Solutions and concentration

A solution is one substance (the solute) spread evenly through another (the solvent). In salt water, salt is the solute and water the solvent. Water dissolves ionic and polar substances well and oils badly, the rule of thumb being "like dissolves like".

The usual measure of concentration is molarity, M: moles of solute per litre of solution. 0.5 mol of sugar made up to 1 L is 0.5 M. Made up to 250 mL (0.250 L) it's 0.5 ÷ 0.25 = 2.0 M. Note it's per litre of finished solution, not per litre of water added.

To make a solution, work out the moles you need (molarity × litres), turn them into grams with the molar mass, dissolve that in some solvent and top it up to the mark in a volumetric flask. For 500 mL of 0.40 M NaCl: 0.40 × 0.500 = 0.20 mol, × 58.5 g/mol = 11.7 g.

Diluting adds solvent but not solute, so the moles stay the same: C₁V₁ = C₂V₂. To turn 100 mL of 6.0 M acid into 1.2 M: V₂ = 6.0 × 100 ÷ 1.2 = 500 mL, so you add 400 mL of water. (With strong acids, always add the acid to the water, not the other way round, because the heat can make water spit.)

Other measures turn up too. Mass percent is grams of solute per 100 g of solution. Very low concentrations, like a pollutant in water, are given in parts per million (ppm): 1 ppm is 1 mg per kg, which for dilute water solutions is about 1 mg per litre.