Molecular Weight Of Substances In Solutions : Osmotic Pressure

If a concentrated solution of copper sulphate contained in the lower part of a cylinder is covered with a layer of water, the copper sulphate molecules (ions) gradually diffuse upwards until the solution becomes homogeneous and of uniform colour. The dissolved molecules behave to some extent like a gas; they are in motion and possess kinetic energy. If we could interpose a partition in the solution which would stop the dissolved molecules but would be freely permeable to pure water above, we should expect the copper sulphate molecules to exert a bombardment on the partition. A partition freely permeable to pure solvent but impermeable to dissolved substances, is called a semipermeable partition, or - since it is usually prepared in the form of a thin film - a semipermeable membrane.

A semipermeable membrane may be regarded as a kind of molecular filter. Just as ordinary filter-paper will stop suspended particles and permit dissolved molecules to pass through, the semipermeable membrane may be regarded as stopping even dissolved molecules and permitting only the molecules of pure solvent to pass. There is, however, a difference between the two cases: in order to squeeze pure solvent through the semipermeable membrane it is necessary to apply a definite pressure to the solution enclosed in it. At lower pressures no solvent percolates through the partition.

Various substances function as semipermeable membranes. Without exception they are slimy, non-crystalline bodies, called colloids, Thus, if a drop of copper sulphate solution is introduced from a pipette into a solution of potassium ferrocyanide, a skin forms over it, composed of copper ferrocyanide, Cu₂FeC₆N₆. This is produced as a reddish-brown, gelatinous precipitate when the two solutions are mixed:

2CuSO₄ + K₄FeC₆N₆ = Cu₂FeC₆N₆ + 2K₂SO₄. The pellicle is semipermeable, because no copper salt diffuses through the drop, as may be seen from the ferrocyanide solution remaining clear. The drop expands or shrinks on standing, owing to passage of water through the pellicle. By holding the drop suspended, with a bright light behind the beaker, the streaks due to changes of concentration may be seen.

Expt. 1. - In a strong solution of sodium silicate place small pieces of ferric chloride, nickel chloride, cobalt chloride, and copper chloride. Observe the formation of pellicles, which assume curious shapes on standing. ("Chemical Garden"; Glauber, 1648.).

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Molecular Weight Of Substances In Solutions
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