C8 - Chemical Analysis

• A pure substance is a single element or compound, not mixed with any other substance.
• Pure elements and compounds melt and boil at specific temperatures. Melting point and boiling point data can be used to distinguish pure substances from mixtures.
• In everyday language, a pure substance can mean a substance that has had nothing added to it, so it is unadulterated and in its natural state, eg pure milk.
• If a substance has a range of boiling points, it is a mixture, and if it has one fixed boiling point, it is pure
• A formulation is a mixture that has been designed as a useful product.
• .Many products are complex mixtures in which each chemical has a particular purpose. Formulations are made by mixing the components in carefully measured quantities to ensure that the product has the required properties.
• Formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.
• Chromatography can be used to separate mixtures and can give information to help identify substances. Chromatography involves a stationary phase and a mobile phase
• Rf = distance moved by substance / distance moved by solvent
• The ratio of the distance moved by a compound (centre of spot from origin) to the distance moved by the solvent can be expressed as its Rf value
• Different compounds have different Rf values in different solvents, which can be used to help identify the compounds.
• The compounds in a mixture may separate into different spots depending on the solvent but a pure compound will produce a single spot in all solvents.
• The test for hydrogen uses a burning splint held at the open end of a test tube of the gas. Hydrogen burns rapidly with a pop sound. (Squeaky pop)
• The test for oxygen uses a glowing splint inserted into a test tube of the gas. The splint relights in oxygen.
• The test for carbon dioxide uses an aqueous solution of calcium hydroxide (lime water). When carbon dioxide is shaken with or bubbled through limewater the limewater turns milky (cloudy).
• The test for chlorine uses litmus paper. When damp litmus paper is put into chlorine gas the litmus paper is bleached and turns white.