Osmosis – The Solution to a Concentration Problem

Throughout my career I have had the privilege to observe a large volume of science lessons.  Two key stage 3 and key stage 4 topics fill me with a sense of dread in those lessons – osmosis and evolution (perhaps a future blog).  The dread stems from my experience that many teachers, with the best intentions, overcomplicate their teaching and confuse the students – all in the name of keeping it simple.  I have had the conversation below with new and experienced teachers, subject specialists and non-specialists.  I probably should have written this a long time ago.

Before the solution, first the problem

The phrase “osmosis is the movement of water from high concentration to low concentration through a partially-permeable membrane” is the problem, specifically “water from high concentration to low concentration”.  Before we go further, yes most exam boards will accept this as a definition of osmosis (but only if worded very carefully and only as an accept… rather than the main marking point) and yes some GCSE biology textbooks use that definition word for word.  Shame on the exam boards and shame on the publishers.

Why do teachers phrase it this way?  Some will see it in the textbooks and mark schemes and just go with it.  Many phrase it this way because they have just taught diffusion (movement of particles from high concentration to low concentration) and are building on that knowledge in their osmosis lesson.  Most key stage 4 schemes do not explicitly include the vocabulary of solutions (solute, solvent, solution, dilute, concentrated) so teachers use the idea of concentration of water as a shortcut to avoid reviewing these terms.

Why is it a problem?  Because it makes absolutely zero sense and requires teachers to invent a new, non-specification definition of concentration.

Why doesn’t it make sense?  When you dissolve a solute in water, you do not change the number of water molecules, therefore you do not change the “concentration” of water molecules.  The volume of the solution does not change (and definitely doesn’t decrease), therefore you do not change the concentration of water molecules.  The “concentration of water” remains constant.  If the “concentration of water” is the same on both sides of the partially-permeable membrane, there will be no net movement of water (which would make osmosis pretty boring).

“Aha”, I hear them say, “but that’s not what we mean by concentration!  We mean the relative number of water molecules compared to the number of solute molecules.  If you define it that way, then you can have higher and lower water concentrations!”  You may think this is an exaggeration or strawman, but it is something I have heard, seen on popular revision websites and on a prominent government funded tree based academy website.

If, in your desire to “keep things simple” you are forced to invent a new definition for an important keyword, a definition that contradicts the one provided in the specification*, then you have gone badly wrong. 

It is a shame that we don’t have terms to describe the situation where there are changes to the relative number of water and solute molecules.  That would simplify everything and get us out of this homonym quagmire.  Oh wait, we do – “dilute solutions” and “concentrated solutions”.

Why didn’t we just use “dilute solutions” and “concentrated solutions” in our definition to begin with?  Almost certainly because we wanted to build on the diffusion lesson and the keyword of “concentration” and didn’t want to overcomplicate or add to the cognitive load of our students by introducing the words “dilute solution” and “concentrated solution”.  I fear it is a common mistake to overcomplicate in our attempts to keep things simple.

Please, if not for me then for the poor potatoes, don’t say “concentration of water”.

Now the diagrams…

Please, if you are drawing diagrams showing water molecules and solvent molecules so you can draw an arrow to show the direction of net flow of water… MAKE SURE THE NUMBER OF WATER MOLECULES IS THE SAME ON BOTH SIDES!

This, the first image that came up on Google images, is an example of what not to do:

First, the reality problem:  In the cup on the left, how is it possible that the water level is the same height if one side has twice as many water molecules as the other?

Second, the pedagogy problem: It is too difficult for students to work out which side of the solution is more concentrated, and which is more dilute.  You are expecting them to count the water and salt molecules (ignoring the “salt molecule” problem), then work out the ratio of each side (holding all of that information in their heads).  Not to mention that the number of water molecules is irrelevant (except of course that it affects the volume of the solution).

An even better approach is to not draw the water molecules at all and just draw the solute.  Something like this:

Now it is immediately obvious which side of the left beaker has the more concentrated solution.  We don’t need to worry about ratios or different coloured particles.  The focus is entirely on the concentration of the solution.  Let’s try and improve the diagrams further.  If I were drawing it, I would use fewer particles.  You’ll inevitably get students counting them to see if they remain the same (time better spent on something else) and you’ll likely get a student who miscounts and then asks where they went/came from – causing the entire class to start arguing about how many particles it started and ended with.

(Also, for both diagrams, I’d use the words “partially-permeable” instead of “semi-permeable”).

In summary, for the love of Poseidon, don’t use the words “water concentration” or “concentration of water” when defining or talking about osmosis.  Don’t draw the water molecules when diagraming osmosis.  And don’t overcomplicate your lessons by inventing new keyword definitions, in an effort to keep things simple.

*The GCSE biology specifications do not include a definition of concentration, but luckily the chemistry specifications do.  For anyone who wants to claim that the chemistry spec definition doesn’t apply to biology, please explain the combined science specification.  Note that none include the idea of relative masses or moles of the solvent compared to the solute.

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