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
2
Freezing point and osmosis
Integrated Science Sec 1 Term 1, Unit 1: pp. 28–32
GoalExplain why a solution freezes below 0 °C, and predict the direction of osmosis.
New words
freezing point · the temperature at which a liquid turns into a solidsemi-permeable membrane · a thin layer that lets water pass but holds back most dissolved particlesosmosis · water crossing a semi-permeable membrane from the side with less dissolved material to the side with moreosmotic pressure · the pressure that arises from a difference in concentration and drives osmosis
Explanation
A solution freezes at a lower temperature than pure water, always, because the dissolved particles get in the way of water molecules joining into ice crystals. Sea water freezes at about −1.9 °C (extra fact). On icy roads in cold places, salt is spread so that rain does not turn into ice at about 0 °C; this works only down to a certain cold. Osmosis is water crossing a semi-permeable membrane from the side with less dissolved material (the dilute solution) to the side with more (the concentrated one). The pressure that drives it is the osmotic pressure, which arises from the difference in concentration. Remember: water moves, not the solute, and it moves towards the more concentrated side, so the concentrations tend to even out. Living cells are surrounded by a semi-permeable membrane, so osmosis happens in every cell.
dilute solution (lots of water)osmosis→water crosses the membrane→concentrated solution (more solute)
Worked examples
A membrane separates a 2% sugar solution (side A) from a 10% sugar solution (side B). Water moves from A to B, because A is more dilute. Side B’s level rises slowly. The sugar does not cross.
In
Water moves
The raisin
plain water
into the raisin
swells
very salty water
out of the raisin
shrivels
A raisin is put in plain water and another in very salty water overnight. In plain water the inside of the raisin is more concentrated than the water, so water moves in and it swells. In very salty water the water outside is more concentrated, so water moves out and it shrivels.
Why is salt spread on an icy road not a way to melt ice at any temperature? Salt only lowers the freezing point, by a limited amount that depends on how much dissolves. When it is far colder, the salty water still freezes.
Class activity
Salt on ice, and raisins, with a grown-up. Put one ice cube on each of two plates and sprinkle a pinch of salt on one. After a few minutes describe which melts faster (touch only, do not taste). Then soak a few raisins or dried chickpeas in plain water and others in very salty water overnight and describe how they look next morning. Explain each result in one sentence using freezing point and osmosis.
Practice
1
Is the freezing point of sea water above, below or equal to that of pure water?
Below it (about −1.9 °C).
2
Side A is 5% salt solution and side B is 1% salt solution, separated by a semi-permeable membrane. In which direction does water move?
From B to A, from the dilute to the concentrated solution.
3
What moves in osmosis, the water or the solute?
The water.
4
Why can salt on a road stop rain from freezing at 0 °C?
It lowers the freezing point of the water, so it stays liquid at 0 °C.
