The most official water costs $120,000 a gallon

We all learned in science class that water freezes at 0 °C or 32 °F at atmospheric pressure. But what water, exactly? Even after you distill out all the dissolved salts and minerals to get pure H2O, not all H2O is created equal – and for the most precise temperature measurements, the differences really matter. Here’s the problem: hydrogen and oxygen atoms aren’t all identical. All atoms of a single element have the same number of protons by definition, but they can have different numbers of neutrons, forming different isotopes. We need to know how much of each isotope to use for our experiments. Credit: OpenStax First, let’s go over the isotopes of hydrogen and oxygen. Hydrogen has one proton and either zero, one or two neutrons forming its isotopes protium, deuterium, and tritium. Oxygen has 8 protons and either 8, 9, or 10 neutrons forming the much less creatively named oxygen-16, oxygen-17, and oxygen-18. Different isotopes of an element behave similarly, but not exactly the same.…