[Shea]

I was doing that lab where you put salt into water to change the temperature at which it boils.

Without salt, it boiled at 97 degrees C, then when I added 1 tbsp. of salt, it boiled at 90 degrees C.  When I added 2 tbsp, it still boiled at 90, and the same when I added 3 and 4 tbsp.

Why was this???

[enahs]

I think maybe you where reading your thermometer wrong or wrote out your question wrong, as salt raises the boiling point of water.

Please clarify your question further.

[mike]

How are you determining when it is "boiling"?

[Shea]

Do you think the type of salt makes a difference? (I was using kosher salt)

I determined that it was boiling by noticing when it started to bubble excessively.

I am positive I didn't misread the thermometer.

I did this 4 times.

Something must be wrong.

Do you think my altitude makes a difference? (5000ft)

[Dan]

the salt should still raise the bp

[Shea]

Well, what could have happened to make the water boil at 90?

Is it entirely impossible?

Somehow, in 4 tries, I got the water to boil at 90 every time.

Maybe I need to try again.

[Dan]

The only (non-ridiculous) thing I can think of right now is that you may have heated the water too fast, and the thermometer hadn't caught up with the temp of the solution, but even that is probably a little far fetched.

[Shea]

I just tried again, with 4 tablespoons of the koshersalt, and the water boiled at about 91 degrees C in about 10 minutes.

It was a rolling boil, too.

I guess the easiest explanation is that my thermometer is bad...

But, when I didn't add salt, it accurately measured the bp of water in the same amount of time. 

Without the salt, it boiled at 97, which at 5000 ft is the right temperature it should boil at.

And I did the regular water at the same heat as the salt-water solution.

Maybe I just keep making a mistake somewhere.   This should be the easiest experiment ever, and I can't get it right.

[english]

Must be a calibration problem with your thermometer.

[Shea]

If so, how did it accurately measure the temperature of the water without salt?

[mike]

It may be the method you are using to measure the temperature. What type of thermometer are you using? If you are using a regular alcohol thermometer and simply sticking the tip into the water this may be inaccurate. You could try boiling the water and immersing a significant proportional if not all of the thermometer in the water, this would give a more accurate temperature reading.

It could be your apparatus, are you using a beaker (large or small) a conical flask (with or without a stopper) distillation apparatus?

Finally you may have disproved the idea that salt raises the boiling point of water?

[Mitch]

For some reason I always thought adding salt had no real affect on water boiling.

[Shea]

I boil the water in a pot on a stove, and I teathered the thermometer to the handle of the pot with a rubber band so that it isn't touching the bottom of the pot, but so that it is about mid-depth in the middle of the water.  This worked for the non-salted water, so it should be right for the salted water.

I think it's the salt I'm using.  I should get a different type of salt and try that.

[enahs]

No “salt” you can even buy that is not from a chemical supply store will lower the boiling point of water. And the salt will only raises it by a few degrees, not 6 as you are getting in the other direction.


Make a solution of ice water and measure the temperature of the ice water. It should be 0C (give it a few minutes, and make sure you have plenty of ice). This will check the calibration of the thermometer since you seem to be having trouble with the upper end.

Then add salt to the ice water and mix, the temperature should go down.


The only thing you have in your house that will lower the boiling point of water would be alcohol.

Note, the thermometer must be vertical (straight up and down). Any tilt more then say 3-5 degrees will throw it off.

[mike]

I doubt that the salt will make a difference.

Maybe try a different thermometer, or use a digital temperature probe.

Maybe the errors involved in the experiment are 10-20 C? and the difference in boiling point is too small to see.

Also maybe the ambient pressure is changing dramatically in your area, is the weather changing rapidly? LOL

Dissolving salt NaCl is endothermic I think (maybe check this) I doubt that this would be noticeable when compared to heating on the stove though.