GCSE Biology - AQA

3.2.4 - Beyond The Syllabus: Advanced Osmosis Information

Beyond The Syllabus: Advanced Osmosis Information

The previous page explained everything you need to know about osmosis for your GCSE exams.

This page contains some more advanced information about osmosis. You do not need to know any of the information on this page for your GCSE exams, so feel free to skip it. It is only included here for interest.

Any solvent can move by osmosis

It is not just water that can move by osmosis. In fact, any solvent can. In order for osmosis to happen all that is required is that there are two solutions with different solute concentrations separated by a membrane which is permeable to the solvent but not to the solute.

(Note: osmosis is also affected by differences in hydrostatic pressure between the two solutions, so the situation described above does not guarantee that osmosis will happen).

In biology, almost all solutions have water as the solvent. Therefore, in biology, it is almost always water that is moving by osmosis rather than some other solvent. This is why you are taught in GCSE biology that osmosis is something that only water does.

However, strictly speaking, it is incorrect to say that only water can move by osmosis. Therefore, a more accurate definition of osmosis would be the one given below.

A move accurate definition of osmosis (DO NOT WRITE THIS IN YOUR EXAMS!!):

Osmosis is the net movement of a solvent through a partially permeable membrane (which is permeable to the solvent but not the solute) from the side with the lower solute concentration to the side with the higher solute concentration.

Why osmosis cannot be explained in terms of water concentration

Some textbooks and videos state that osmosis happens because the concentration of water is different on the two sides of the membrane. This is incorrect.

To understand what is wrong with the 'water concentration' explanation for osmosis, we have to start by thinking about what concentration is.

As explained earlier in the course, concentration is a measure of how closely packed together the particles of a substance are. It is usually defined as the number of particles of the substance per unit of volume.

Diagram illustrating the relationship between concentration and volume. Two cubes are shown - one on the left labelled "Low Concentration" and one on the right labelled "High Concentration". The low concentration cube contains a small number of red circles, with large spaces between them. Underneath it, there is a label that says "Small number of particles in a volume of 1 litre". The high concentration cube contains a large number of red circles closely packed together. Underneath it, there is a label that says "Many particles in a volume of 1 litre".

Concentration is often defined as the number of particles of a substance per litre.

Therefore, in order to measure the concentration of a substance, we need to know two things:

  • The number of particles of the substance that are present
  • The volume of space that those particles occupy

Think about what happens when you take pure water and dissolve some solutes in it (or when you take an existing solution and dissolve even more solutes in it).

Clearly, the number of solute particles present increases, since we have added more solute.

The number of water particles present, on the other hand, stays the same. No water has been added or removed.

What happens to the volume of the solution?

Many people assume that the volume increases since there is more matter present. However, it is more complicated than that.

The water molecules in a solution have spaces between them. As solute particles are added, they go into these spaces between the water molecules. The interactions between the water molecules and the solute particles might cause the water molecules to get very slightly closer together or very slightly further apart (depending on the nature of the solute). However, in either case, the change is usually very, very small.

Therefore, when solute is dissolved in water, the volume might very, very slightly increase or decrease. However, this change in volume is so small that it is negligible. So, we can effectively say that the volume does not change.

Diagram about concentration and volume. The title at the top is "Dissolving solutes in a liquid does not usually have much effect on its volume". Under this there is an illustration of the formation of a solution. This starts, on the left, with a diagram of a beaker containing water. This is labelled, "1 litre of water". Then there is a plus sign. Then a drawing of two spoonfuls of sugar, labelled "Sugar". Then an arrow, then drawing of the same beaker but now with sugar molecules spread out within the water. It is labelled "1 litre of solution".

The volume of a solution is usually roughly the same as the volume of the solvent. Therefore, adding more solutes to water does not significantly affect the concentration of the solution.

So to summarise, when solute is dissolved in water or more solute is dissolved in an existing solution:

  • The number of solute particles present increases
  • The number of water molecules present stays the same
  • The volume of the solution (effectively) stays the same

Since there are now more solute particles in the same volume, the solute concentration has increased.

However, there are still the same number of water molecules in the same volume. Therefore, the water concentration has not changed!

This means that even if two solutions have very different solute concentrations, they will both have the same water concentration.

Osmosis cannot be due to a difference in water concentration between the two solutions, because there is no difference in water concentration! It is only the solute concentration that varies.

(It is actually possible to calculate the concentration of water if you know how to do mole calculations. Water has a density of 1 kg/dm3, which means that 1 dm3 (1 litre) of water has a mass of 1kg (1000g). Water has a relative formula mass of 18.0 (to 3sf), which means that the amount of water molecules in 1kg of water is 1000 รท 18 = 55.6 moles (to 3sf). Therefore, the concentration of water is 55.6 mol/dm3).

The actual explanation for osmosis is complicated and is beyond the scope of this course. For GCSE biology, you do not need to be able to explain why osmosis happens, you simply need to know the definition and be able to use it to predict which way water will move by osmosis in a given situation.


Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.


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