Question

Desalinating salt water is one way arid regions are able to obtain drinking water. However, it’s not an easy process.

a. Which has more entropy: a container of salt water or a container of fresh water plus a box of salt?

b. Why does your answer to part a indicate that the desalination process requires either the conversion of ordered energy into thermal energy or the movement of heat from a hotter region to a colder region?

c. One technique for desalination is reverse osmosis, in which salt water is squeezed through a special membrane that permits only pure water to pass. Show that squeezing water through this membrane takes work (ordered energy).

d. Another technique for desalination is distillation, in which salt water is boiled in a hotter region to form water vapor that is then condensed to pure water in a colder region. This desalination process is a heat engine that separates the water from the salt. Explain why it’s a heat engine.

e. Still another desalination technique is freezing the salt water into pure ice and salt crystals. In this case the salt water has to be cooled to very low temperatures to freeze it, then warmed to a much higher temperature to melt it. Show that this technique is also using a heat engine to drive the desalination process.

Answer 1

a.

We cannot drink sea water, as sea water is salty. Human kidneys can filter only a minute amount of salt by production of urine. If we drink sea water having lots of dissovled salts, we would die of dehydration as we need to urinate often to get rid of the excess salts. We can desalinate small amounts of water for survival by boiling the sea water and condensing the pure water vapor. This procedure leaves behind the salts once all the water has been boiled off. We can also use a Reverse Osmosis plant, where in a semipermeable mebrane allows only the pure solvent to pass through it and get filtered out.

b.

Sea water has water as a solvent with dissolved salts as solutes. When we are desalinating it we have to increase the free energy of the system as energy is required to break the solvent solute (ionic) interactions and separate out pure water. So, we are giving energy either in the form of work or temperature. Hence, desalination process requires either the conversion of ordered energy into thermal energy or the movement of heat from a hotter region to a colder region.

c.

When two salt solutions of varying concentrations are separated by a semipermeable embrane, wee observe osmosis that is the movement of water molecules from dilute to concentrated solutions. This takes place spontaneously till both the solutions become equally concentrated. In reverse osmosis, we have the movement of of water molecules from the concentrated saline waters. This can be achieved by applying high pressures towards the saline water side. This facilitates the movement of pure water molecules out of the saline water (high pressure) area. Reverse osmosis is energy efficient as it just requires ordered energy (work) in the form of pumps that pump out sea waters at high pressure.