Science 7–10 · Years 9–10

Burning magnesium in a crucible: the mass goes up

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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

Mass is conserved in a reaction, so when magnesium combines with oxygen from the air the solid product weighs more than the metal by the mass of oxygen it took in.

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

burning magnesium is intensely bright and can damage eyesight if stared at
hot crucible and tongs
magnesium oxide smoke irritates

Controls

never look directly at the burning ribbon; view from the side
tongs for the lid; crucible cools on the mat
eye protection; hair tied back; learners stand

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.

What you need

magnesium ribbon weighed to about 0.20 g (15 to 20 cm, depending on ribbon thickness), cleaned with emery paper and coiled loosely
crucible with lid, pipe-clay triangle, tripod, Bunsen burner, tongs, heat-resistant mat
balance reading to 0.01 g

How to do it

Weigh the crucible and lid, then weigh again with the coiled magnesium inside; record both.
Heat strongly with the lid on. Every 30 seconds lift the lid a little with tongs to let air in, then replace it before the white smoke escapes.
When no more glow is seen, heat for a further 2 minutes, cool on the mat and weigh.
Reheat for 1 minute, cool and weigh again until the mass is constant.
Calculate the mass gained and the ratio of product mass to magnesium mass; compare with the ratio predicted from the equation 2Mg + O2 gives 2MgO.

What you should see

The ribbon burns with a brilliant white flame to a white powder. Theory: MgO weighs 1.658 times the magnesium, so 0.20 g of magnesium becomes 0.332 g of magnesium oxide, a gain of 0.132 g (66 percent). Class results usually fall below this gain because some oxide escapes as smoke and some magnesium combines with nitrogen instead: magnesium nitride weighs only 1.384 times the magnesium. The learner knows it worked when the final mass is higher than the start and stays constant on reheating.

What changes

What you change: mass of magnesium
What you measure: mass of magnesium oxide formed
What you keep the same: lid technique
heating time to constant mass
same balance

Common misconceptions

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

Burning always makes things lighter because something is used up.
The white ash is what is left of the magnesium after part of it burnt away.
Air has no mass, so it cannot add to the product.

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)SC5-RXN-01 SC5-WS-05
Australian Curriculum v9AC9S9U07AC9S10U07

Sources

The pages the author read to write this activity.

All Concept Studio activities