Chemical Analysis

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infoWhy this? Chemical analysis allows us to identify substances, assess purity, and distinguish a pure chemical from a useful formulation. Chromatography and gas tests develop the use of observable evidence to check claims about composition and product quality.

scheduleWhy now? Following Using Resources, we can distinguish chemically pure substances from mixtures that are suitable for a particular use. Earlier work on electrolysis and atmospheric gases gives gas tests meaningful contexts, and these methods support later investigations of hydrocarbon reactions in Organic Chemistry.

In chemistry, a pure substance is a single element or a single compound that is not mixed with any other substance.
A pure substance has a sharp melting point and a sharp boiling point at fixed temperatures.
An impure substance or mixture melts and boils over a range of temperatures rather than at one fixed temperature.
Impurities often lower the melting point of a solid and make its melting range wider.
A formulation is a mixture designed as a useful product, with components present in carefully measured amounts.
Examples of formulations include fuels, medicines, paints, cleaning products, fertilisers, alloys and foods.
A soluble substance dissolves in a solvent to form a solution, and an insoluble substance does not dissolve in that solvent.
Solubility is the maximum mass of solute that dissolves in a given volume of solvent at a particular temperature.
Chromatography separates substances because different substances have different solubilities in the solvent and different attractions to the stationary phase.
In paper chromatography, the mobile phase is the solvent and the stationary phase is the paper.
A pure substance produces one spot on a chromatogram in a particular solvent, while an impure substance or mixture produces more than one spot.
The Rf value is calculated using `R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent front}}`.
Hydrogen gives a squeaky pop when tested with a lit splint.
Oxygen relights a glowing splint.
Carbon dioxide turns limewater cloudy or milky because calcium carbonate forms.
Chlorine bleaches damp litmus paper, usually turning blue litmus red and then white.

rocket_launchYou must be able to

Use melting point apparatus by heating a small sample in a capillary tube and recording the temperature range over which it melts.
Interpret melting point data by identifying a sharp melting point as pure and a broad melting range as impure.
Carry out paper chromatography by drawing a pencil baseline, placing a sample spot on the line, standing the paper in solvent below the baseline, and marking the solvent front before it dries.
Calculate an Rf value by measuring from the baseline to the centre of the spot and from the baseline to the solvent front, then substituting into `R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent front}}`.
Test for gases safely by using a lit splint for hydrogen, a glowing splint for oxygen, limewater for carbon dioxide, and damp litmus paper for chlorine.

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