Diffusion and Osmosis


Diffusion is the movement of water from an area of higher concentration to an area of lower concentration down a concentration gradient.

Diffusion shows that a system tends to move from a state of order to a state of disorder if left alone. Because of the random motion of molecules in a liquid or gas (Brownian Motion), a concentrated solution in a liquid or scent in the air will spread out to fill the entire container or room.

Diffusion can be demonstrated experimentally by many different methods. In this post we will examine diffusion in a liquid. You will need a clear container filled filled with water, food colouring and a timer. The darker the colour the better.

To start, using a dropper or any other suitable method, place a drop of food colouring in the liquid. Start the timer and note the time it takes for the drop to spread to the other side of the container.

After you have made note, repeat the experiment with the water after it has been warmed.

What you should notice, is that when the water is warm, diffusion should occur faster. This is as a result of the particles of water moving faster thereby moving the molecules of food colouring faster.

Diffusion in the air will happen in a similar manner. A perfume may be used as the scented object. The warmer the air the faster the scent will spread throughout the room, the colder the air the the slower it will spread.


Osmosis is the movement of water from a an area of lower concentration of solute to a higher concentration of solute across a semi-permeable membrane.

This process can be demonstrated experimentally with the use of a potato, salt and water. This can be done at home. You will need: two plates (ensure that the edges of the plate are elevated to prevent water overflow), one raw potato, a knife, a tablespoon, salt and water.

Lay out the plates and fill them both with the same amount of water. Measure a tablespoon of salt and sprinkle it into one of the dishes. Ensure that you label the dishes correctly so you know which has salt and which doesn’t.

Slice the potato in half and place both halves face down in either dish. Allow about three to four hours for the potatoes to soak in their respective dishes.

After some time, you should notice that the potato which was soaking in the water has now become more dense, and it may or may not become whiter. This is because the potatoes have absorbed the water. The potatoes which were soaking in the salt solution should be shriveled and much softer in comparison to the potato that was soaking in the water. This is because osmosis occurred via the water leaving the cells in the potato. The water in which the potato was soaking had a higher solute concentration than what was in the potato. So water moved to where the solute concentration was highest across the semi-permeable membrane of the potato (the cell wall) from where the solute concentration was the lowest.

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