Atoms, elements and compounds · Contrast
Compounds
Iron and sulfur in a dish. Same two elements before and after heating — and afterwards the magnet is useless. What changed?
Start here
The magnet worked five minutes ago.
Grey iron filings and yellow sulfur powder, stirred together. Hold a magnet over the dish and the iron jumps out of it. Stir it back in, heat one corner until it glows, and the glow spreads by itself through the whole dish. What is left is a dull grey-black solid — and the magnet now does nothing at all.
Where has the iron gone?
Every iron atom is still in the dish; not one has been lost. They are now joined to sulfur atoms, and the substance they are part of is not iron. It has its own properties, and being magnetic is not one of them.
Mixing changes nothing about the substances — it just puts them in the same place. A chemical reaction joins their atoms together, and what comes out is a new substance: a compound.
At the bench · the same two elements, twice
Test the mixture. Heat it. Test it again.
1 of 4 tests run
Four tests, two states. Three of the four give an interesting answer in both states, and only one of them settles anything.
Commit first. In the mixture you can use any amounts you like. What about after heating?
The dish
How much of each
Two kinds of particle in the same dish, in whatever proportion you chose. Nothing is joined to anything.
Look at it
Before heating
Grey specks and yellow powder, clearly two different things side by side.
After heating
A dull grey-black solid, the same all the way through.
Settles nothing. Interesting, and it settles nothing. Grind the mixture finely enough and it looks uniform too.
Hold a magnet over it
Before heating
The grey specks jump out and cling to the magnet. The yellow powder is left behind.
After heating
Nothing moves at all.
Settles it. This one does settle it. The iron can be pulled out of a mixture because it is still iron; it cannot be pulled out afterwards because there is no longer any iron there — only iron atoms, joined to sulfur.
Weigh what combines
Before heating
Any amounts at all. Nine parts iron to one of sulfur, or the other way round — it is still a mixture of iron and sulfur.
After heating
Always 7 g of iron to every 4 g of sulfur. Add extra of either and the extra is left over, unreacted.
Settles it. The quiet test, and the strongest. A mixture can be any proportion. A compound has one, and it is not negotiable.
Add dilute acid
Before heating
Bubbles of a gas that burns with a squeaky pop, and the smell of nothing much.
After heating
Bubbles of a gas that stinks of rotten eggs.
Settles it. Different substance in, different substance out. The mixture gives hydrogen because the iron in it reacts as iron; the compound gives hydrogen sulfide, which nothing in the dish could have given before.
Key fact
A mixture can be any proportion and can be separated again. A compound is one fixed proportion, joined in a reaction, and behaves like neither element that made it.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Your turn · four substances
Mixture or compound?
0 of 4 decided
One question decides all four: does its composition have to be exactly that, or could it be anything?
Sea water.
Mixture. The saltiness varies from sea to sea and from place to place in the same sea. Boil it off and the salt is back, unchanged.
Carbon dioxide, the gas that comes out of a fire extinguisher.
Compound. Always one carbon atom to two oxygen atoms, wherever it comes from — a volcano, your lungs or a factory. It puts out fires, which neither carbon nor oxygen does.
Steel, made by adding a little carbon to iron.
Mixture. The amount of carbon is chosen by the steelmaker and changes the properties. Anything whose recipe can be adjusted is a mixture.
Sugar, however carefully it is purified.
Compound. Carbon, hydrogen and oxygen in a fixed ratio. It is not a mixture of those three: carbon is black, hydrogen and oxygen are gases, and sugar is a sweet white solid.
Think again
“The iron is still in there, so it must still be magnetic — the sulfur is just covering it up.”
Every iron atom that went into the dish is still in the dish. Commit to what that does and does not mean.
The atoms are all present and the element is not. Being magnetic is a property of iron — of iron atoms arranged in iron, with nothing else joined to them. In iron sulfide each iron atom is bonded to a sulfur atom, and the substance those atoms now make is not magnetic, not shiny, and does not behave like either of the things that went in.
This is the whole idea of a compound, and it is why sodium — a metal that catches fire in water — and chlorine — a poisonous green gas — join to make the salt on your chips. The properties belong to the substance, not to the shopping list of atoms in it.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What is a compound?
Rung 2 · The one that catches people
Iron and sulfur react in a fixed ratio of 7 g to 4 g. A student heats 7 g of iron with 8 g of sulfur. What do they get?
Rung 3 · Explain
Explain how you could tell, using two different tests, whether a grey-black powder is a mixture of iron and sulfur or the compound iron sulfide. Say what each test would show in each case.
Rung 4 · Take it somewhere new
A mixture of hydrogen and oxygen explodes when a spark is applied. Water is made of the same two elements and is used to put fires out. Explain how both can be true, and say what a chemist would have to do to turn one into the other.
Key note
A compound is two or more elements chemically joined, in a fixed proportion, making a substance with its own properties. A mixture is the same elements in the same place, in any proportion, each still behaving as itself — and it can be separated again without a reaction.
Going further
A 2:1 mixture of hydrogen and oxygen gas is one of the most dangerous things you can have in a laboratory: a spark and it detonates. Join the same atoms chemically, in the ratio of two to one, and you get water — which you use to put fires out. Same elements, same ratio by atoms, and the difference between the two is only whether the atoms are bonded. That difference is worth more than almost anything else in chemistry.
Before this lesson
Next in this unit
At GCSE this becomes
- Bonding — what actually holds the atoms of a compound together, and why that changes everything about the substance.
Where to next
Ask Mr Badmus AI
Not sure why the magnet stops working?
Heating iron and sulfur produces a small amount of hydrogen sulfide, which smells of rotten eggs and is toxic. This one belongs in a fume cupboard, done by a teacher.
Lesson content © MrBadmusAI.