Surface Tension of Water Vs. Mineral Oil

Suppose someone were to walk up to you and strike a question - a question that you’d first think, “Well it’s obvious. The answer is (blank answer).” But after a few moments of pondering, you might think, “Well, actually, it might not be so obvious. Maybe the answer is (other blank answer).” Questions that conclude with this sort of format often keep people up at night, unable to clear their mind enough to find some decent sleep. So today, we’ll be tackling one of those questions and it’s: Do all liquids have the same surface tension? For example, if you had identical surfaces, temperatures, methods of transferring liquids, basically every control group properly maintained, would plain water have more or less surface tension than mineral oil?

This question was solved during a typical day of Mrs. Wixom’s chemistry class - at least as typical chemistry class can be… To solve this question, the class conducted a semi-basic lab activity in which students collected results of the amount of water drops that could be placed on the surface of a US penny. This was done 3 times before conducting the same acts, but with mineral oil. Once this had been completed, averages for both the water and the mineral oil was calculated.

It was found that more water could be placed on the surface of a US penny than mineral oil could be. Below, you’ll see a table of data recordings from the lab activity that illustrate the stated findings:

At first, it might seem strange as to why more water drops can be placed on a predetermined surface area than mineral oil drops, but it can all be explained with intermolecular forces (IMF).

There are three different things you’ll need to know about when talking about IMF: hydrogen bonds, dipole-dipole interaction, and London Dispersion Forces.

Hydrogen Bonds

• Strongest intermolecular force

• Occurs when a hydrogen atom is attracted to a highly electronegative atom (always either a nitrogen atom, an oxygen atom, or a fluorine atom)

Dipole-dipole bonds

• An attraction between oppositely charged regions of polar molecules

London Dispersion Forces

• The weakest force that all covalent molecules experience

• Electrons from these molecular forces are constantly in motion and sometimes they collect in one area of the molecule that creates a temporary dipole moment. This induces a dipole moment in the molecule next to it, which produces a weak interaction between the molecules.

In the situation of this lab, the reason more water drops are able to sit atop of a US penny than mineral oil can is because water molecules(H2O) have stronger and tighter hydrogen bonds between them. Since hydrogen bonds are the strongest intermolecular force, the molecules of water are going to have higher surface tension than the molecules of the mineral oil. Mineral oil is non-polar, therefore the only interaction it can achieve would be a London Dispersion Force, resulting in super low surface tension. So to conclude this all, do all liquids have the same levels of surface tension? The answer would be no, at least based off of the data that has been given above.