[mooeypoo]

Okay, this should've been very simple: Two cups with the same amount of hot water at the same temperature (75 Celsius).
Add about 5 tablespoons of salt to one of those cups and stir until the salt is dissolved. Measure temperature of both cups. Wait 20 minutes, measure again.

The salt-water SHOULD cool faster.


I'm staring at those two cups at the moment, and they're both the same. Not only that, but this experiment says that the first two runs they did -- the temp dropped the same, and was different only at the third trial.

WTF. Why does this not work? Salt should decrease the thermal retention rate of the water ... sooooo.... what's going on?






Also, as a side note, if two times fail and once succeed, I wouldn't call it an experimental victory. Just saying.


What am I doing wrong, can anyone fare a guess? They both have thermometers in them and are, after 10 minutes, stable on 50 degrees. Grr.

~mooey

This post has been edited by mooeypoo: 7 June 2011 - 07:13 PM
Reason for edit: (sorry, forgot the link to the experiment)

[Moontanman]

I read the link, the experimenter seems to be saying that the hypothesis is that salt water will cool faster than freshwater. I think this is flawed, salt water is denser than fresh and so a liter of salt water would be heavier than a liter of fresh so the salt water should cool slower. The salt water should cool slower due to evaporation from the salt water being slower than evaporation of fresh water as well.

[mooeypoo]

Salt lowers the boiling point of water, and the water SHOULD cool faster. Regardless, it seems we both agree there should at least be a difference in how fast they cool, and there is not so far.

[Moontanman]

mooeypoo, on 7 June 2011 - 07:26 PM, said:

Salt lowers the boiling point of water, and the water SHOULD cool faster. Regardless, it seems we both agree there should at least be a difference in how fast they cool, and there is not so far.

From your link

Quote

However, I wanted to see more dramatic results so on my 5th, 6th, and 7th trial. At this point, I added the amount of salt needed to completely saturate the water. This only raised the boiling point of the water.

[John Cuthber]

Perhaps the difference just isn't big enough for you to detect?

[swansont]

mooeypoo, on 7 June 2011 - 07:26 PM, said:

Salt lowers the boiling point of water, and the water SHOULD cool faster.

Salt actually raises the boiling point.

Salt has a lower heat capacity, so you should lower the composite heat capacity of the system, but by how much? A few percent?

[Moontanman]

swansont, on 7 June 2011 - 08:08 PM, said:

Salt actually raises the boiling point.

Salt has a lower heat capacity, so you should lower the composite heat capacity of the system, but by how much? A few percent?

Are you saying salt has a lower heat capacity or that salt water has a lower heat capacity?

It might be very true that your instruments are not sensitive enough to measure the difference, try super saturating the water with salt and see what happens. Although salt crystallizing out of solution as it cools will have an effect I would think. (are you using sodium chloride or sea salts?)

This post has been edited by Moontanman: 7 June 2011 - 08:16 PM

[John Cuthber]

According to this
http://www.ornl.gov/...45605501588.pdf
salt water has a lower heat capacity (measured by weight) than fresh water.
(page 8 has graphs of heat capacity vs temp for different salinities.)
Even for a 12% w/w solution the change is only about 14%.

Also, the method used isn't well designed.
If you had the same mass of brine and fresh water and compared the rates of temperature loss of each you would get a better view of the effect. In this case you have different amounts of materials cooling.

[swansont]

Moontanman, on 7 June 2011 - 08:16 PM, said:

Are you saying salt has a lower heat capacity or that salt water has a lower heat capacity?

Both, though the heat capacity of NaCl is probably not be the value that matters, because it's in ionic form in solution.

[mississippichem]

Sounds like a job for a temp probe. I also think that measuring the rate of cooling would be easier to observe than a temperature difference after some elapsed time.

[mooeypoo]

Meh, I always get those confused; Salt RAISES the boiling point, which is why it takes longer for saltwater to boil. Sorry. I get those mixed up all the time.

BTW, thanks everyone for your help on this. It's very frustrating.

Moontanman: I did read what the girl wrote, and I added a ton of salt (oversaturated my mixture) and it still didn't work. I'm starting to think you guys are right, and that the difference is too weak to actually see in the naked eye.

Sadly, the "naked eye" (a thermometer) is the only thing I can use -- these are supposed to be home experiments, so no probes for me. If it isn't strong enough to measure directly, it's out. I'm going to have to try and think of another experiment showing different heat capacities. Maybe I'll go with oil and water, though I would rather not tell kids to mess with hot oil.

Any ideas, btw, will be welcomed

It must be a home experiment with stuff you can get in your kitchen (or can *easily* buy for relatively cheap).

Thanks for your help!

~mooey