Mole Day

What the day is

Mole Day is observed on October 23, specifically between 6:02 in the morning and 6:02 in the evening. Written in American date format, that is 6:02 10/23, which encodes 6.02 times ten to the twenty-third: Avogadro's number.

It is a chemistry teachers' holiday, marked in schools with puns, posters and a great deal of enthusiasm for a constant.

Origin and background

The mole is one of the base units of the international system, and it solves a practical problem. Atoms and molecules are far too small to count and too light to weigh individually, but chemical reactions happen in whole-number ratios of particles. The mole bridges the two: it is a fixed number of particles, so a chemist can weigh out a quantity in grams and know how many particles are present.

That number is Avogadro's constant, now fixed by definition at 6.02214076 times ten to the twenty-third. Since the 2019 redefinition of the SI base units it is exact, in the same way the speed of light is exact, rather than being a measured value with uncertainty attached.

Amedeo Avogadro, an Italian scientist working in the early nineteenth century, did not calculate the number and never saw it named after him. His contribution was the hypothesis that equal volumes of gases at the same temperature and pressure contain equal numbers of particles, which was largely ignored during his lifetime and became foundational afterward.

The scale of the number resists intuition. A mole of sand grains would bury a continent; a mole of seconds is many times the age of the universe. A teaspoon of water contains rather more molecules than there are stars in the observable universe.

How to take part

• Work out how many moles are in something ordinary. A glass of water is about 28.
• Do the arithmetic on how large a mole of a common object would be. The answers are genuinely startling.
• Make a bad chemistry pun. This is traditional and expected.
• Thank a science teacher, who has almost certainly built a lesson around this date.
• Look up the 2019 SI redefinition, which changed how several familiar units are defined.