Question

A student is performing an experiment for surface tension. She is dropping two different substances with a dropper onto two pennies. 15 drops of oil are dropped onto one penny until it spilled over. 20 drops of rubbing alcohol were dropped onto another penny until it spilled over. Which substance had stronger electrical forces - oil or isopropyl alcohol? How do you know?

Answer 1

If one substance is more polar than another, it will be more resistant to gravity. The mutual attraction of water molecules is called cohesion. This cohesion pulls a drop into a spherical shape, because that’s the shape that allow for the maximum number of bonds.

For the oil we have:

$15_{\text{ }}drops$

For the alcohol we have:

$20_{\text{ }}drops$

So:

$15<20$

We can conclude oil is less polar than alcohol, it can’t resist gravity, and all of the molecules are pulled down into a flat sheet over the penny’s surface. This flat sheet can accommodate many fewer drops of oil than the hydrogen-bond induced sphere that’s on the penny with alcohol.