Chemical Analysis

• A pure substance is a single element or compound, not mixed with any other substance.
• Pure substances melt and boil at specific temperatures, which can be used to distinguish them from mixtures.
• In everyday language, a pure substance is a substance that has had nothing added to it, so it is unadulterated and in its natural state, for example, pure milk.
• 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.
• Examples of formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.
• Chromatography is used to separate mixtures and give information to help identify substances.
• Chromatography involves a stationary phase and a mobile phase, and separation depends on the distribution of substances between the phases.
• Rf value is the distance moved by a substance divided by the distance moved by the solvent.
• Different compounds have different Rf values in different solvents, which can be used to help identify the compounds.
• 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.
• Paper chromatography is an analytical technique separating compounds by their relative speeds in a solvent as it spreads through paper.
• The more soluble a substance is, the further up the paper it travels.
• Paper chromatography separates different pigments in a coloured substance.
• Test for hydrogen uses a burning splint held at the open end of a test tube of the gas, which creates a ‘squeaky pop’ sound when hydrogen burns rapidly.
• Test for oxygen uses a glowing splint inserted into a test tube of the gas, which relights in oxygen.
• Test for carbon dioxide involves bubbling the gas through limewater (calcium hydroxide (aq)), which will turn milky (cloudy) when carbon dioxide is present.
• Test for chlorine uses damp litmus paper, which bleaches and turns white when put into chlorine gas.
• Flame tests can be used to identify metal ions, with different colours indicating different ions.
• Aluminium, calcium and magnesium ions form a white precipitate with NaOH, while only aluminium’s precipitate dissolves when excess NaOH is added.
• Copper(II) produces a blue precipitate, iron(II) produces a green precipitate, iron(III) produces a brown precipitate, and equations can be used to identify the metal ions in the solution and measure their concentrations.
• Carbonates react with dilute acids to create carbon dioxide, which can be bubbled through limewater, if the limewater goes cloudy, the gas is CO2.
• Halides can be identified using dilute nitric acid and silver nitrate solution, with chloride giving a white precipitate, bromide giving a cream precipitate, and iodide giving a yellow precipitate.
• Sulfates can be identified using dilute hydrochloric acid and barium chloride solution, with a white precipitate forming when sulfate ions are in this solution.
• Elements and compounds can be detected and identified using instrumental methods, which are accurate, sensitive and rapid, making them advantageous compared to chemical tests.