Two chemistry inquiries
$1000 \, \text{g}$ of water occupy a volume of (approximately) $1 \, \text{L}$ because the density of water is (approximately) $1000 \, \text{g/L}$.
If you had $1000 \, \text{g}$ of mercury, whose density is $13534 \, \text{g/L}$, it would occupy a volume of $0.0739 \, \text{L}$.

Usually you can only dissolve so much of a solute into a solvent until a certain threshold is reached.
When this threshold is reached you have a saturated solution.
After that point, it is thermodynamically more favourable (under the given conditions, e.g. temperature, pressure, etc.) for any extra solute not to get dissolved in the solvent but clump together and precipitate.
But if you use some special techniques you can prepare a supersaturated solution that contains more solute than it should, thermodynamically speaking.
A supersaturated solution is a meta-stable state, meaning that it is only a local energy minimum for the system, not the global minimum.
But it would like to get to the global minimum if it gets the chance.
The only thing preventing that, is a small energy barrier that has to be surmounted.
So, if you disrupt the supersaturated solution in any way, like shaking it or adding a crystal seed, the solution can surmount the energy barrier and attain its global minimum state, i.e. the saturated state.