Diffusion vs Osmosis: Worked Examples and Practice

A membrane separates two sugar solutions. Water can cross; sugar can't. Drawing a sugar arrow toward the more dilute side looks reasonable until you notice that the membrane blocks its path. The concentration difference alone doesn't tell you what can move.

For diffusion vs osmosis questions, write down the moving substance before drawing the arrow. Then check whether it can cross. The examples below use that small habit to separate a correct prediction from a lucky guess.

What the two terms mean

In the simple solutions below, diffusion produces net movement from higher to lower concentration of the moving substance. That difference is its concentration gradient. Diffusion can happen with or without a membrane. Osmosis is a special case involving water crossing a selectively permeable membrane, which lets some substances through and blocks others. Both processes are passive: the cell needn't spend ATP to power them. OpenStax's passive transport chapter explains the relationship.

Net movement describes the overall balance. Molecules still travel both ways. At dynamic equilibrium, those opposing movements balance on average; the molecules haven't stopped.

For all examples, assume dilute solutions at the same temperature and initially equal pressure, with no bulk flow. The only solute (dissolved substance) is the named neutral substance, which doesn't split into ions or react. We're predicting initial net movement, before concentrations or pressures change appreciably. Membrane permissions are stated separately.

Work through three setups

These are illustrative scenarios, not reported experiments. For each, make a four-part record: substance → permission → initial net direction → reason.

A dye spreads without a membrane

Imagine a neutral dissolved dye concentrated at one end of a narrow, still water-filled channel. There is no partition. Ignore currents and stirring.

Your record is: dye → unobstructed → away from the concentrated region → dye concentration is lower elsewhere. This is diffusion. Calling it osmosis because water is present identifies the solvent but misses the moving substance and the required membrane.

If someone stirs the channel, they add bulk mixing. Don't attribute everything that happens after stirring to diffusion alone.

Sugar cannot cross, but water can

Side A contains 0.04 mol/L sucrose; side B contains 0.12 mol/L sucrose. The membrane passes water and blocks sucrose. A mole is a measure of particle amount; mol/L tells you how much is dissolved per liter of solution.

Substance	Can cross?	Initial net direction	Reason
Water	Yes	A → B	At equal pressure, B's higher sucrose concentration gives it lower water potential
Sucrose	No	No crossing	The membrane is impermeable to sucrose

The water movement is osmosis. Water potential describes water's tendency to move: net movement is toward lower water potential. At equal pressure, adding sucrose lowers it. A's remaining solution becomes more concentrated as it loses water, even though no sucrose enters A. B becomes more dilute as it gains water. The number of sucrose molecules on each side stays unchanged.

Don't extend that initial arrow into a claim that water must move until both sugar concentrations match. A pressure difference can oppose and eventually balance the osmotic effect. In this setup, water potential accounts for both solute and pressure. OpenStax's water transport chapter explains how those effects combine.

The missing membrane information matters

Now a worksheet gives A = 0.04 mol/L and B = 0.12 mol/L of substance X, but says only “a membrane separates them.”

You can't complete the permission column. Ask whether water and X can cross. If water passes and X doesn't, the previous reasoning applies. If X passes, its own initial net diffusion would be B → A. If neither passes, neither crosses. “Selectively permeable” alone still doesn't tell you which substances pass.

Eight questions before you look at the answers

Keep the earlier temperature, pressure, and solute assumptions unless a question explicitly changes them. For direction questions, name the substance and give a reason. Use paper; you don't need an app.

1. A neutral dye is unevenly distributed in still water with no membrane. Is its spreading diffusion, osmosis, or both?
2. A has 0.02 mol/L sucrose and B has 0.09 mol/L. Only water crosses the membrane. Predict initial net movement for water and sucrose separately.
3. Reset question 2, but exchange A's and B's solutions. Does the water arrow change?
4. Both sides contain 0.06 mol/L sucrose. Only water crosses. Does zero net movement mean no water molecules cross?
5. A has 0.03 mol/L X and B has 0.08 mol/L X. X is neutral and can diffuse across the membrane. Predict X's initial net direction. Is that movement osmosis?
6. A question supplies unequal sucrose concentrations but no membrane permissions. A student draws a water arrow. What information is missing?
7. A flexible model cell contains 0.10 mol/L sucrose; its surroundings contain 0.04 mol/L. Its membrane passes only water. Does it initially gain or lose water, and is the outside hypotonic or hypertonic relative to the cell?
8. A water-permeable, sucrose-impermeable membrane separates unequal concentrations. The more concentrated side now has enough extra pressure to exactly balance the osmotic effect. Must net water movement continue?

Answers and reasoning

1. Diffusion. The question tracks dye spreading, with no selectively permeable barrier for osmosis.
2. Water: A → B. Sucrose: no crossing. B has the higher sucrose concentration, but sucrose itself lacks permission to cross.
3. Yes: B → A. The more concentrated solution is now in A. The letters don't determine direction.
4. No. Water crosses both ways at equal average rates. Zero net movement describes the balance, not individual molecular motion.
5. X: B → A, by diffusion. Compare X's concentrations. A solute crossing a membrane isn't osmosis.
6. Whether water and sucrose can cross. Concentrations don't establish permeability. You can give a conditional answer: “If only water passes, its initial net movement is toward the higher sucrose concentration.”
7. It gains water; the outside is hypotonic. Under these assumptions, the outside has less solute that cannot cross the membrane. Hypertonic surroundings would instead produce water loss; isotonic surroundings produce no sustained volume change. Tonicity describes an outside solution's effect relative to a particular cell, not a label that concentration carries alone. The HACC osmosis lab uses this relative comparison.
8. No. Pressure balances the osmotic effect, so net water movement is zero despite unequal sucrose concentrations. Water still crosses both ways. This question deliberately removes the equal-pressure assumption.

Make cards for the errors you repeat

If you missed a direction question, first redo it with the sides swapped. Then save the reasoning you forgot. These cards keep enough conditions on the front to have a definite answer.

Front	Back
Dilute sucrose solutions: A = 0.02 mol/L; B = 0.09 mol/L. Same temperature, equal pressure, no bulk flow; only water crosses. Initial net water direction?	A → B. B has lower water potential under these conditions.
Water crosses a membrane at dynamic equilibrium. What is zero?	Net transfer, not molecular movement. Opposing transfers balance on average.
A sugar gradient exists across a membrane. What must you check before drawing a sugar arrow?	Whether that membrane permits sugar to cross.
Can unequal sucrose concentrations coexist with zero net osmosis across a water-only membrane?	Yes. An opposing pressure difference can balance the osmotic effect.

Keep only the cards that address your mistakes. The practice-question flashcard workflow helps turn a missed explanation into a focused prompt. On your next attempt, write the permission column before the direction column and check whether that fixes the error.