Singapore-Cambridge O-Level Chemistry

Measuring apparatus

Time is measured with a stopwatch or stop clock, temperature with a thermometer, and mass with a balance. Volume is measured with different pieces of apparatus depending on how precise the measurement has to be.

A burette is read at the bottom of the meniscus with the eye level with it, and the volume delivered is the difference between the final and initial readings.

Collecting and drying gases

The way a gas is collected depends on how it behaves. A gas that is not very soluble in water, such as hydrogen or oxygen, can be collected over water by displacement, which also shows how fast it is being made. A gas syringe collects any gas and measures it at the same time. A gas that is denser than air, such as carbon dioxide or chlorine, is collected in a gas jar by downward delivery (upward displacement of air). A gas that is less dense than air and very soluble in water, such as ammonia, is collected by upward delivery (downward displacement of air), with the jar held upside down.

A gas made in solution is usually damp. It is dried by passing it through a drying agent, and the agent has to be one that does not react with the gas.

For rate experiments, the apparatus has to allow something to be measured as time passes: a gas syringe or an upturned measuring cylinder of water for the volume of gas, or a balance under the flask for the loss in mass as gas escapes.

Separating mixtures

The method depends on which physical property differs between the substances, so the first step is to decide what kind of mixture it is.

Solid from liquid

An insoluble solid is separated from a liquid by filtration. The mixture is poured through filter paper in a funnel, the solid stays on the paper as the residue, and the liquid that passes through is the filtrate.

A dissolved solid is recovered by heating the solution. If the solid is stable and is not damaged by heat, all the solvent can be evaporated. If the solid decomposes on strong heating or contains water of crystallisation, the solution is heated only until it is saturated, which is when a drop placed on a cold glass rod forms crystals. The hot saturated solution is then left to cool, crystals form, and they are filtered off, washed with a little cold distilled water and dried between filter paper. This is crystallisation.

Solid from solid

If one solid dissolves in a solvent and the other does not, the mixture is stirred with the solvent, filtered, and the dissolved solid is recovered from the filtrate by evaporation or crystallisation. A mixture of sand and salt is separated this way, with water as the solvent.

Sublimation separates a solid that changes straight from solid to gas on heating from one that does not. Ammonium chloride sublimes and sand does not, so heating the mixture drives off the ammonium chloride, which condenses again on a cold surface above the dish.

Liquid from liquid

Immiscible liquids, such as oil and water, form two layers. They are separated with a separating funnel: the tap is opened and the denser, lower layer is run off into one container, and the upper layer is then poured out or run into another.

Miscible liquids mix completely and have different boiling points. A mixture of two of them is separated by distillation if only one of them needs collecting, for example obtaining pure water from salt solution: the solution is boiled, the vapour passes into a condenser cooled by water running in at the bottom and out at the top, and the pure liquid that condenses is the distillate.

Fractional distillation separates a mixture of liquids with similar boiling points, such as ethanol and water. A fractionating column is placed between the flask and the condenser. Vapour rising in the column condenses on the packing and runs back, and is vaporised again by the hotter vapour below it. This happens repeatedly, and each time the vapour becomes richer in the liquid with the lower boiling point, so that liquid reaches the top of the column and the condenser first. The thermometer reading stays close to that liquid's boiling point while it is being collected, and then rises when it has mostly gone and the next fraction begins to arrive.

Paper chromatography

Paper chromatography separates substances, often dyes, that dissolve in the same solvent, because they are carried along a sheet of paper at different speeds. A small spot of the mixture is placed on a pencil line, known as the origin, near the bottom of the paper. The line is drawn in pencil because ink would dissolve and travel up with the solvent. The paper is stood in a solvent so that the solvent level is below the origin, and the solvent rises up the paper by capillary action carrying the substances with it. A substance that is more soluble in the solvent and less strongly held by the paper moves further. When the solvent front is near the top, the paper is removed and the position of the front is marked at once, because it will otherwise evaporate.

The result is a chromatogram. A pure substance gives one spot. The spots from an unknown mixture can be compared with spots from known samples run on the same paper: a spot at the same height as a known sample, with the same colour, is probably that substance. The retention factor is

and its value is always between 0 and 1. For a given solvent and type of paper, each substance has a characteristic value, so the value can be used to identify it. For example, a spot $\pu{3.6 cm}$ above the origin when the solvent front has moved $\pu{9.0 cm}$ has $R_f = 3.6 \div 9.0 = 0.40$.

Colourless substances, such as amino acids, cannot be seen on the paper. The chromatogram is sprayed with a locating agent, a substance that reacts with the colourless compounds and forms coloured products, so that the spots appear. The particular agents do not need to be known.

Melting point, boiling point and purity

A pure substance melts and boils at fixed temperatures, and these can be looked up and compared with measured values to identify it. A pure solid melts sharply at one temperature, and a pure liquid boils at one temperature that stays constant while it boils.

An impurity lowers the melting point of a solid and raises the boiling point of a liquid, and it makes melting and boiling happen over a range of temperatures instead of at one. So a sample that melts over a range, or at a temperature below the expected value, is impure, and the closer the melting range is to the known value the purer it is.

The purity of substances used in everyday life matters because impurities may be harmful or may change how the substance works. A drug with impurities may have side effects or contain less of the active ingredient than the label says, and contaminated food may be toxic. Melting and boiling point measurements, and chromatography, are used to check purity.