Osmosis
To introduce the process known as osmosis.
What students learn
The science behind this experiment
Osmosis is responsible for water absorption in animal and vegetable cells. It can be described as the movement of molecules through a semi-permeable membrane due to a gradient of concentration between two solutions. The solvent exerts a pressure on the membrane called osmotic pressure.
Equipment used
- Iron stand — 5404.52
- Universal clamp — 5416.20
- Right angle clamp — 5401.16
- Beaker 500 ml — 1118.50
- Osmotic pressure apparatus — 4182.62
- Osmotic membrane — 4182.61
- Rubber band
Chemicals and reagents
- Sodium chloride solution
- Distilled water
- Methyl blue dye — R:20/21/22
Safety notes
Questions for students
- Observe what happens inside the capillary and try to give an explanation. What do you think caused the level variation inside the capillary?
What to expect
<p>The level of liquid inside the tube rises to a determined height and will then stop.</p><p>Osmotic pressure is a result of the natural tendency of solutions of different concentrations to mix to a uniform concentration.</p><p>If two solutions are separated by a semipermeable membrane the system tends to reach a constant concentration. Water molecules exert a pressure on the membrane and on the solution at lower concentration.</p><p>The essence of an osmosis experiment is a membrane separating solutions of differing concentrations. This membrane must be selectively permeable, that is, it must be permeable to the solvent but impermeable to the solute.</p><p>Thus the sodium chloride remains within the thistle tube but water is free to pass in either direction. A high concentration solution will always move to a more concentrated solution.</p><p>The membrane you are using is taken from dialysis tubing that blocks the passage of molecules whose molecular weight exceeds 10,000 amu. At a mere 18 amu, water has no difficulty with such a membrane, but macromolecules such as proteins can have molecular weights that easily exceed this value. (1000 amu = 1 kiloDalton or kD.).</p><p>The apparatus is built so that as the water diffuses into it, the solution is forced up the tube against gravity. Since the solute always stays within the tube, it can never reach the zero concentration of the pure water solution on the other side of the membrane. The solution will not rise indefinitely.</p><p>The "free energy of mixing" is countered by the gravitational energy associated with the solution's rise until the two solutions establish equilibrium. If the solutions are ideal (or nearly so) the pressure is given by the osmotic pressure expression. The measured pressure, in mm Hg, is the mm of solution scaled by the ratio of the densities of the solution and mercury. The density of Hg = 13.6 g/cc.</p><p> = (n / V) RT = C RT.</p>
Disposal
Dispose of the liquids into the acids and bases waste container.
Run this experiment in your classroom
This experiment is part of the ATP Mobile Lab — a self-contained laboratory that turns an ordinary classroom into a working science lab, with over 200 experiments in physics, chemistry, biology, robotics and engineering. Gali, the AI tutor built into ATP Connect, guides students through each step and answers their questions at the bench.
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