Science 7–10 · Year 8

Iron and sulfur: a mixture you can un-mix and a compound you cannot

Solutions and mixtures

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

Two elements mixed keep their own properties and can be separated, but once they react the compound formed has new properties and the elements can no longer be pulled apart physically.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

sulfur can boil and burn to give sulfur dioxide, a toxic and corrosive gas that can trigger asthma
hot tube and glowing solid
iron powder is flammable and an eye hazard

Controls

all heating inside a fume cupboard and under 10 g of mixture in total (CSIS Appendix F, iron); ignition tubes and test tube plugged with mineral wool, never an open tin lid
learners with asthma kept well away from the fume cupboard during heating, since some asthmatics are particularly sensitive to sulfur dioxide
eye protection; tube left to cool in the holder inside the fume cupboard
the mixed powders are heated only in the fume cupboard, never on the open bench

Note

NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1.7 (risk assessment before use) and Volume 2 Appendix D (generic assessment advice, user codes and DoE categories). Complete a RiskAssess or CSIS site-specific risk assessment before the lesson. CSIS Appendix F lists iron filings or powder and sulfur under chemicals to be used with caution; it states that the iron and sulfur reaction can be carried out safely on a small scale (under 10 g of total material) in a fume cupboard, and that experiments which give off sulfur dioxide are carried out in a fume cupboard.

What you need

a 7 : 4 mixture by mass of iron powder and powdered sulfur, as in the RSC method (made up as 7 g of iron and 4 g of sulfur and shared out), about 0.5 g per pair for the magnet tests
borosilicate test tube and ignition tubes (75 mm by 10 mm), mineral wool plugs, test-tube holder, Bunsen burner and heat-resistant mat, all used inside a fume cupboard
bar magnet in a plastic bag, watch glass, spatula, hand lens
for the heating: about 2 g of the mixture for the teacher demonstration, or 0.2 g per ignition tube for a group working at the fume cupboard

How to do it

Look at each element with the hand lens and record colour and texture. Hold the magnet under the watch glass of iron and then of sulfur; record which is attracted.
Mix the two powders on the watch glass. Pass the magnet underneath and record what happens; note that the mixture can still be separated.
In the fume cupboard, the teacher heats about 2 g of the mixture in the borosilicate test tube plugged loosely with mineral wool (or a group heats 0.2 g in an ignition tube no more than a quarter full, at the fume cupboard under the teacher's eye) until an orange glow spreads through the solid; heating then stops and the tube cools in the fume cupboard.
Tip the cooled solid onto a clean watch glass. Record its colour and test it with the magnet.
Write the word equation and the symbol equation (Fe + S gives FeS) and list three properties that changed.

What you should see

The 7 g : 4 g ratio is 1 : 1 by amount (0.125 mol iron to 0.125 mol sulfur), so almost none of either element is left over. Before heating the magnet lifts the grey iron out of the yellow mixture; after heating a dark grey-black brittle solid (iron(II) sulfide) remains that no longer shows separate yellow and grey grains and that the magnet attracts far less than it attracted the mixture; the RSC notes that showing the product is non-magnetic is not always successful, and iron left unreacted is one reason. The glow that travels through the tube after the burner is removed shows the reaction releases its own heat. The learner knows it worked when the product is attracted far less than the mixture was and shows none of the yellow of sulfur.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Mixing two substances is the same as reacting them.
The compound is a very fine mixture of the two elements.
Iron sulfide should still be magnetic because it contains iron.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC4-SOL-01 SC4-CHG-01
Australian Curriculum v9AC9S8U06AC9S8U07

Sources

The pages the author read to write this activity.

All Concept Studio activities