Purity and Separating Mixtures
Pure Substances in Chemistry
In everyday language, “pure” often means natural or uncontaminated. In chemistry, the meaning is very specific: a pure substance is one that contains only a single element or a single compound. For example, pure water contains only \(H_2O\) molecules and nothing else. Orange juice labelled “pure” in a supermarket is not pure in the chemical sense because it is a mixture of water, sugars, citric acid and many other compounds. This distinction between the everyday and scientific meanings of “pure” is frequently tested in the exam.
You can test whether a substance is pure by measuring its melting point. A pure substance has a sharp, precise melting point — it melts at one exact temperature. An impure substance (a mixture) melts over a range of temperatures, and its melting point is lower than expected. Similarly, a pure substance has a fixed boiling point, while an impure substance boils over a range.
In chemistry, a pure substance contains only one element or one compound. A pure substance melts and boils at a single sharp temperature. An impure substance melts over a range of temperatures.
Relative Formula Mass
The relative formula mass (\(M_r\)) of a compound is found by adding up the relative atomic masses (\(A_r\)) of all the atoms in its formula. For example, for water \(H_2O\): \(M_r = (2 \times 1) + 16 = 18\). For calcium carbonate \(CaCO_3\): \(M_r = 40 + 12 + (3 \times 16) = 100\).
You can also deduce the empirical formula of a compound from the relative numbers of atoms present or from a model or diagram. The empirical formula shows the simplest whole-number ratio — for example, glucose \(C_6H_{12}O_6\) has the empirical formula \(CH_2O\) because the ratio of C:H:O is 1:2:1. You may be asked to deduce the empirical formula from a model, a diagram, or data about the relative numbers of atoms present.
Formulations
Many useful materials are formulations — mixtures that have been carefully designed so that each component is present in a measured quantity to give the product specific properties. Examples include alloys (mixtures of metals), paints, medicines, cleaning products and fuels. An alloy such as steel is a formulation of iron with small amounts of carbon and sometimes other metals such as chromium (in stainless steel). Other examples include paints (pigments, binders, solvents in precise ratios), medicines (active ingredient mixed with fillers and flavourings), and even concrete (cement, sand, gravel, water).
Separation Techniques
Because the components of a mixture are not chemically bonded, they can be separated by physical methods. The choice of technique depends on the properties of the substances in the mixture.
Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper in a funnel — the liquid passes through (the filtrate) and the solid stays behind (the residue). Crystallisation separates a dissolved solid (solute) from a solution. The solution is heated gently to evaporate the solvent until crystals begin to form, then left to cool so that more crystals grow. This is used to produce pure, dry samples of salts.
Simple distillation separates a solvent from a solution — for example, water from salt water. The solution is heated until the solvent evaporates, then the vapour is cooled and condensed back into a liquid in a separate container. Fractional distillation separates a mixture of liquids with different boiling points. The mixture is heated in a flask fitted with a fractionating column. The liquid with the lowest boiling point evaporates first, rises to the top of the column, and is collected. Each liquid is collected separately as the temperature rises.
Chromatography
Chromatography separates dissolved substances. It works because different substances have different affinities for a stationary phase (such as paper) and a mobile phase (such as a solvent). In paper chromatography, a spot of the mixture is placed on a pencil line near the bottom of chromatography paper. The paper is placed in a solvent, which rises up the paper carrying the dissolved substances with it. Substances that are more soluble in the mobile phase travel further up the paper.
The \(R_f\) value identifies a substance and is calculated as: $$R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent front}}$$ Each substance has a unique \(R_f\) value for a given solvent and stationary phase. A pure substance produces a single spot; a mixture produces multiple spots.
Other chromatographic methods include thin layer chromatography (TLC), which uses a thin layer of silica or alumina on a glass plate as the stationary phase, and gas chromatography, which uses a gas as the mobile phase. Locating agents (such as UV light or ninhydrin spray) may be needed to make colourless spots visible on the chromatogram. Gas chromatography uses a gas as the mobile phase and can separate complex mixtures that would be impossible to analyse by paper chromatography alone.