Experimental techniques: Class 11 Chemistry

Key points

Most substances found in nature or made in the laboratory are impure. Chemists first separate the wanted substance from its impurities, then test how pure it is. The method chosen depends on the physical state of the mixture and on differences in properties such as solubility, boiling point and volatility.

Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper held in a funnel. The solid stays on the paper and is called the residue. The clear liquid that passes through is called the filtrate. Sand from water and chalk from water are common examples. A hot filtration, using a warm funnel, is used when a solution must be filtered without crystals forming in the funnel.

Crystallisation purifies a solid that is soluble in a hot solvent. The steps are:

• Dissolve the impure solid in the minimum amount of hot solvent to make a nearly saturated solution.
• Filter the hot solution to remove insoluble impurities.
• Allow the filtrate to cool slowly. The solubility falls and pure crystals form, while soluble impurities stay in the solution (the mother liquor).
• Filter off the crystals, wash them with a little cold solvent and dry them.

A good solvent for crystallisation dissolves the compound well when hot but only slightly when cold. It must not react with the compound, and it should be easy to remove by drying. Water is used for salts and sugar. Ethanol is used for many organic solids. Fractional crystallisation separates two solids whose solubilities change differently with temperature.

Sublimation is the change of a solid directly into vapour on heating, without passing through the liquid state. The vapour condenses back to a solid on a cold surface. It purifies solids that sublime, such as ammonium chloride, iodine, camphor and naphthalene, from non-volatile impurities such as sand or salt. The mixture is heated in a china dish covered with an inverted funnel, and the pure solid collects on the cooler funnel walls.

Simple distillation separates a liquid from a dissolved non-volatile solid, for example pure water from salt solution. The liquid is boiled in a flask, the vapour passes through a water-cooled Liebig condenser, and the condensed liquid (the distillate) is collected. The thermometer bulb is placed at the side arm so that it reads the temperature of the vapour.

Fractional distillation separates a mixture of miscible liquids whose boiling points are close, for example ethanol and water, or the fractions of petroleum. A fractionating column packed with glass beads is fitted above the flask. Repeated condensation and evaporation on the column enrich the vapour in the more volatile liquid, which reaches the top first. Steam distillation is used for substances that are immiscible with water and decompose at their own boiling point, such as aniline and essential oils. The mixture boils below 100 °C, so the substance is not damaged.

Solvent extraction removes a dissolved substance from its solution by shaking it with another solvent that does not mix with the first one. The substance is more soluble in the second solvent, so it moves into it. A separating funnel is used and the two layers are run off. The distribution of a solute between two immiscible liquids at a fixed temperature follows the partition law: the ratio of its concentrations in the two solvents is constant. Several small extractions are more efficient than one large extraction.

Chromatography separates the components of a mixture because they move at different rates over a stationary phase carried by a moving phase. The mobile phase is a liquid or gas. In paper chromatography a drop of the mixture is placed on a baseline on filter paper, and the paper is dipped in a solvent without touching the spot. The solvent rises by capillary action and carries each component a different distance. Water held in the paper fibres acts as the stationary phase.

In thin layer chromatography (TLC) the stationary phase is a thin layer of silica or alumina on a glass or plastic plate. In column chromatography it is packed in a glass tube and the mixture is washed down with a solvent. Colourless spots can be seen with ultraviolet light or by spraying a developing agent. The result is described by the Rf value, the ratio of the distance moved by the solute to the distance moved by the solvent front. Rf is always less than 1, has no unit, and is constant for a substance under the same conditions.

Purity is tested by physical constants. A pure solid has a sharp melting point, melting over a range of about 1 °C. A trace of impurity lowers the melting point and widens its range. A pure liquid boils at a fixed temperature at a given pressure, and it is a single spot in chromatography. The boiling point changes with external pressure, so it must be recorded with the pressure.

A pure substance has a fixed composition and fixed properties, while a mixture has variable composition and its parts keep their own properties. Always match the method to the mixture: filtration for a solid in a liquid, sublimation for a subliming solid, distillation for liquids, chromatography for small samples of similar substances.

Study tip: Learn each method as a named apparatus diagram with labels, then write one sentence on the principle and one example. In numerical questions, show the formula first, then substitute. For Rf and percentage recovery, always check that the answer is sensible: Rf below 1 and recovery below 100 %.

Definitions and flashcards

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Common mistakes

• Placing the solvent level above the baseline in chromatography, so the sample dissolves in the solvent instead of rising with it.
• Writing a unit for Rf or giving a value greater than 1. It is a ratio of two distances.
• Putting the thermometer bulb in the boiling liquid during distillation. It must be at the side arm to read the vapour temperature.
• Using a large amount of solvent in crystallisation, which gives a dilute solution and very few crystals.
• Saying that sublimation works for every solid. It only works for solids that turn to vapour on heating, such as iodine and camphor.

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