Vanishing Volume

DESCRIPTION:
When 50mL of water and50mL of ethanol are combined in a 100mL graduated cylinder, the resulting mixture has a volume of ~97mL. Intermolecular interactions between water molecules in a pure sample involve hydrogen bonding, though the details of these complex interactions are not completely understood.  Some models predict transient ice-like structures within liquid water, where the local density of the sample is temporarily diminished. Ethanol and water molecules are also attracted to each other through hydrogen bonding. The ethanol likely interrupts the transient ice-like structures present in liquid water, resulting in a slight contraction of the sample. This activity demonstrates that volume isn’t always conserved.

TOPICS COVERED:
intermolecular forces
hydrogen bonding
physical change
miscibility
mixtures

MATERIALS NEEDED:
– two 50mL graduated cylinders
– 100mL graduated cylinder
– 95% ethanol
– distilled water

PROCEDURE:
1. Measure out exactly 50mL of 95% ethanol in a 50mL graduated cylinder
2. Measure out exactly 50mL of water in a 50mL gradated cylinder
3. Pour both liquids into a 100mL graduated cylinder

ADDITIONAL COMMENTS:
Food coloring can be added to help visualize. Get students involved by having them read the graduated cylinders, it’s a good time to reinforce how they are read.

SAFETY:
Safety goggles should be worn at all times.

REFERENCES:
Meszaros, M. “The Lost Volume.” Caroline Biological Supply Company. Accessed May 31, 2013. http://www.carolina.com/teacher-resources/Interactive/chemistry-lost-volume-demonstration/tr10785.tr

STORY:
Ask students what 50 + 50 is equal to. This sets up an expectation for them.

Handout