[robotowilliam]

My class were trying to construct the solubility curve for sodium chloride in water at different temperatures. Obviously, the correct answer is that solubility rises, but not steeply.

My group's result was a consistent decrease. I want to know why that might have happened, given our method:

• Saturate the water with salt (confirmed by presence of undissolveable salt crystals)
• Measure the temperature at that point
• Measure 10ml of the solution and pour into a crucible
• Boil away the water (with lid on crucible to catch splatter)
• Weigh the salt (weight of crucible+lid+salt minus weight of clean crucible+lid)

So the data we collected was grams of salt in 10ml of saturated saltwater solution at various temperatures. We measured 5 temperatures from 18 degrees C to 100, and got one pretty obvious outlier at 50 degrees which we discarded. The other 4 data points decrease almost in a straight line from 3.0g to 1.8g.

Trying to figure out why it decreased.

The only thing I can think of is that thermal expansion meant that the 10ml of solution at the higher temperatures was less dense, therefore had less water AND salt in it. Does water expand enough for this, between 18 degrees and 100?

[Meter]

Could you post a picture of your graph?

[jeffmoonchop]

NaCl is not a great compound to be doing solubility curves in a class. It hardly changes from the temps you mentioned. Your results may just be down to variability in measuring 10ml. Did you filter before measuring the volume? Did you refresh the solution each time you took 10ml? 100 degrees is also a pretty hard temperature to measure solubility with this method.

[Borek]

Solubility is typically reported per 100 g of solvent - this way changes in the density of the solution (with both temperature and concentration) can be ignored.

[robotowilliam]

Could you post a picture of your graph?

Attached, thanks.

[robotowilliam]

NaCl is not a great compound to be doing solubility curves in a class. It hardly changes from the temps you mentioned. Your results may just be down to variability in measuring 10ml. Did you filter before measuring the volume? Did you refresh the solution each time you took 10ml? 100 degrees is also a pretty hard temperature to measure solubility with this method.

The downwards trend we measured seems suspiciously consistent for any of those issues. No we didn't control things all that carefully, but I was wondering if there was a reason with our method why the mass of dissolved salt would decline with temperature.

[Borek]

Four more or less consistent point can be easily a coincidence (especially with one outlier that was completely off).

How was the 10 mL measured?

Have you dried the NaCl to the constant mass, or just for some time in hope it will be dry?

[jeffmoonchop]

Easy to become impatient with boiling off water, especially if you started off with the water at 18C. takes a while, maybe it was still wet. Normally these types of analyses are done baking in the oven until constant weight.

[AWK]

Sodium chloride increases solubility as temperature increases from ~36 at 18°C to ~40 at 100°C.

[robotowilliam]

I guess it was probably coincidence then. Thanks for your input, all