Magnesium carbonate decomposes when heated to form magnesium oxide and carbon dioxide gas. The equation for this thermal decomposition is:

\text{MgCO}_3(\text{s}) \rightarrow \text{MgO}(\text{s}) + \text{CO}_2(\text{g})

A student uses the following method to investigate this reaction:

Step 1: Weigh a clean, dry crucible and lid, and record the mass.
Step 2: Add a sample of white magnesium carbonate powder.
Step 3: Reweigh the crucible, lid, and contents, and record the mass.
Step 4: Heat the crucible and contents strongly for five minutes.
Step 5: Allow the crucible to cool and then reweigh.
Step 6: Repeat steps 4 and 5 until the mass of the crucible and contents does not change.

The student performs the experiment four times (Trial A to Trial D). The table below shows the results:

Mass measurements (in $g\text{g}$ )	Trial A	Trial B	Trial C	Trial D
Mass of empty crucible and lid	22.40	21.15	23.05	21.85
Mass of crucible, lid, and magnesium carbonate before heating	27.50	25.85	28.25	26.65
Mass of crucible, lid, and contents after heating for 5 minutes	25.40	24.10	26.15	24.80
Mass of crucible, lid, and contents after heating for a total of 10 minutes	24.84	23.40	25.55	24.14
Mass of crucible, lid, and contents after heating for a total of 15 minutes	24.84	23.40	25.10	24.14

State why the mass of the crucible and contents decreases during heating.

State the reason for Step 6.

Explain in which trial the student should have heated for a fourth period of five minutes.