Lesson 2 of 4

Osmosis.

About 20 minutes · 5 practice questions

When only the water can cross: which way it flows, what happens to the concentrations and volumes, and when the flow stops.

Step 1 of 5 · Only water

Step 1 of 5

Only water

The same tank, but a different membrane: its channels are so narrow that only water molecules, the small ones, get through. The sugar molecules are too big. A membrane like that, which lets some substances through and not others, is called partially permeable. The cell membrane is one.

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Step 2 of 5

What osmosis is

Look inside is open. The blue arrow shows the net flow of water, and the amber sugar molecules each stay on their own side.

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Step 3 of 5

The concentration at the end

The water does not flow for ever. Every moment water comes into the left side, it becomes more dilute, and the right side, losing water, becomes stronger. The marks on the tank wall show the liquid levels.

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Step 4 of 5

How much water

Concentration is the amount of solute divided by the volume. When the solute cannot cross, the amount on each side is fixed, so every change in concentration is a change in volume.

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Step 5 of 5

In balance

Right side is free, and holds pure water. The left side stays at 300 mOsm/L.

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Interactive scene

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Each step sets up the Scene for you, and a control marked “fixed” stays put for that step.

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Practice · Predict, then look

Now the membrane lets only water through. The left holds 300 mOsm/L of sugar, the right 100 mOsm/L. What will happen to the liquid levels?

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Practice · Choose

What is osmosis?

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Practice · Calculate

The left holds 300 mOsm/L, the right 100 mOsm/L, and only water crosses. What will the concentration on the left be, in mOsm/L, once the water stops flowing?

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Practice · Calculate

In the same experiment: how many times bigger has the left side’s volume become by the time the water stops?

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Practice · Reach the target

The left holds 300 mOsm/L, and the right pure water. Change Right side until there is no net flow of water either way, then press Check.