Solute Effect on Vapor Pressure A Classroom Demonstration ARTHUR W . DEVOR' Adelphi College, Garden City, New York
for demonstrating the effect of temperAMETHOD ature upon the vapor pressure of a solvent is described in a previous article.% It was used in the general chemistry classes. When demonstrating osmosis to beginners the author found it helpful and also interesting to the students to demonstrate qualitatively the effect of a solute upon the vapor pressure of the solvent as described below. The apparatus was made from a pyrex test tube 25 X 250 mm. by bending and blowing i t into a U-shape; making the two T-connections, sealing off the open end, and blowing i t out into the desired shape. Tight-fitting rubber tubes with stopcocks were forced over the openings at A and B. Boiling stones (porous chips) were added and distributed so that both sides of the tube contained several ~~~-~~ small pieces. Ten milliliters of distilled water were introduced through the tube into the side C. Ten milliliters of saturated magnesium sulfate solution (saturated a t 60°C.) were then poured through the tube into the side D, being careful that none of the magnesium sulfate mixed with the distilled water. The air was eliminated from the system by boiling the water and the solution for a minute or two. Then the stopcock a t A was closed, the heat was removed, and the stopcock a t B closed very carefully so that no air would get back into the system. The apparatus was lowered into a beaker of hot water. The students could see the distilled water boil while the solution did not show any signs of boiling. The entire apparatus was left untouched until the next class period. When it was exhibited to the class they could see that most of the distilled water was gone and that the solution was nearly twice the original volume. Eliminating the air by applying suction seemed more confusing to students who had had no physics. When evacuating by boiling, i t was also found advisable to connect the tube a t B to a trap containing steam (no ~
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'Present address: University of Southern California, Los Angeles, California. a J. CHEM. EDUC.,22,144 (1945).
air) and boiling water. In this way the instructor could wait until there was absolutely no danger of causing pressure inside the system before closing the second screw. clamp. When saturated calcium chloride solution was used the distilled water boiled more vigorously than in the case of magnesium sulfate solution, and no distilled water was left by the next period. It was found that the water boiled more smoothly if new boiling stones were used for each demonstration.
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