stays clear.
hydrochloric acid + magnesiummakesmagnesium chloride + hydrogen
Vigorous again, and just as fast. The tube warms and the strip is gone within the minute.
The same metal doing the same thing, because the reactivity series does not care which acid the hydrogen came out of. What changes is the salt left behind: sulfuric acid gives a sulfate, so this tube ends up holding magnesium sulfate.
sulfuric acid + magnesiummakesmagnesium sulfate + hydrogen
Steady. A constant stream of small bubbles and a tube that warms gently over several minutes.
Zinc is above hydrogen too, but not as far above it as magnesium — so the same reaction runs at a pace you can watch rather than one you have to catch. The salt is zinc chloride.
hydrochloric acid + zincmakeszinc chloride + hydrogen
Steady, the same as with the other acid. Small bubbles, several minutes, a tube that warms.
This is the tube a school uses when it wants a jar of hydrogen: fast enough to fill one and slow enough to control. The salt is zinc sulfate, and it is the same zinc sulfate whether you make it from the metal or from zinc oxide.
sulfuric acid + zincmakeszinc sulfate + hydrogen
Slow. A few bubbles clinging to the filings, and it takes most of a lesson to see much change.
Iron is only just above hydrogen, so it does displace it — but barely, and you need patience to see it. That narrow margin is the whole reason iron rusts away in wet air over years rather than in minutes.
hydrochloric acid + ironmakesiron chloride + hydrogen
Slow again, and the liquid turns a pale green as the filings go into solution.
Slowest of the three that work, in either acid, which places iron at the bottom of the reacting metals here. The pale green is the iron sulfate dissolving — the metal has not vanished, it has changed into something that dissolves.
sulfuric acid + ironmakesiron sulfate + hydrogen
Nothing. Not in twenty minutes, not in a week.
Copper sits BELOW hydrogen in the reactivity series, so it cannot push hydrogen out of an acid. There is no slow reaction here to wait for — there is no reaction at all, and warming it, stirring it or leaving it out overnight all give the same result.
Nothing again. The turnings are as bright at the end as they were at the start.
Changing the acid changes nothing, and that is the point of running the second copper tube at all. What decides this is where copper sits relative to hydrogen, and that is a fact about copper — so no acid you could reach for on this bench would give a different answer.
Two patterns, not one.
Read down the columns and the speed falls in the order magnesium, zinc, iron, copper — the reactivity series again, arrived at by a different route. Read across the rows and the metal decides the first word of the salt while the acid decides the second: chloride from hydrochloric, sulfate from sulfuric.
Copper is the row that proves the rule. It is below hydrogen in the reactivity series, so it cannot push hydrogen out of an acid — and no amount of waiting will change that.
Key fact
acid + metal makes salt + hydrogen. The hydrogen comes out of the acid, not the metal, and a lit splint gives a squeaky pop.
Three judgements · testing the gas
A squeak, a lit splint and a stoppered tube
0 of 3 decided
Commit to each before reading. The third one is a genuine hazard, not a trick question.
A tube of gas from magnesium and acid is tested with a lit splint and squeaks. Does that prove the gas is hydrogen?
Yes — the squeaky pop is the standard test for hydrogen, and no other gas you will meet at this level does it. The squeak is the sound of the hydrogen burning explosively with the oxygen in the tube, making water.
A student holds the splint to the mouth of the tube while it is still fizzing hard. Is that a good idea?
No. The tube needs to be collected and stoppered first, and tested away from the reaction. Holding a flame over a vessel that is actively producing hydrogen means the flame can run back down into it — and a tube of hydrogen mixed with air does not squeak, it bangs.
The same test is tried on the gas from copper and acid, and nothing pops. Does that mean the test failed?
No — it means there was no gas to test. Copper sits below hydrogen in the reactivity series, so it cannot displace hydrogen from an acid. A test that correctly reports nothing is not a failed test, and this one has just told you where copper sits.
Think again
“The bubbles are the metal turning into gas.”
The magnesium does vanish, and bubbles do come off. Commit before you read on.
Two different things are happening and they are easy to run together. The bubbles are hydrogen, which came out of the acid. The metal has gone into solution as a salt — dissolved, invisible, and still every atom present.
You can prove both halves. The gas pops with a lit splint, which magnesium vapour would not do. And evaporating the liquid leaves white magnesium chloride crystals, which were not there when you started. The metal did not become the gas — it swapped places with the hydrogen in the acid.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
Zinc is added to sulfuric acid. What are the products?
Rung 2 · The one that catches people
Copper is left in dilute hydrochloric acid for a week and nothing happens. What is the reason?
Rung 3 · Explain
Magnesium is dropped into hydrochloric acid. Describe what you would see, name both products, and explain where the gas comes from.
Rung 4 · Take it somewhere new
You are given four unlabelled metals and a bottle of dilute hydrochloric acid. Explain how you would put the metals in order of reactivity, and what you would do to make it a fair comparison.
Key note
A metal above hydrogen in the reactivity series reacts with an acid to give a salt and hydrogen gas. The more reactive the metal, the faster the fizzing. Hydrochloric acid gives chlorides, sulfuric acid gives sulfates. Hydrogen is tested with a lit splint: it burns with a squeaky pop.
Going further
This reaction is why the hulls of ships are fitted with slabs of zinc bolted to the steel below the waterline. Seawater is full of dissolved salt and attacks iron steadily. Bolt on something more reactive and the seawater attacks that instead: the zinc corrodes away and the hull does not. The slabs are called sacrificial anodes, and replacing them is routine dry-dock work.
It is also the reason nobody stores acid in a metal can. Concentrated acids arrive in glass or plastic, and the tanker lorries that carry sulfuric acid are lined — because a steel tank full of acid is a hydrogen generator, and hydrogen and air together only need a spark. The gas that gives a tidy squeak from a test tube is the same gas that took down the Hindenburg.
Before this lesson
Next in this unit
At GCSE this becomes
- Redox: the metal loses electrons and hydrogen ions gain them, plus rates of reaction measured by the volume of gas collected.
Where to next
Ask Mr Badmus AI
Still not sure why copper does nothing?
Hydrogen mixed with air explodes rather than squeaking. The gas is collected in a tube, stoppered, and carried away from the fizzing flask before a lit splint goes near it — never tested at the mouth of a tube that is still reacting.
Lesson content © MrBadmusAI.