The Lost Volume
By Mark W. Meszaros, PhD
Vice President, Core Product Management and Innovation
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
This demonstration is best performed 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. A 3rd alternative is using 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 all 3 options, the volume of each liquid—water and ethyl alcohol—should be ½ 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
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 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.
Start the demonstration by asking students to predict what will occur when an equal volume of water and ethyl alcohol are mixed. Specifically, ask them if they think the mass and volume will be additive. The following table is the answer key to the Water-Alcohol Demonstration Worksheet located at the end of this activity. During the demonstration, students record data to their worksheet table.
Water-Alcohol Demonstration Table Worksheet answer key
|Water||250 g||250 mL||1.00 g/mL|
|Ethyl Alcohol||197.5 g||250 mL||0.79 g/mL|
|Solution—Prediction||447.5 g||500 mL||0.90 g/mL|
|Solution—Actual||447.5 g||480 mL||0.93 g/mL|
Warning: Ethyl alcohol is a flammable liquid and is toxic by ingestion and inhalation.
- Read all MSDS and labels for chemical hazards.
- Wear chemical-resistant apron, gloves, and goggles.
- Dispose of waste according to local, state, and federal regulations.
- 250 mL Water
- 250 mL Ethyl Alcohol, 95%
- Food Coloring, Blue and Yellow
- 2 Volumetric Flasks, 250 mL
- Volumetric Flask, 500 mL
- Stopper or Cap for 500-mL Volumetric Flask
- Graduated Cylinder, 25 mL
- Utility Funnel
- Disposable Pipet
- Electronic Balance, 1500 g
- Mass an empty 250-mL volumetric flask. Record the data.
- Add a few drops of blue food coloring to this flask.
- Fill the flask to the volume line with water.
- Mass the volumetric flask containing the tinted water. Record the data.
- Calculate the mass of the tinted water.
- Ethyl Alcohol
- Mass the second 250-mL volumetric flask. Record the data.
- Add a few drops of yellow food coloring to this flask.
- Fill the flask to the volume line with ethyl alcohol.
- Mass the volumetric flask containing the tinted ethyl alcohol. Record the data.
- Calculate the mass of the tinted ethyl alcohol.
- Record Data/Predict Outcome Have students fill in their data table for the mass and volume of the flask of tinted water and the flask of tinted ethyl alcohol, then calculate the density of each. Students should also predict the mass, volume, and density of the water and alcohol solution when it is poured into the 500-mL volumetric flask.
- Water-Alcohol Mixture
- Mass the empty 500-mL volumetric flask. Record the data.
- Pour the tinted water into the 500-mL volumetric flask, using a funnel if needed.
- Pour the tinted ethyl alcohol into the 500-mL volumetric flask, using a funnel if needed.
- Note the level of the liquid in the 500-mL volumetric flask. (It should be very close to the 500-mL volume line.)
- Place a stopper or cap on the 500-mL volumetric flask and invert the flask several times.
- Remove the stopper or cap to prevent a vacuum from occurring inside flask.
- Replace the cover and continue mixing the solution by inverting the flask until a green color is consistent throughout the solution.
- Allow the solution to settle, then observe the volume of liquid. (It should be far below the 500-mL volume line.)
- Mass the 500-mL volumetric flask containing the water-alcohol solution. Record the data.
- Calculate the mass of the water-alcohol solution.
- Determine how much volume was lost in the creation of this solution. Using a disposable pipet, add water from a graduated cylinder to the volumetric flask containing the water-alcohol solution. Subtract this volume from 500 mL to obtain the final volume. Record the data.
- Answer Questions Have students fill in the remainder of their data table and answer the questions on the Water-Alcohol Demonstration Worksheet.