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What's wrong with this experiment for measuring boiling point elevation?

Hi, I'm doing a lab on the effect of salt on the boiling point of water... The procedure is to fill a metal pot 3/4 full with water, add salt, place a thermometer in for 5 minutes, then place the pot on the stove and begin heating the pot. I was hoping you could give me some information on what happens.
Gianna Massouras

Gianna,

There are some flaws in this experiment that you'll have to correct if you want to use the results to show how salt affects the boiling point of water.

The molecules in the liquid are in constant motion. They constantly collide with each other and with the walls of their container; every now and then, a collision will cause one of the molecules to be ejected from the liquid into the air above it, creating a vapor pressure above the liquid. Heating the liquid increases the average speed of the molecules, increases the number of molecules ejected, and increases the vapor pressure.

A liquid boils when its vapor pressure becomes equal to atmospheric pressure. Low atmospheric pressure causes the boiling point to go down; high pressure drives it up. Atmospheric pressure varies a bit from day to day, depending on the weather, and it varies from place to place, depending on the altitude. So it's quite possible that you'll get a boiling point for the salt water that's less than 100°, which seems to contradict what your textbook predicts (namely, that salt increases the boiling point temperature).

You can look at the effect of salt without the pressure effect if you measure the boiling point of the water before adding the salt. That way you can compare the boiling point of the water and the solution at the same pressure.

Another thing that affects the vapor pressure of the water is the relative number of water molecules in the solution. The higher the percentage of water, the more molecules will escape into the vapor, and the higher the vapor pressure will be. A salty solution has a lower percentage of water molecules than pure water does. So dissolving salt in water decreases the vapor pressure of the water. The more salt you dissolve, the lower the vapor pressure of the water becomes. You'll have to heat the solution to a higher temperature than before to get its vapor pressure equal to atmospheric pressure

You must carefully measure the amount of salt you use to be sure that it's salt and not something else that is affecting the boiling point. The boiling point goes up by roughly 1°C for every mole of NaCl per liter- it's a very small effect, and one you're likely to miss completely if you're sloppy or your thermometer isn't very precise.

Finally, make sure that the temperature you're measuring is really the boiling point. The first bubbles that appear aren't steam- they're dissolved air, which comes out of solution as the temperature rises. You won't be at the boiling point until the bubbles forming at the bottom rise all the way to the top and burst on the surface. Don't wait too long; as the liquid boils, some will evaporate, increasing the salt concentration. The boiling point temperature will slowly climb as the experiment proceeds!

Author: Fred Senese senese@antoine.frostburg.edu



General Chemistry Online! What's wrong with this experiment for measuring boiling point elevation?

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/solutions/faq/boiling-pt-elevation-experiment.shtml