Puns abound in our Valentine’s day video, but we’re not about to let a holiday get in the way of talking about cool science!
These cool color changes are brought about by chemistry, specifically by a little number known as pH. While it is pretty common knowledge that this number indicates how acidic something is, understanding acidity is a little more complicated but surprisingly elegant! Let’s start by exploring a surprising fact about water! This precious little molecule known as dihydrogen monoxide has a secret: it doesn’t always stay as H2O! As these tiny water molecules collide, they sometimes exchange hydrogen atoms, like this:
Animation by Dr. Walt Volland
This is an equilibrium, meaning the reaction is constantly going both directions. The H30+ ions are called hydronium and cause water to be more acidic. The OH– ions are called hydroxide and make water more alkaline or basic.
CAUTION: MATH AHEAD!
In distilled water, which doesn’t contain anything else, this happens so that each of these has a concentration of 10-7, which is very very small. pH is just the negative power of the concentration of hydronium in this concentration, so distilled water has has a pH of 7, which is neutral. All of this is based on the equilibrium in the animation above.
Conveniently, the concentrations of the two always have their powers add up to -14. This means if you add a bunch of acid to some water to get a pH of 1 (which would be a concentration of 10-1), the concentration of the OH- ions would be 10-13. This should make some sense: If you pour a bunch of acids and bases together, you don’t end up with something that is an acid and a base. Instead, they combine to form water, until there is a little of one left over.
Unfortunately, just looking at a pool of water isn’t enough to tell if it is acidic or basic, since our eyes can’t see the tiny ions that are responsible floating around. Instead, we used something called Universal Indicator, which changes color depending on the pH of the solution it is in. Those colors can reveal what the solution is like by using the chart below.
The preliminary blue heart matches up with about a pH of 10. Since this is greater than 7, we know the solution is basic (or alkaline). Next, a spritz of vinegar is added, resulting in a change of heart. The color shifts to a pinkish color, which is on the acid side of the scale.
Universal indicator is not the only way to see pH. The second heart starts with a clear liquid, which actually contains an indicator called phenolphthalein. This indicator remains clear at all pH’s above 8.2, and between 10 and 13 it changes to a very holiday appropriate fuschia! To get this result, we simply added a dilute ammonia solution!
Happy Valentine’s Day!