Iron + Sulfur: When Two Elements Become Something Completely New
One of Chemistry's Simplest Reactions — and One of Its Most Important
Some chemistry experiments impress because they produce flames, dramatic colour changes or clouds of gas.
Others are valuable because they reveal an idea so clearly that it becomes difficult to forget.
Heating iron filings with sulfur is one of those experiments.
At the beginning, we have two familiar elements:
iron, a grey metallic solid;
sulfur, a yellow non-metallic solid.
Mix them together and, at first, surprisingly little has actually happened.
The iron is still iron.
The sulfur is still sulfur.
Most importantly, I can demonstrate that very easily with a magnet.
Pass a magnet close to the mixture and the iron filings are attracted towards it. With sufficient care, we can even separate much of the iron from the sulfur again.
Then we heat the mixture.
What we obtain afterwards behaves very differently.
We have made a new substance: iron sulfide.
Fe + S -> FeS
And one of the most memorable pieces of evidence is that the magnetic behaviour of the original iron has effectively disappeared.
That simple observation opens the door to some fundamental chemistry.
Before Heating: It Is Only a Mixture
Suppose I put some iron filings and powdered sulfur together in a small container.
The iron is grey.
The sulfur is yellow.
Even after mixing them thoroughly, close inspection may still allow us to distinguish the two materials.
Chemically, nothing has yet changed.
We have made a mixture.
That distinction matters enormously.
In a mixture:
the substances are not chemically bonded together;
each substance retains its own chemical properties;
the proportions can vary;
the components can often be separated using physical methods.
The magnet provides a particularly elegant demonstration.
Iron is strongly attracted to a magnet.
Sulfur is not.
Bring a magnet close to the mixture and the iron responds while the sulfur does not.
This is physical separation in action.
No chemical reaction is required.
Then We Add Energy
The situation changes when the mixture is heated strongly.
In a properly equipped laboratory, a small quantity of iron and sulfur can be placed in a suitable ignition tube and heated carefully.
This should be carried out with appropriate eye protection and good ventilation, preferably in a fume cupboard where available.
The tube must never be sealed.
As the mixture becomes hot enough, the reaction begins.
Once initiated, something particularly interesting may be observed: the reaction can continue through the mixture even after the strongest external heating has been reduced.
That tells us something else important.
The reaction releases energy.
Iron atoms and sulfur atoms are rearranging and forming a new substance.
The overall reaction is:
Fe + S -> FeS
Iron + sulfur -> iron sulfide
The product is no longer simply iron mixed with sulfur.
It is a compound.
The Magnet Test: A Beautiful Piece of Chemical Evidence
This is the part of the experiment I particularly like.
Before heating, I can demonstrate the presence of iron immediately.
I bring a magnet towards the mixture.
The iron moves.
After the reaction has taken place and the product has cooled completely, repeat the test.
The dramatic magnetic response associated with the original iron filings is no longer there.
Why?
Because the iron atoms are no longer present as metallic iron.
They are now chemically combined with sulfur as iron sulfide.
This provides a wonderful opportunity to ask students:
Where has the iron gone?
It has not vanished.
The iron atoms are still present.
But they are now part of a different substance.
That distinction between an element being present as an element and its atoms being present inside a compound is one of the most important ideas in chemistry.
The Atoms Have Not Disappeared
Students occasionally interpret chemical reactions as substances somehow disappearing and being replaced.
That is not what happens.
Before the reaction we have iron atoms and sulfur atoms arranged within two separate elemental substances.
After the reaction, those same types of atoms are present, but they have been rearranged into iron sulfide.
No iron atoms have magically vanished.
No sulfur atoms have magically appeared.
The atoms have been reorganised.
This leads naturally into the law of conservation of mass.
In a closed chemical system:
mass of reactants = mass of products
The appearance and properties may change dramatically, but the atoms themselves are conserved.
Mixture Versus Compound
The experiment gives us an almost perfect comparison.
Iron and sulfur before heating
This is a mixture.
The iron:
remains magnetic;
retains its metallic properties;
can potentially be physically separated.
The sulfur:
remains yellow;
retains its characteristic properties;
is not chemically bonded to the iron.
The proportions could also be changed.
We could mix more iron with less sulfur, or more sulfur with less iron.
It would still simply be a mixture.
Iron sulfide after heating
Now we have a compound.
The iron and sulfur are:
chemically combined;
present in a fixed chemical relationship;
no longer easily separated by physical methods;
part of a substance with properties different from either starting element.
That last point is crucial.
Compounds do not have to resemble the elements from which they are made.
A Compound Can Behave Completely Differently
This concept extends far beyond iron sulfide.
Sodium is a highly reactive metal.
Chlorine is a toxic gas.
React them appropriately and we obtain sodium chloride — ordinary table salt.
Hydrogen is a flammable gas.
Oxygen supports combustion.
Combine hydrogen and oxygen chemically and we can produce water.
Chemistry continually reminds us that knowing the properties of the elements does not automatically tell us the properties of the compound they will form.
Iron sulfide is another excellent example.
Iron is magnetic.
Sulfur is yellow.
Iron sulfide is neither simply "magnetic iron mixed with yellow sulfur".
It is a new substance.
Why Does Heating Matter?
Another useful question is:
If iron wants to react with sulfur, why doesn't the reaction happen immediately when we mix them?
The answer introduces activation energy.
Particles must have enough energy for successful reactions to occur.
Heating supplies the initial energy needed to get the reaction started.
At A Level, we can describe this using the idea of an activation energy barrier.
The reactants need sufficient energy to reach the transition towards products.
Once the reaction begins, energy is released as new chemical interactions form.
So this very simple GCSE experiment can become the starting point for much deeper A Level discussion.
What Does the Equation Really Mean?
The symbolic equation is:
Fe + S -> FeS
At GCSE, students need to understand that the symbols represent substances and atoms.
Fe represents iron.
S represents sulfur.
FeS represents iron sulfide.
At a simple particle level we can think of one iron atom combining with one sulfur atom in the formula unit.
Using approximate relative atomic masses:
Fe = 56
S = 32
So the mass ratio for the equation is:
56 : 32
which simplifies to:
7 : 4
That does not mean that large quantities should be used for the demonstration. Practical laboratory experiments should use suitably small quantities following the laboratory's risk assessment.
But mathematically it tells us something very important.
For every 7 parts by mass of iron required by this idealised equation, we require 4 parts by mass of sulfur.
That takes us directly into stoichiometry.
What Happens If We Use Too Much Iron?
Suppose there is more iron than required.
The sulfur may become the limiting reactant.
Once all the available sulfur has reacted, some iron could remain unreacted.
What might happen if we then bring a magnet towards our final material?
We might still detect some magnetic material.
That does not necessarily mean that iron sulfide itself has suddenly retained all the properties of metallic iron.
It may indicate that some unreacted iron remains.
This is a useful reminder that real laboratory results are sometimes messier than textbook diagrams.
At A Level, that becomes an interesting discussion about:
limiting reactants;
excess reactants;
completeness of reactions;
purity;
theoretical yield;
experimental evidence.
A GCSE practical has suddenly become an A Level chemistry lesson.
A Simple Investigation Rather Than Just a Demonstration
Instead of telling a student what will happen, I much prefer turning the experiment into a sequence of questions.
Give the student the iron and sulfur mixture before heating.
Ask:
What evidence shows that iron is still present?
Try the magnet.
Then ask:
Has a chemical reaction happened just because we mixed the powders?
No.
Next heat the mixture safely.
Allow it to cool completely.
Then repeat the observations.
Ask:
Can we still separate the iron with a magnet?
Does the product look like either original substance?
What evidence suggests that a new substance has formed?
Now the student is not simply watching chemistry.
They are reasoning from experimental evidence.
That is a much more powerful way to learn.
Physical Change or Chemical Change?
This experiment also provides an excellent way of distinguishing physical and chemical changes.
Mixing iron and sulfur is essentially a physical process.
No new substance has been produced.
Heating them sufficiently causes a chemical reaction.
Evidence includes:
energy being released during the reaction;
formation of a substance with different properties;
inability to recover the original iron simply using a magnet;
a substantial change in appearance and behaviour.
Students often learn lists such as:
"colour change = chemical reaction"
or
"temperature change = chemical reaction".
Those can be useful clues, but chemistry is more subtle.
The strongest evidence is that a new substance with new properties has formed.
Could We Simply Reverse the Reaction?
Before heating, separating the mixture is relatively straightforward because the substances retain their identities.
After the reaction, separating iron from sulfur is no longer a matter of simply using a magnet.
The atoms are chemically combined.
Breaking a compound apart usually requires another chemical process.
That is another fundamental distinction:
Mixtures are separated by physical processes.
Compounds require chemical processes to separate them into chemically different substances.
Filtration, evaporation, distillation, chromatography and magnets can separate suitable mixtures.
They do not simply dismantle chemical compounds into their constituent elements.
A Useful GCSE Exam Question
A typical question might say:
A student mixes iron filings with sulfur powder. Before heating, a magnet attracts the iron. The mixture is heated strongly and forms iron sulfide. Explain why the product is different from the original mixture.
A strong answer might include:
Before heating, iron and sulfur form a mixture in which both elements retain their individual properties. Heating causes a chemical reaction and produces the compound iron sulfide. The iron and sulfur atoms are chemically combined, so the product has different properties from the original elements and the iron can no longer simply be separated using a magnet.
Notice that the answer is not merely:
"because a reaction happened."
It uses the observations to explain the chemistry.
Taking It Further at A Level
For an A Level student I would push the discussion further.
Why is energy required initially?
What determines whether collisions lead to reaction?
What is happening energetically as bonds and interactions change?
Which reactant would be limiting if the quantities were altered?
Could the yield be less than expected?
How would we determine the purity of the product?
How might the behaviour of the material differ from the simplified model used at GCSE?
A good experiment should create more questions than it answers.
That is one reason I continue to value practical chemistry so highly.
Important Safety Considerations
This is a genuine heating experiment and should be treated accordingly.
It should be performed in a properly equipped laboratory, rather than casually attempted at home.
Suitable precautions include:
wearing eye protection;
using only small quantities;
using heat-resistant apparatus intended for strong heating;
keeping the ignition tube pointed away from people;
never sealing the tube;
using appropriate ventilation, ideally a fume cupboard where available;
avoiding inhalation of any fumes;
allowing the apparatus and product to cool fully before handling.
Sulfur and sulfur-containing materials should not be heated carelessly. Burning sulfur can produce irritating sulfur dioxide.
It is also unwise to improvise further reactions with the iron sulfide product. In particular, adding acids to metal sulfides can release hazardous hydrogen sulfide gas.
The educational value comes from the carefully controlled iron-sulfur reaction itself.
Why I Like This Experiment So Much
There are more spectacular experiments in chemistry.
There are reactions with brighter flames, louder noises and more dramatic colour changes.
But iron and sulfur does something particularly valuable.
It allows a student to test an idea before and after a reaction.
Before heating:
"There is iron here. I can prove it with a magnet."
After heating:
"Something fundamental has changed."
That is chemistry made tangible.
Students are not simply being told that compounds have different properties from their constituent elements.
They can see it.
They can test it.
And they can explain it.
For me, those are the experiments that students tend to remember.
From Two Elements to One New Substance
Iron filings and sulfur begin as two separate elements.
Mixing them does not change their identities.
Heat them sufficiently, however, and their atoms become chemically combined.
Fe + S -> FeS
The grey magnetic iron and yellow sulfur are replaced by a material with its own properties.
That simple transformation demonstrates:
elements;
mixtures;
compounds;
chemical reactions;
conservation of atoms;
activation energy;
energy changes;
stoichiometry;
limiting reactants;
experimental evidence.
All from a small quantity of iron, a little sulfur, an ignition tube — and a magnet.
Sometimes the best chemistry experiments are not the biggest.
They are the ones that allow a student to say:
"I can prove that something new has been made."
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