The Lost Volume Demonstration
What happens when an equal volume of water and ethyl alcohol mix? Is the mass additive? Is the volume additive? The Law of Conservation of Mass dictates that the mass is conserved, but is there a Law of Conservation of Volume?
No, if you mix 250 mL of water with 250 mL of ethyl alcohol, only about 480 mL of solution results. What caused the volume reduction, and what happened to the lost volume?
Bonding models of liquids
Bonding models for liquids are more difficult to predict due to their significant molecular motion and strong intermolecular forces. Bonding models for liquid water, although extensively studied, are still not fully understood. Recent studies suggest that some regions of pure water may temporarily resemble the solid structure of ice, which has a very open structure to maximize hydrogen bonding. The open structure of solid ice is the reason ice floats in water.
When water and ethyl alcohol mix, several forces come into play. First, between the water and ethyl alcohol is strong hydrogen bonding that draws the different molecules close together. Second, open spaces in the liquid are much less likely because the ethyl alcohol interferes with any temporary open structures that are similar to those of solid water. The 2 different molecules pack closer together than in pure solution, resulting in a reduction of volume.
About the demonstration
There are several ways to perform this demo:
- The preferred way is using 3 volumetric flasks, which provide a good opportunity to
instruct students in the purpose and use of volumetric flasks. However, you can use
whatever glassware is available. Using volumetric flasks makes this a very
quantitative demonstration. The larger the volumetric flask, the easier it is for
students to observe the difference in volume. - A lower-cost alternative to volumetric flasks is using 3 graduated cylinders.
- Use a large-diameter tube (e.g., 10 to 25 mm inside diameter) with 2 rubber stoppers
to seal each end. With this alternative, a vapor bubble appears as the solution is
mixed and the volume reduces.
For each option, the volume of each liquid—water and ethyl alcohol—should be 1/2 the
total volume of the final solution. Use food coloring to provide a visual indicator of the
mixed solution. If needed, add a piece of black electrical tape at the volume line of each
volumetric flask to assist students with their visual observation.
A Discrepant Event Phenomenon
The demonstration is very easy to perform:
Fill a 250-mL volumetric flask with water and another 250-mL volumetric flask with ethyl alcohol. Then, pour both liquids into a 500-mL volumetric flask and observe the difference in volume.
This demonstration is a discrepant event phenomenon because students don’t normally expect the volume to be different and usually cannot explain the phenomenon. This engages students and sets the stage for further exploration, explanation, and expansion of the properties of liquids and intermolecular forces.
Pre-Demonstration
Water-Alcohol Demonstration Table Worksheet Answer Key
| Mass | Volume | Density | |
|---|---|---|---|
|
Water |
250g |
250ml |
100g/ml |
|
Ethyl Alcohol |
197.5g |
250ml |
0.79g/ml |
|
Solution-Prediction |
447.5g |
500ml |
0.90g/ml |
|
Solution-Actual |
447.5g |
480ml |
0.93g/ml |
Questions
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