Demonstration pubs.acs.org/jchemeduc
Simple Demonstration of Vapor Pressure Lowering Wayne H. Pearson* Chemistry Department, United States Naval Academy, Annapolis, Maryland 21402, United States ABSTRACT: A simple, qualitative demonstration of the vapor pressure lowering of aqueous solutions is presented. The demonstration consists of comparing two liquids, deionized water and a salt solution, in closed, glass containers. The difference in the degree of condensation inside of the two containers illustrates the difference in the vapor pressures of the two systems.
KEYWORDS: First-Year Undergraduate/General, High School/Introductory Chemistry, Demonstrations, Inquiry-Based/Discovery Learning, Phases/Phase Transitions/Diagrams, Physical Properties, Solutions/Solvents Allowing the flasks to sit for a period of time in the same environment allows for enough thermal cycling to result in significant condensation in the flask containing the DI water. Once established, the flasks can be used repeatedly over a period of years with no further preparation. The amount of condensation that results in both flasks will depend upon the ambient temperature. To increase the amount of condensation, simply place both of the flasks in a refrigerator for an hour prior to use. Photographs of two Florence flasks containing the different liquids are shown in Figure 1.
C
olligative properties are physical properties of solutions that have interesting and useful applications. The four properties that are generally discussed in introductory chemistry courses are freezing point lowering, boiling point elevation, osmotic pressure, and vapor pressure lowering. There are simple ways that can be used to demonstrate the first three of these properties. Instructors can show that salt solutions freeze at temperatures below the normal freezing point of pure water and boil at temperatures higher than the normal boiling point. Osmotic pressure can be shown to cause vegetables to shrivel up in salt solutions and swell in pure water. However, the vapor pressure lowering phenomenon is a bit more difficult to demonstrate and to relate to everyday life. In the past, several demonstrations have been developed that clearly show vapor pressure lowering either by measuring the pressure with a manometer1,2 or using a decrease in color of Br2 gas mixed with chloroform to demonstrate the decrease in the vapor pressure of the bromine. 3 The main advantages of this new demonstration are the very simple nature of the demo, the connection to observations that students may have already seen in water bottles, and the use of aqueous chemistry that is nonhazardous and familiar to beginning students.
Figure 1. Florence flasks containing DI water and a salt solution.
■
■
APPARATUS The demonstration involves two sealed, glass containers containing different liquids. One liquid is deionized water, whereas the other liquid is a 6.0%, by weight, NaCl(aq) solution. More concentrated solutions could be used but the 6.0% solution works well. Any sealed, glass container could be used, but plastic soda bottles should be avoided. If the salt solution comes into contact with the sides of a plastic soda bottle, the solution does not disperse well over the surface of the plastic. Drops can form that hang onto the sides of the plastic that are not the result of a condensation process. For the best results, it is recommended that flasks be prepared at least a couple of days before the demonstration. This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society
HAZARDS There are no chemical hazards associated with the DI water and salt solutions. The only possible hazard lies in handling the glassware. In the event of breakage, cleanup would consist of properly disposing of the glass while flushing solutions down the drain.
■
OBSERVATIONS The clear difference between the two flasks is the amount of condensation that appears on the sides of each flask. In a small
A
dx.doi.org/10.1021/ed300419x | J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Demonstration
classroom holding 20 students, the effect is visible from the back row. For larger lecture halls, the flasks can be projected using a document camera or overhead projector. In an ideal setting with flasks that have reached thermal equilibrium, there should be no condensation inside of the flask that contains the salt solution. The vapor pressure would not be high enough to support condensation and the subsequent water−water vapor equilibrium. However, depending upon the thermal history of the salt water flask, there may a slight amount of condensation present. There will always be more condensation in the flask containing the distilled water.
■
DISCUSSION The instructor can discuss the demonstration in a number of ways. The demonstration could simply be explained to the class but a question-and-answer session can be very effective. Once the class has agreed that there is more condensation in one flask than the other, a major conclusion that needs to be reached is that the mechanism of condensation occurs through the gas phase. More condensation would be consistent with more water molecules in the gas phase. The presence of more water molecules in the gas phase would result in higher vapor pressure. An alternative approach would be to have students reach these conclusions in small group discussions. An instructor can identify the composition of the liquids from the beginning of the demonstration or simply label them as A and B in order to introduce a bit of mystery. There are several ways to identify the two liquids. A taste test is all that is needed to identify the salt solution. If an instructor is uneasy about tasting a liquid that is poured from a piece of chemical glassware, alternatives exist for testing the solutions. Equal volumes of liquids could be delivered into graduated cylinders and weighed. A conductivity apparatus could also be used. Once the identities of the two liquids are known, the discussion can shift to why the salt solution has the lower vapor pressure.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Brandou, J. R. A Qualitative Demonstration of Raoult’s Law. J. Chem. Educ. 1961, 38, A545. (2) Koubek, E. A Simple Apparatus Designed To Measure Vapor Pressures and Demonstrate the Principles of Raoult’s Law. J. Chem. Educ. 1983, 60, 1069. (3) Wilson, A. S. A Visual Demonstration of Raoult’s Law. J. Chem. Educ. 1990, 67, 598.
B
dx.doi.org/10.1021/ed300419x | J. Chem. Educ. XXXX, XXX, XXX−XXX
