In a high school science class, Riley was given 50 milliliters of water and 50 milliliters of ethanol. His task was to mix them together and then run an experiment on the mixture.
When the teacher came to check on him, however, Riley's mixture only contained 94 milliliters. The teacher accused him of drinking some of the mixture in an attempt to get intoxicated and immediately sent him to the principal. Riley swore that he did NOT drink the mixture and he didn't lose any of the liquids by any means.
What happened to the other 6 milliliters and how could Riley prove his innocence?
Liquids evaporate roughly in proportion to their vapor pressure and the exposed surface area.
At room temperature, water will evaporate from a cylinder (drinking glass size) of 6 cm diameter, dropping at about 2 mm per hour. Likewise, for a wider container: a liter beaker of 10 cm diameter, the level will drop about 6 mm per hour. In 1/3 this time, 20 min, the water level will therefore drop about 2 mm in this beaker.
Ethanol has about 2.5 times the vapor pressure of H2O (5 kPa vs. 2.3 kPa), so the ethanol level will drop about 5 ml in 20 minutes from the same beaker. This is in-line with the teacher's observations. (Note: my "liter beaker" is actually a 10 x 10 x 10 cm cube for ease of calculation.) Of course other variables play-in here: humidity, air movement. Mixing the liquids doesn't mater. The evaporation effect is fairly well represented even as simply as above.
Also, the school really needs a better chemistry teacher.
Edited on April 16, 2022, 9:43 am
soln
