By which method is a solute obtained from a solution?

A solution is a homogeneous mixture in which the solute is dissolved uniformly at the molecular or ionic level throughout the solvent, so it cannot be separated by any technique that relies on particle size, density, or settling. Recovering the solute from a solution requires removing the solvent itself, most commonly by evaporation, where heating vaporizes the solvent and leaves the dissolved solute behind as a solid residue.

Here the mixture given is a solution, so the solute is spread evenly through the solvent and does not exist as separate particles that can settle, be poured off, or be trapped by a filter. Heating the solution drives off the solvent as vapor; once all the solvent has evaporated, only the solute remains behind as a residue. Evaporation is therefore the method that yields the solute from the solution.

Contrast with the other techniques:

• Sedimentation lets a denser, insoluble solid settle to the bottom under gravity; it separates a suspension, not a solution, so it cannot act on a dissolved solute.

• Decantation simply pours off the liquid above an already-settled solid; it needs the solid to be undissolved and separate, which is not true of a solute in solution.

• Filtration passes the mixture through a porous medium that traps particles larger than its pores; a dissolved solute exists at the molecular or ionic scale and passes straight through any filter along with the solvent.

Because only evaporation removes the solvent rather than the solute, it is the technique that isolates a solute from a solution.