You don't have to be a rocket scientist to understand how distillation works, but it does help to know a little about chemistry - the science of atoms and molecules. Everything is made up of atoms, and those atoms are joined together into millions of different kinds of molecules. Molecules are combinations of atoms, and each different combination forms a unique substance. If a molecule contains more than one type of atom, it is called a compound. We only have to concern ourselves with the properties of a few compounds (water, ethanol, and some congeners), so we can learn what we need to know quite quickly.
The molecules of compounds have their own characteristics, such as size, shape, mass, and electrical charge, which determine the properties of the substance. Painstaking research has determined the relative masses of all atoms, and hence of all molecules. Since atoms and molecules are so extremely tiny, their mass can't be meaningfully discussed in terms of grams or ounces; a new reference for mass was needed. Over the last 100 years, chemists have changed the standard of reference for the mass of an atom or molecule three times, as they gained more and more knowledge about how matter is constructed. The first standard of mass was Hydrogen, then Oxygen was selected, and finally in 1971, Carbon was settled on, and it remains the standard to this day.
The differences between these standards is in fact very small, usually found in the second or third decimal place. For example, in the Oxygen base system, water has a molecular mass of 18.016. In the Carbon system, the mass is 18.015.
For the purposes of calculating anything to do with distilling, rounding to the first decimal place is usually more than enough accuracy, so this really doesn't affect us. Here are the relative masses of some of the compounds you'll encounter when distilling ethyl alcohol:
water 18.016 atomic mass units, methanol 32.042
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