I still remember learning about osmosis at school many years ago. I don’t know why that particular memory has stayed with me so strongly, maybe because it was hard to understand or maybe because that was the year where I discovered my love of biological science which led to my study choices later on. Whatever the reason, osmosis is a term I’ve never forgotten the meaning of.
Osmosis is when water moves from a less concentrated solution to a more concentrated solution, through a semi permeable membrane.
The important thing to remember is that osmosis is the movement of WATER ( or other solvent ) not the particles dissolved in the water. For example if you split a beaker of water into two halves with a semi permeable membrane and added salt to one side, water would move from the side of the beaker with no salt until the two concentrations of salt water were the same.
Try soaking a raisin in water, what happens? It should swell up a little, this is because the water moves from it’s own point of low concentration to into the cells of the raisin, which has a high sugar concentration to rehydrate the cells. Water keeps moving by osmosis until equilibrium is reached, this is when the concentration of both solutions is the same.
Another fun way to illustrate osmosis is with eggs. Eggs are perfect as under the shell is a semi permeable membrane.
How to remove the shell of an egg?
First we need to remove the shell to expose the semi permeable membrane of the egg. You can do this very simply by submerging the eggs in vinegar for about 24 hours. After this time rub gently under running water to remove the shell, if some still remains just soak again.
Prick the egg from the water with a fine needle and watch a jet of water shoot out!
How do you think you could rehydrate your shrunken egg?
Why does this happen?
Our concentrated solution was the sugar solution. The dissolved sugar molecules cannot pass through the semi permeable membrane of the egg, but the smaller water molecules can. Water moves from the less concentrated egg solution to the more concentrated sugar solution until the equilibrium is reached. Therefore water moves from the egg to the sugar solution via osmosis, shrinking the egg as the water leaves it.
To rehydrate the egg, place into plain water. In this instance the concentration of the liquid inside the egg is higher than the water, so water moves from the water to the egg.
When we pricked the egg soaked in water, water shot out of the egg. This is because the egg had absorbed water, increasing the pressure inside it.
Why does eggshell dissolve in vinegar?
The egg shell dissolves in the vinegar as the acetic acid in the vinegar reacts with the calcium carbonate of the the shell, Carbon dioxide is given off during this reaction so you should see bubbles of gas escaping.
Extra for older children
Can you weigh the eggs at each stage to monitor the loss/gain of water?
Try addding food colouring to the water and watch as the eggs absorb the coloured water.
What do you think would happen if you left an egg in a glass of golden syrup?
Don’t forget to wash your hands after handling raw eggs!
This post is part of the A to Z Science Series with Frogs, Snails and Puppy Dogs Tails.