Final answer:
The residue is likely magnesium carbonate or other impurities, while the magnesium sulfate remains in the saturated solution and recrystallizes upon cooling.
Step-by-step explanation:
When preparing magnesium sulfate, the residue after filtering in step 2 is not the desired product, magnesium sulfate, but rather impurities such as magnesium carbonate or other insoluble materials. The saturated solution from step 3 would contain dissolved magnesium sulfate in the solvent, usually water. During step 4, the crystals of magnesium sulfate form because as the solution cools, the solubility of magnesium sulfate decreases, and it precipitates out of the solution in a more pure crystal form. This solidification process driven by cooling is known as recrystallization.
In summary, the correct statements are: b) The residue is magnesium carbonate, and crystals form due to heating.
The residue after filtering in step 2 and the saturated solution in step 3 when preparing magnesium sulfate are both magnesium sulfate. Crystals form during step 4 due to cooling. When a saturated solution is cooled, the solute becomes less soluble and starts to precipitate out of solution, forming crystals.