Density

Density is mass per unit volume, $\rho = m/V$ — how much material is packed into a given space. It is an intensive property, meaning it does not depend on how much of the substance you have: a drop of water and a lake full of it have the same density, because doubling the mass doubles the volume and the ratio does not move. That is what makes it useful as a fingerprint of a material rather than a measure of a sample.

Water is the reference point everyone carries: 1000 kg/m³, which is the same number as 1.000 g/mL and 1.00 kg/L. So a 20 L pail of 12% sodium hypochlorite with a specific gravity of 1.20 holds $m = 1.20 \times 20 = 24$ kg of solution, not 20 kg. Mild steel runs about 7850 kg/m³, aluminium 2700, concrete 2400, and dry air a mere 1.2 — which is why a cubic metre of air weighs about as much as a large apple and is easy to forget entirely.

Rearranged for volume it is how a dosing calculation gets from a required mass of chemical to a number of litres to pump, and it is the density term in hydrostatic pressure, $P = \rho g h$, and in buoyancy. The temperature dependence has consequences well beyond the arithmetic: water is at its densest not when frozen but at 3.98 °C, at 999.97 kg/m³, so ice floats and lakes freeze from the surface downward instead of solidifying from the bottom up.

The mistake that ruins dosing calculations is confusing the density of a solution with the concentration of what is dissolved in it. A 50% sodium hydroxide solution at 1.53 kg/L delivers 1.53 kg of solution per litre and only 0.765 kg of actual NaOH; treating the litre as though it were all caustic doubles your dose. A related slip is confusing density with specific gravity — specific gravity is a dimensionless ratio against water, so it has no units and 1.20 SG is 1200 kg/m³, not 1.20 of anything. Two more: density shifts with temperature, so a chemical metered by volume while hot delivers less mass than the same volume cold, which matters for fuel and for concentrated solutions; and "pounds per gallon" is ambiguous unless you say which gallon, because the imperial gallon is about 20% larger than the US one.