Dissolving pure sugar in pure water supplies many mobile ions, making the solution a good electrical conductor.

Sodium chloride dissolves in water until its grains are no longer visible. What happened to the dissolved salt?

• Its sodium and chlorine atoms changed into water molecules
• Its ions separated and became surrounded by water
• Its particles stopped existing when the grains became invisible

Oil is mostly nonpolar. Why does it form a separate layer instead of mixing evenly with water?

• Oil-water contacts do not favour extensive mixing
• Oil is denser than water, so its molecules cannot mix
• Water changes oil into charged particles which float away

In a supervised sodium acetate 'hot ice' demonstration, a cooled supersaturated solution crystallises after a seed crystal is added. What does the seed do?

• It removes enough heat to freeze all the water in the solution
• It turns the dissolved sodium acetate into a different substance
• It supplies a starting surface where crystals can grow

A nonpolar solid is poorly soluble in water. Which solvent type is usually a better choice to try, if no chemical reaction occurs?

• A polar solvent, because opposite polarities often favour mixing with nonpolar solutes
• Salt water, because its ions often improve contacts with poorly soluble nonpolar solutes
• A nonpolar solvent, because similar interactions often favour mixing

A solubility curve gives 70 g at 80°C and 40 g at 30°C per 100 g water. A saturated solution with 100 g water cools from 80°C to 30°C without water loss. Crystals form and equilibrium is reached. The crystals contain no water. How many grams of solute crystallise?

Two unknown solutions contain either 5 g sodium chloride or 5 g sugar per 100 g distilled water, at the same temperature. Both are fully dissolved. Which supervised test distinguishes them?

• Compare total masses: the sugar solution must weigh more
• Use a battery-powered conductivity tester: salt water conducts better
• Filter both solutions: dissolved salt stays on the filter paper

A student sees sugar grains remain after stirring sugar into oil and concludes that sugar cannot dissolve in any liquid. What is the problem with that conclusion?

• One solvent test cannot establish insolubility in all solvents
• Heating any solvent always dissolves every solid added to it
• Remaining grains prove that sugar became a different substance