The properties of the stuff itself — the elements and what they are used for, how a fluid behaves as it warms, and what actually sets the strength of a concrete mix. The numbers other calculations start from.
Twenty-six molecules you can turn in 3D — and the shapes are worked out from electron repulsion, not looked up. Water bends to 104.5°, methane is exactly 109.47°, and C60 is built from the golden ratio.
The pH of a strong or weak acid or base with the particles drawn, a titration with the indicator that suits it, a buffer and how it holds, and which form of an acid is in the beaker at any pH.
All 118 elements on real grid coordinates, a nucleus and electron shells you can spin, five uses and five compounds each, and the table coloured by any property you like.
Type any formula for its molar mass and composition, convert grams to moles, molecules and gas volume, and work out how to make, dilute and measure solutions, from a bottle of stock acid.
Type a chemical equation and it is balanced for you; then see which reagent runs out first, how much product you can make, what is left over, the percentage yield and the atom economy.
Density, dynamic and kinematic viscosity for water, oils, air and more, across temperature — the numbers every pipe, bearing and heat calculation starts from.
How dense air and ten other gases are at any temperature and pressure — humid air, what rises and what sinks, and where a gas would turn to liquid.
Squeeze, heat and add to a gas and watch the molecules, solve PV = nRT for any unknown in any units, see how fast the molecules go, and see where a real gas stops behaving like the ideal one.
Cement, sand and aggregate by grade, with the water-cement ratio that actually decides the strength — and how much the dry ingredients shrink when you mix them.