Question

To make a solution for an experiment, Gunther needs to add 40 g of a solute to 100 g of water. When making the solution at room temperature, he could only add 34 grams before the solute settled out.

What could he do to dissolve the remaining 6 grams of the solute?

Put the solution in an ice bath, dissolve the solute, and let the solution return to room temperature.
Heat the solution, dissolve the solute, and let the solution cool verifying nothing settled out.
Add more water, boil the solution, and dissolve the solute until the some of the water evaporates.
Keep the solution at room temperature, add more water, and dissolve the excess solute

Answer 1

Correct choice: Keep the solution at room temperature, add more water, and dissolve the excess solute.

Reason: at room temperature the solubility is 34 g per 100 g water, so to dissolve 40 g you need
water mass = 100*(40/34) ≈ 117.6 g. So add about 17.6 g (≈18 g) more water and the full 40 g will dissolve.

Why the others are wrong:
- Cooling (ice bath) lowers solubility, so even less will dissolve.
- Heating can dissolve more while hot but on cooling the extra will usually precipitate (unless you make a metastable supersaturated solution).
- Adding water then boiling/evaporating defeats the purpose: evaporating water will concentrate the solution and cause precipitation on cooling.