Practical skills Study Guide

Chromatography is a powerful laboratory technique used to separate and identify substances within a mixture. It relies on the principle that different components in a mixture will have varying affinities for a stationary phase (a solid or a liquid supported on a solid) and a mobile phase (a liquid or a gas that moves through the stationary phase). As the mobile phase carries the mixture through the stationary phase, substances that are more soluble in the mobile phase or less adsorbed by the stationary phase will travel further and faster. This differential movement causes the components to separate, creating distinct bands or spots. The resulting pattern is called a chromatogram.

Paper chromatography, a common type, uses a strip of filter paper as the stationary phase and a solvent as the mobile phase. A spot of the mixture is applied to a baseline on the paper, which is then lowered into a solvent bath, ensuring the solvent level is below the spot. The solvent moves up the paper by capillary action, carrying the mixture components with it. Each component travels a characteristic distance, which can be quantified by its Rf value (retardation factor). Rf = (distance travelled by substance) / (distance travelled by solvent front). This value is unique for a given substance under specific conditions and helps in its identification. For example, if a substance travels 5 cm and the solvent front travels 10 cm, the Rf value is 0.5.

Safety is paramount when performing chromatography. Solvents can be flammable or toxic, so experiments should be conducted in a well-ventilated area, preferably a fume cupboard. Appropriate personal protective equipment (PPE), such as eye protection and gloves, must be worn to prevent contact with chemicals. Care must also be taken when handling glassware to avoid breakage and spills. Disposal of chemical waste must follow laboratory guidelines to ensure environmental protection and safety.

A reactivity series ranks metals by their ease of losing electrons to form positive ions. More reactive metals readily lose electrons and can displace less reactive metals from their compounds. This principle forms the basis for investigating reactivity trends using displacement reactions. In a typical experiment, a metal is placed into a solution containing ions of another metal. If the solid metal is more reactive, it will displace the ions of the less reactive metal from the solution, resulting in the formation of a new solid metal and a metal salt solution. For example, if zinc metal is added to a copper(II) sulfate solution, zinc (being more reactive) will displace copper, forming zinc sulfate and solid copper. The observation of a displacement reaction, such as a change in the appearance of the metal strip or the solution, indicates that the added metal is more reactive than the metal in the compound. By systematically testing different combinations of metals and metal salt solutions, a reactivity series can be established. Safe handling of reagents is crucial as some metal salts can be irritants or harmful, and direct contact should be avoided. Control variables, such as the concentration of solutions, the surface area of the metals, and the temperature, must be kept constant to ensure a fair comparison and reliable results. Visual observations, such as the formation of a new solid metal coating or a colour change in the solution, are key indicators of a displacement reaction.