☰ Year plan · Science

Solutions, osmosis and the kinds of cells

Week 5 · 1-4 Properties of Aqueous Solutions: solution, solvent and solute, colligative properties (lower vapour pressure, higher boiling point, lower freezing point, osmosis and osmotic pressure); start of 1-5 Biological Importance of Water: water in cells, prokaryotes and eukaryotes, the four kingdoms · 3 lessons
1

Dissolved solutes: vapour pressure and boiling point

Integrated Science Sec 1 Term 1, Unit 1: pp. 28–32
GoalExplain why dissolved particles lower vapour pressure and raise the boiling point, and what boiling point depends on.
New words
solution · a uniform mixture of a solvent (in nature usually water) and one or more solutescolligative properties · properties of a solution that depend on the number of dissolved particles, not on what the solute isvapour pressure · the pressure of the vapour above a liquid when evaporation and condensation balanceboiling point · the temperature at which the vapour pressure of a liquid equals the pressure above it
Explanation

Water in lakes and oceans is never pure: it is a solution, with salts and other solutes dissolved in the solvent water. Some properties of a solution depend only on how many dissolved particles it holds (the solute does not evaporate easily), not on what the particles are. These colligative properties are a lowered vapour pressure, a raised boiling point, a lowered freezing point and an osmotic pressure. In a closed flask, molecules leave the liquid and return from the vapour at equal rates: this is dynamic equilibrium, and things still happen, but the amounts stay steady. The vapour then exerts the vapour pressure. Dissolved particles take up part of the surface and attract water molecules, so fewer water molecules escape and the vapour pressure is lower, in proportion to the number of particles. A liquid boils when its vapour pressure equals the pressure above it. Pure water boils at 100 °C at standard pressure, at a lower temperature on a mountain and at a higher one in a pressure cooker. A solution has a lower vapour pressure, so it must be hotter to boil: at standard pressure it boils above 100 °C. Salt does not make water boil faster; it raises the boiling point only slightly.

Worked examples
Why do water molecules still “move” at equilibrium? In a closed flask, the number leaving the liquid each second equals the number returning from the vapour, so the amounts of liquid and vapour stay constant, while individual molecules keep moving.
HeightBoiling point, about
0 m100 °C
1500 m95 °C
3000 m90 °C
Estimate the boiling point at 1500 m. A rough rule (extra): the boiling point of water drops about 1 °C for every 300 m. 1500 ÷ 300 = 5, so water boils at about 100 − 5 = 95 °C. It is only a rough rule. Why? The air pressure is lower, so the vapour pressure reaches it sooner.
Which has the bigger effect on vapour pressure: 0.1 mol of sugar or 0.1 mol of NaCl in the same amount of water? One NaCl gives two ions (Na⁺ and Cl⁻), so 0.1 mol gives 0.2 mol of particles, about twice as many as the sugar, which does not split. NaCl has the bigger effect.
Class activity

Surface dots, on paper. Draw a square surface of 20 dots (5 × 4) to be water molecules at the top of a liquid. Now redraw it with 6 of the dots replaced by crosses for solute particles. Count how many water dots remain at the surface and work out the share: 14 out of 20. Write one sentence on what this means for how many water molecules can escape, and for the vapour pressure.

Practice
1
At what temperature does pure water boil under standard atmospheric pressure?
2
Why does water boil at a lower temperature on a high mountain?
3
Which solution has the lower vapour pressure: one with 0.3 mol or one with 0.1 mol of dissolved particles in the same water?
4
True or false: salt makes water boil faster. Correct it if it is false.