Page 41 - Curriculum Visions Dynamic Book

Semi-permeable GLUCOSE membrane SOLUTION AND
New level
WATER
DYE
Start level
Glucose and dye (solute) molecules are too large to go through the membrane.
Water (solvent) molecules are small enough to get through the holes in the membrane.
Water in high concentration
The net diffusion of water molecules is down the concentration gradient and into the glucose solution.
Water in low Water has been lost to concentration the glucose solution.
Glucose solution has been diluted.
solution by a semi-permeable membrane. The sodium and chloride ions in the salt cannot get through the membrane. However, the water can move from the dilute side (where the water is in greatest concentration) to the concentrated side in order to balance the concentrations. However, in doing this, the water builds up in what was the concentrated side.
The height of water that can be supported by this process is quite considerable. It is used by plants to get water into their roots and up to the leaves. It is also used in animals, including us, for example, to extract food from the gut.
Osmosis can work in industry too. It is used, for example, to concentrate fruit juice without boiling the juice (by placing the natural, weak juice next to a concentrated salt solution), for desalting seawater, and for helping clean up sewage.
(Above) In this experiment a piece of special viscose tubing has been used to demonstrate osmosis. Glucose solution mixed with a dye was placed
in the tube and lowered into a beaker of distilled water. It took only a
few minutes for the level of the liquid in the tube to rise by about 1 cm. The water molecules have moved through the membrane from the beaker, where they are in high concentration, and into the glucose solution, where they are in low concentration in order to address this imbalance.
The reverse osmosis process is
a membrane process used to extract fresh water from brackish inland water, the salt content
of which, though undesirable,
is considerably below that of seawater. Brine, subjected to pressure, is forced against a semi-permeable membrane; fresh water passes through, while the concentrated mineral salts remain behind. The membranes are made from suitable polymer materials such as cellulose acetate.
The reverse osmosis process
is not as commonly used as the distillation desalination processes available – see page 50.
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