Blow the Lid Off of Magnesium Oxide Lab

Magnesium reacting with oxygen in a red-hot crucible
Photo by Julia Paneyko

Every chemistry student makes magnesium oxide in the lab in their introductory course.  My students do it twice in my lab program:  once in the study of chemical reactions (types of chemical reactions lab) and then again in the mole unit.  High school chemistry students around the world are determining the chemical formula of magnesium oxide in this classic experiment.  I use this experiment as my first "mole concept application".


The procedure is very simple:  heat magnesium in a crucible until it is fully reacted with oxygen.  Use the mass data of the magnesium before reaction and product after reaction to calculate the mole ratio of Mg and O in the compound.  This mole ratio is the experimental chemical formula of magnesium oxide.  My "go to" procedure for this lab is found in ChemTopic Labs Volume 7:  Molar Relationships & Stoichiometry by Flinn.  In Flinn's version of the lab, there are detailed instructions to heat the magnesium with the lid on, then open the lid every three minutes, and then heat it some more with the lid at a tilt.  All of this manipulation of the crucible lid invariably results in at least one dropped and broken lid each lab period.  And, the results of the lab are usually not that great, with many groups getting ratios other than the expected 1:1.

My students are heating their crucibles with lids off.

This year I decided to try the lab with no lid on the crucible during the heating process.  It seems only obvious to let the maximum amount of air into the reaction by just leaving it open for the whole reaction.  I was a little nervous that burning magnesium might pop out of the crucible, but that didn't happen all day.  The hardest part of the experiment was adjusting the bunsen burner so that it would get hot enough to start the reaction.  Most of the reactions burned at a steady rate, without a big flare up of bright light.  Once the reactions were completed without any safety issues, my students crunched the numbers.

Here's where the big payoff was obvious.  The lab results were much better this year, with most groups getting the expected 1:1 ratio without the need for sad stories about experimental errors in the conclusion.  Taking the lid off resulted in better results, more interesting observations during the reaction, and better results for the chemical formula for magnesium oxide.