Chemistry 11–12 · Year 11

Burning magnesium in a crucible: mass gain and the empirical formula of magnesium oxide

Module 2: Introduction to Quantitative Chemistry

Practical, model not builtMedium risk

School laboratory, not for home

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

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Elements combine in a fixed whole-number mole ratio; the mass of oxygen gained by a weighed strip of magnesium gives that ratio directly.

Safety card

Medium riskLearners carry it out, with a teacher supervising

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

Hazards

intense white light from burning magnesium
hot crucible and tripod
fine magnesium oxide dust

Controls

do not stare at the burning ribbon
tongs for every transfer
lid kept on except brief lifts
cool on the mat before the balance
eye protection

Note

NSW Department of Education Chemical Safety in Schools (CSIS) package, 2021 Technical Update: Section 1.7 (risk assessment) and Volume 2 Appendix D (generic assessment advice and DoE chemical categories); record a RiskAssess (riskassess.com.au) risk assessment before the lesson and check the school's hazardous chemical register (CSIS Section 1.9) for local restrictions.

What you need

Magnesium ribbon, 30 cm cleaned with emery paper (about 0.3 g)
Porcelain crucible with lid, pipe-clay triangle, tripod, Bunsen burner, heat-resistant mat
Crucible tongs
Electronic balance reading to 0.001 g (0.01 g acceptable)
Wash bottle of deionised water, dropper

How to do it

Heat the empty crucible and lid for two minutes, cool and weigh them together (mass 1).
Coil the magnesium loosely, place it in the crucible, replace the lid and weigh (mass 2).
Heat strongly with the lid slightly ajar; lift the lid briefly with tongs every 30 seconds to admit air; continue until the ribbon no longer glows when the lid is lifted (8 to 12 minutes).
Cool, add three drops of water to convert any magnesium nitride to hydroxide, and reheat for three minutes to drive off the water; cool and weigh (mass 3).
Reheat for two minutes, cool and reweigh; repeat until the mass is constant within 0.005 g.
Calculate the mass of magnesium, the mass of oxygen gained, moles of each and the simplest whole-number ratio; pool the class ratios and discuss the spread.
Enter the measured masses in the simulation and compare with the predicted product mass.

What you should see

For 0.300 g of magnesium the product mass is close to 0.497 g, an oxygen gain of 0.197 g: moles Mg 0.01234, moles O 0.01234, ratio 1.00 to 1.00, formula MgO. A Mg:O ratio noticeably above 1:1 means too little oxygen was recorded: magnesium oxide escaped as smoke or some metal was not fully burned (losing 0.010 g of oxide smoke from 0.300 g of magnesium gives 1.05). A ratio below 1:1 means the product weighed too much, for example water left behind after the nitride step. The white ash is magnesium oxide, with any grey unreacted metal visible.

What changes

What you change: mass of magnesium burned
What you measure: mass of oxygen gained (g)
What you keep the same: heating to constant mass
lid technique to keep smoke in
same crucible and balance

Common misconceptions

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

Burning always makes things lighter (the ash is heavier because oxygen from the air is bonded in).
The lid must stay off so the magnesium can get air (a lifted lid admits air; a lid off lets the oxide smoke escape, the oxygen gain reads low and the Mg:O ratio rises above 1).
Mass ratio equals mole ratio (24.3 g of magnesium pairs with 16.0 g of oxygen; the mole ratio is 1:1 only after dividing by molar masses).

Curriculum references

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

Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH11-9 CH11/12-4 CH11/12-6
Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document); a Working Scientifically outcome not among those the syllabus targets in Module 2, which it allows in any moduleCH11/12-3
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-11-02 CH-11WS-03 CH-11WS-04
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities