Desalination of seawater is done by using reverse osmosis. The pressure applied to the solution is
- A.larger than osmotic pressure
- B.smaller than osmotic pressure
- C.equal to osmotic pressure
- D.equal to atmospheric pressure
Correct answer
Explanation
The correct answer is A, larger than osmotic pressure. Only a pressure greater than the osmotic pressure of seawater can push pure water backwards through the membrane.
In ordinary osmosis, water moves through a semipermeable membrane from the dilute side to the concentrated side. The osmotic pressure is the extra pressure that must be applied on the concentrated solution just to stop this flow. If an even higher pressure is applied, the flow reverses: water molecules are forced out of the salty solution through the membrane, while the salt ions are held back. This is reverse osmosis. Seawater has an osmotic pressure of roughly 25 atmospheres, so desalination plants use pumps working at about 50 to 80 atmospheres with membranes of materials such as cellulose acetate. Household RO water purifiers work on the same idea.
Option B is wrong because a smaller pressure would only slow down normal osmosis. Option C is wrong because a pressure equal to the osmotic pressure just stops the flow and produces no fresh water. Option D is wrong because atmospheric pressure is far below the osmotic pressure of seawater.
Exam tip: pressure equal to osmotic pressure = no flow; pressure greater than it = reverse osmosis.
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