Question

Explain reverse osmosis and write any one example of artificial semipermeable membrane. (1+1=2)

Hint:

Apply pressure greater than the osmotic pressure so solvent flows out of the solution; cellulose acetate is a common artificial membrane.

Answer 1

Step 1: Ordinary osmosis. In osmosis, solvent molecules flow through a semipermeable membrane from the region of pure solvent (or dilute solution) into the more concentrated solution. This natural flow can be stopped by applying a pressure equal to the osmotic pressure on the solution side.
Step 2: Reverse osmosis (definition). If a pressure greater than the osmotic pressure is applied on the solution side, the direction of solvent flow is reversed: solvent now moves from the solution (concentrated side) into the pure solvent through the semipermeable membrane. This process is called reverse osmosis.
Step 3: Use. Reverse osmosis is used for the desalination of sea water, where applying high pressure on sea water pushes pure water out through the membrane, leaving the salts behind, giving drinking water.
Step 4: Example of an artificial semipermeable membrane. Cellulose acetate is a commonly used artificial semipermeable membrane in reverse osmosis (it allows water to pass but not the dissolved salts). Another acceptable example is copper ferrocyanide, \( Cu_2[Fe(CN)_6] \).
\[\boxed{\text{RO: solvent forced from solution to pure solvent by pressure } > \pi;\ \text{membrane: cellulose acetate}}\]