(The experiment described below can also be done using chopped up copper foil instead of magnesium, but because copper does produce smoke when heated the crucible lid is not needed)

Experiment to see if magnesium changes in mass when heated

Diagram

Method
About 20cm of magnesium ribbon is cleaned, chopped up into short lengths and placed in a crucible with a lid which has been weighed.

The crucible, lid and magnesium are weighed again and then heated strongly.

The lid of the crucible is raised every few minutes.

When the reaction is over the apparatus is allowed to cool and then reweighed. All results are recorded.

Results:

Mass of empty crucible + lid = 19.65 g

Mass of crucible, lid + magnesium = 21.11 g

Mass of crucible + lid after heating = 21.17 g

Questions

What mass of magnesium was used:?     21.11-19.65 =     1.46g

By how much did the magnesium gain in mass?    21.17-21.11=    0.06g

Why did the magnesium gain in mass?   The magnesium gained in mass because it combined with oxygen from the air.

Why did the crucible need a lid ?  To stop the smoke escaping.
The lid also slows down the reaction by limiting the available oxygen.

Why was the lid raised every few minutes?  To let air into the crucible so that the magnesium had oxygen.

How can you tell when the reaction is over?   The reaction is over when the magnesium stops glowing.

What chemical is left behind in the crucible?   magnesium oxide.

What does this reaction tell us?   This reaction shows us the amount of oxygen used up when magnesium burns.

What is the word equation for the reaction which takes place when the magnesium burns.

magnesium + oxygen    —> magnesium oxide