There are three states of mater; solid liquid and gas. The sold state is the difficult to compress while the gaseous state is quite easy to compress.
A gas is easily compressed because the particles in a gas are far apart from each other. A solid is difficult to compress because the particles of a solid are close together. From all the above statements, it is easily deducible that the compressibility property of a substance in a particular state of matter depends on the proximity of the particles to each other, hence the answer above.
You need to monitor the solution to see when the crystals start gathering at the bottom of the solution. You can slowly add alum until it stops dissolving. The point just before it starts its sedimentation at the bottom is the point when the solution is saturated.
