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Q & A: Fresh water from salt water

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Most recent answer: 09/06/2009
Q:
Can you get fresh water from salt water by evaporation. If yes,how?
- Anonymous (age 12)
A:
Sure -- that's a process known as "distillation", although evaporation is only half of the story. Evaporation from salt water gives nice, pure water vapor, but to produce water again requires condensation. This can be done by flowing the water vapor (+ any air it might be mixed with) past a surface that is cooler than it, and necessarily cooler than the boiling point of water. The cooler it is, the faster it will condense water out of the vapor. The condensing surface is tilted so the water that forms runs down the surface into a container. If the condensing surface is colder than 32 degrees Fahrenheit (0 Celsius) some water will freeze on the condensing surface.

Note -- the salt from the seawater (and anything else that might have been in it) stays in the original container holding the seawater. Eventually this may cake up with salt and it will have to be cleaned out. Some people buy sea salt at the grocery store and claim it is tastier than the kind you get out of salt mines (I cannot tell the difference however).

Tom

(published on 10/22/2007)

Follow-Up #1: Biblical water

Q:
I hear and read that the water supply in the area Jesus taught was polluted and scarce so the people drank wine. Did the inhabitants of that area know how to produce clean water from brackish water and saltwater?
- Bob Sims (age 67)
Huntsville, Alabama, USA
A:
This is an interesting question. I've searched around the Web for info on ancient water purification.  Although there were elaborate public works to gather, store, transport, and to some extent purify water in much of the ancient world, including Roman Judaea, desalination seems not to have been known. So far as I can tell, the first known way to desalinate water,  distillation,  was developed by an Arab alchemist named Geber in the 8th century AD.
It's also an interesting question how much the widespread use of wine and beer was due to their intoxicating qualities and how much due to their relative sterility, compared to many ancient water supplies.

Here's some links:






(published on 09/06/2009)

Follow-up on this answer.