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Topic: boiling water  (Read 13326 times)

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Offline Shea

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Re: boiling water
« Reply #15 on: September 20, 2006, 10:51:32 PM »
I didn't have any string, so I attached a bunch of paperclips and attached it to the thermometer so that it hung vertically.

I accidentally forgot about it for a while, (I was watching TV,) but when I came back, it was really boiling, and it wasn't any higher than the water without salt.

So I don't think salt does anything.

Offline mike

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Re: boiling water
« Reply #16 on: September 20, 2006, 11:06:23 PM »
You should also test different amounts of salt, maybe you haven't added enough to notice a difference yet.

You could also try adding sugar instead of salt and see if there is a change.
There is no science without fancy, and no art without facts.

Offline Yggdrasil

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Re: boiling water
« Reply #17 on: September 20, 2006, 11:34:06 PM »
Based on the boiling point elevation constant listed on wikipedia, a 5M solution of salt (~30g in 100mL) will only change the boiling point by about 5oC (this is also assuming 100% dissociation, which is not always correct with such a high concentration of salt).  So, Shea probably doesn't have enough salt in there for the change in boiling point to be noticible with the thermometer he has.

Offline Shea

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Re: boiling water
« Reply #18 on: September 21, 2006, 10:52:20 PM »
Someone told me this, "Salt is composed of NaCl - Sodium and Chlorine. Sodium is a metal which conducts heat very quickly. The salt added to the water will allow the water to reach boiling point faster, not increase it's boiling temperature. (unless you pressurize it)
."

Does this have any truth in it?

Offline mike

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Re: boiling water
« Reply #19 on: September 22, 2006, 12:35:59 AM »
Quote
Someone told me this, "Salt is composed of NaCl - Sodium and Chlorine. Sodium is a metal which conducts heat very quickly. The salt added to the water will allow the water to reach boiling point faster, not increase it's boiling temperature. (unless you pressurize it)."

This is not correct. Well some of it is not correct.

Common table salt is NaCl (sodium chloride), comprised of Na+ and Cl- atoms (not elemental sodium Na, and chlorine gas Cl2). The properties of NaCl are very different to the properties of Na metal.

Water boils when heated when its vapour pressure equals atmospheric pressure. Adding salt to the water alters its vapour pressure. Indeed, adding NaCl will decrease its vapour pressure and thus more heat is required to achieve boiling.
There is no science without fancy, and no art without facts.

Offline Yggdrasil

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Re: boiling water
« Reply #20 on: September 22, 2006, 12:45:11 AM »
Here's a thermodynamic explanation of why salt raises the boiling point of water.

Water boiling at its boiling point is a reversible reaction.  For a reversible reaction, ?G = 0.  Therefore:

?G = ?H - T?S = 0

where ?H is the change in enthalpy of vaporization, ?S is the change in entropy of vaporization, and T is the boiling point.  Rearranging this expression, we get:

T = ?H / ?S

Now, when we add salt to pure water, we increase the entropy of the liquid water.  Since adding salt does nothing to the entropy of the gaseous water, ?S = Svapor - Sliquid is lower.  Since ?H is not changed significantly, the decrease in ?S will increase T (the boiling point of water).

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