To separate, filtration would work best. Sand particles and water molecules differ drastically in size. So when filtered, the sand would not flow out of the device as the water would. This will effectively separate the water and sand with the smallest amount of material lost.
Depends, does the teacher just care about the water and sand being separated or does the teacher want you to separate them and still have both substances in their original form. If the teacher just cares about the separation I'd use evaporation. It was still water and sand the only thing is the water changed forms and became a gas. If the teacher wants both substances in their original state then I'd use filtration. You can separate the sand from the water and still have them both.
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