The big idea: When a solute (like salt or sugar) dissolves in water, the water molecules gather around each solute particle and hold it in solution — this is called solvation.
Because some of the water is now busy surrounding solute particles, a concentrated solution effectively has fewer free water molecules than a dilute one.
Osmosis is the net movement of water across a partially permeable membrane, and it always moves water from where there is more free water to where there is less.
Water moves by osmosis from a higher water potential (dilute solution) to a lower water potential (concentrated solution): into a cell in a hypotonic solution, out of a cell in a hypertonic solution, with no net movement in an isotonic one.
Interactive diagram
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- Solvent
- The liquid that does the dissolving. In living things the solvent is almost always water.
- Solute
- A substance dissolved in the solvent (for example salt, sugar or glucose).
- Solvation
- The process in which water molecules surround and separate each dissolved solute particle, holding it in solution.
- Partially permeable membrane
- A membrane that lets water molecules pass through but blocks (most of) the larger dissolved solute particles.
- Osmosis
- The net movement of water molecules across a partially permeable membrane, from a region of higher water potential to a region of lower water potential.
- Water potential
- A measure of how freely water can move out of a solution. Pure water has the highest water potential; adding solute lowers it.
What 'water potential' really tells you: Think of water potential as a ranking of how 'free' the water is to leave.
Pure water = the highest water potential.
Add solute → water potential drops (the more solute, the lower it goes).
So a dilute solution has a higher water potential than a concentrated one — and water always flows from high to low water potential.
Osmosis is not random — it has a direction, and water potential is what sets it.
Water always moves from the side with the higher water potential (the more dilute side) to the side with the lower water potential (the more concentrated side).
Another way to say the same thing: water moves toward the side with more solute, spreading itself out until both sides have the same water potential.
The rule, in one line: Water moves by osmosis from a higher water potential to a lower water potential — that is, from the dilute side toward the concentrated side.
It keeps moving until the two sides are equal (isotonic), at which point there is no net movement — water still crosses both ways, but in equal amounts.
Higher water potential
- The dilute side (less solute)
- More free water molecules
- Water tends to leave this side
- Closest to pure water
Lower water potential
- The concentrated side (more solute)
- Fewer free water molecules
- Water tends to enter this side
- Furthest from pure water
| Side of the membrane | Solute concentration | Water potential | Net water movement |
|---|---|---|---|
| Dilute side | Low solute (lots of water) | Higher water potential | Water leaves this side |
| Concentrated side | High solute (less water) | Lower water potential | Water enters this side |
| Overall | — | Higher → lower | Water moves toward the more concentrated side until water potential is equal |
The conditions osmosis needs: For osmosis to happen across a membrane, two things must be true:
1. A partially permeable membrane — it must let water through but hold back the solute.
2. A difference in water potential — the two sides must differ in solute concentration, so there is a gradient for water to move down.
If the solute could cross freely, or if both sides were already equal, there would be no net osmosis.
| Condition for osmosis | What it means | What happens without it |
|---|---|---|
| A partially permeable membrane | A membrane that lets water through but blocks (most of) the dissolved solute | If solute could cross freely, the two sides would simply mix — there would be no osmosis |
| A difference in water potential | The two sides differ in how dilute they are (a water concentration gradient) | If both sides are equal (isotonic), there is no net movement of water |
| Liquid water on both sides | Water molecules must be free to move across the membrane | With no free water there is nothing to move by osmosis |
A memory hook: Water follows the solute. Wherever the solute is more concentrated, that side has the lower water potential, so water moves toward it.
And osmosis is passive — it needs no energy (ATP); the water simply moves down its own concentration gradient.
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How this is tested: A very common Paper 3 data question gives a membrane experiment and asks you to outline the conditions required for osmosis to take place across a membrane — worth 2 marks, so give two separate points: a partially permeable membrane and a difference in water potential (a solute-concentration gradient).
On Paper 1B the same idea is dressed up as a data question: you are given two solute concentrations, or a graph of how a tissue's mass changed, and asked to predict or explain which way the water moved.
The key always comes back to one rule — water moves from a higher to a lower water potential (dilute → concentrated).
IB-style question — outline the conditions for osmosis
A scientist sets up a glass tube with a sugar solution separated from pure water by a membrane. Outline the conditions required for osmosis to take place across the membrane. [2]
How to score both marks
- State the membrane condition. There must be a partially permeable membrane — one that lets water molecules pass through but holds back (most of) the dissolved solute.
- State the gradient condition. There must be a difference in water potential (a difference in solute concentration) between the two sides, so water has a gradient to move down. (Award 1 mark for the partially permeable membrane; 1 mark for the water-potential / concentration difference.)
Final answer
Osmosis needs a partially permeable membrane and a difference in water potential (solute concentration) between the two sides.
✓ Why this scores full marks: Both required conditions are given as separate points — the membrane and the gradient.
A 2-mark 'outline' needs two distinct scoring ideas, not one idea (such as 'a membrane') written two ways. Naming the membrane as partially permeable (not just 'a membrane') is what earns the first mark.