MadSci Network: Chemistry

Re: Does adding table salt to H20 make it boil at a higher temp?

Date: Mon Jan 31 16:26:52 2000
Posted By: Dr. Michael Gallagher, Senior Rsearch Chemist, J.R. Simplot Company
Area of science: Chemistry
ID: 948991619.Ch

Dear Minda,

This is a wonderful subject on which to run some experiments. You should be able to get some very accurate results with only a little bit of work and a bunch of fun.

You are absolutely correct in that the definition of boiling is when the vapor pressure of the liquid equals the ambient atmospheric pressure. To explain what those terms means, we need to use our imagination.

First of all, imagine a regular cooking pan that is six inches across, about half full of water, that is sitting on your stove, just about to boil. Now imagine the column of air that is six inches across above the pan, extending from the surface of the water, up through the ceiling and roof, the going way, way, up in the sky, about 60 miles straight up.

Even though air does not weigh very much, a 60 mile high column of air would have a considerable amount of weight. In fact, at sea level, it would weigh 14.7 pounds per square inch. This is the atmospheric pressure. With our six inch pot of water, that means a total force of (3)^2 x (pi) x 14.7, which equals 415.6 pounds over the surface of the pot of water. At this pressure, water boils at 212 F or 100C.

At the point of boiling, the water just begins to form bubbles that detach from the bottom of the pan and drift to the surface. The pressure of the steam in the bubble is just a tiny bit above the pressure of the air pushing down on it, plus the pressure of the depth of the water in the pan, which is not much since our pan is not very deep. When you see rapid boiling and steam shooting out like when the tea kettle whistles, then the pressure of the steam is greater than the pressure of the air above the kettle.

Now the big question! What happens when the atmospheric pressure changes?? When you change altitude the atmospheric pressure changes. For example at Death Valley, California, the altitude is about 280 feet below sea level, which means that the column of air is 60 miles plus 280 feet. That doesnt sound like much, but there is a pretty big change in the pressure in the first couple of miles above the earths surface. This gives a pressure of about 14.85 pounds per square inch. At that pressure the boiling point of water is going to increase somewhat, about 0.6F to 212.6F or 0.3C to 100.3C

On the other hand, going up a mountain, the atmospheric pressure drops to a lower value and so does the boiling point. At the top of Mount Everest, the altitude is 29,028 feet above sea level and the atmospheric pressure is about 4.65 pounds per square inch. At this pressure water boils near 70C or about 158F ! ! You would have to boil potatoes or pasta a very long time to cook them. If you had a huge burner to boil the water, it would not boil at a higher temperature, but would only make water boil faster. This is because of the constant pressure.

Now that we understand boiling, we can begin to think about your experiments on the effects on the boiling point of water by adding table salt to the solution.

You are again absolutely correct that you need an accurate thermometer to measure the temperature of boiling. Since we have constant pressure the important thing is to allow the thermometer itself to come to a steady reading without boiling too much of the water away, or you will change the amount of salt in the solution. (concentration)

You should ask your teacher to find a thermometer that has the smallest divisions between markings that can be found. You need to be able to make measurements to the nearest 1/5th C or * F, even if you have to estimate the 1/5ths or *s.

As to how much water to use, I would suggest that you make up a series of concentrations of table salt in water in 2 liter soda bottles. This will allow you to use a small pan and run several experiments so any errors in measuring will mostly average out.

As far as how much salt to add to each solution, the maximum amount of table salt that can be dissolved in cold water is about 36 grams of table salt into 100 milliliters of water.
(? How much in a 2 liter bottle?)

I would try at least two different amounts of salt in addition to just plain water. Record the temperature every few seconds as the solutions get close to boiling, and keep going after they are until you get the same reading several times in a row. If you go too long, (like a couple minutes past boiling) the reading will begin to rise, except for just plain water. (??WHY is that so??).

If you need further background material I would suggest a high school textbook on chemistry or physical science., Look under Boiling Point Elevation or Freezing Point Depression. These chemical properties fall under the term Colligative Properties. If you try an internet search using those words, you will get a zillion hits, but most of the pages seem to be written for high school and college students.

I hope this discussion helps you to understand what results you are going to get in your experiments.

Make certain to have a bunch of fun!

Dr. Michael M. Gallagher
Senior Research Chemist
J.R. Simplot Co.

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