Mixtures and Solutions

Abstract

The world around us is made not of pure substances but of mixtures. The air is a mixture of dinitrogen and dioxygen and small amounts of other gases and vapors. The blood flowing through our veins and arteries is a mixture of water and many thousands of other molecules. When we add salt, NaCl, to water and let it dissolve we call this mixture a solution. The solid NaCl is the solute and the liquid water is the solvent. We measure salt in grams (or kilograms) and water in milliliters (or liters). Another way to express the amount of both the solute and the solvent is to convert them to moles. The ratio of the amount of salt to the amount of water is a fixed property for each solution; we call it concentration. Like any other ratio, concentration is constant throughout the solution: every single, smallest drop of a solution has the same ratio of salt and water. The same with your coffee, only stir that sugar at the bottom of the cup well so it all dissolves. Concentration is therefore an intensive property; in general, two concentrations do not add.