Solutions: common misconceptions

If your child says… “A percentage compares sugar with just the water.”

Ask together

Does the dissolved sugar contribute to the solution's total mass?

What’s true

Percentages and parts per million compare with the whole solution, using the stated mass or volume basis.

If your child says… “A bigger Henry's constant means more gas dissolves.”

Ask together

At the same pressure, does dividing by a larger constant give a larger answer?

What’s true

Dissolved mole fraction equals the gas's partial pressure divided by Henry's constant.

If your child says… “Just add the two pure-liquid pressures.”

Ask together

Does either liquid occupy the whole mixture?

What’s true

Each liquid's contribution must first be weighted by its liquid mole fraction.

If your child says… “Enough distillation will separate an azeotrope.”

Ask together

How could collecting vapour enrich a liquid if both have the same composition?

What’s true

At an azeotrope, boiling provides no change in composition to exploit.

If your child says… “Divide the pressure fall by the solution's pressure.”

Ask together

Should a relative fall be compared with the original pressure or the reduced pressure?

What’s true

Relative lowering compares the pressure fall with pure-solvent pressure at the same temperature.

If your child says… “Adding solute raises the freezing point too.”

Ask together

For a freezing-point drop, should we add to or subtract from the pure solvent's freezing temperature?

What’s true

A non-volatile solute raises boiling temperature but lowers freezing temperature in this dilute solution.

If your child says… “The solute crosses the membrane while water stays put.”

Ask together

Which particles cross the membrane in the diagram?

What’s true

The membrane lets solvent cross, while solute stays on the solution side.

If your child says… “Splitting particles makes the calculated molar mass too large.”

Ask together

If splitting suggests more moles, what happens to mass divided by moles?

What’s true

Dissociation increases the measured effect, so ignoring it makes the inferred molar mass too small.