The definition of 'Osmosis' is expressed as being the diffusion of a solvent (usually water) through a semi-permeable membrane, from a solution of low solute concentration (high water potential) to a solution with high solute concentration (low water potential). There are several experiments to show diffusion in action. This practical involving an egg and salt solution is effective and relatively simple to conduct.
This practical involves de-shelling an egg by placing in acid for 24 hours then placing the soft egg into beakers of salt solution. The egg is weighed before and after being in contact with the solution. Over time, some of the water in the solution will pass through the egg's soft semi-permeable membrane and this will be reflected in the weight of the egg.
Equipment typically required (per set):
- De-shelled egg
- Salt solutions (sodium chloride in water at several concentrations eg 0%, 10%, 20%)
- 250ml beaker per solution
- Accurate balance
- Paper towels
Some technician preparation time is required before this practical can be conducted. Several eggs need to be shelled very carefully using acid at least 24 hours before the experiment. 2M Hydrochloric acid should be poured into a beaker and a hen's egg carefully placed into it. Make sure the egg is completely covered with the acid. You may need to place a spatula on the egg to keep it weighed down. This should be left until the shell has dissolved completely leaving the egg in a soft membrane. The egg should be removed, rinsed under gently running water and dried using a paper towel.
Students will require one egg per salt solution so ensure you have enough prepared for each student.
The salt solution may also need to be prepared in advance of the lesson. Simply dissolve the correct amount of sodium chloride in deionised (distilled) water to make up approximately 200ml of each concentration for each student. Teachers may request specific concentrations. Alternatively make up solutions of 0% (just water) 10%, 20% sodium chloride.
Students typically weigh and then place an egg in each of the solutions. A set period of time, such as 45 minutes is observed or the egg is left until the next lesson, after which time the egg is removed, dried and weighed again. Students should see that the egg has gained a certain amount of weight after being submerged into the 0% solution.