Chemical Analysis: GCSE AQA Combined Science

Welcome to this course on chemical analysis. In the following modules, we will look at pure substances, formulations, chromatography and the identification of common gases.

Course Introduction This course covers Topic 5.8 Chemical Analysis which is examined on Chemistry Paper 2 of the GCSE AQA Combined Science Course. Topics covered include: Pure substances Formulations Chromatography Identification of common gases This course contains one specified practical: Investigate how paper chromatography can be used to separate and tell the difference between coloured substances and calculate Rf values.

In nature, elements are often blended together to greater or lesser degrees. Compounds are substances that are chemically bonded together. Mixtures are elements/compounds that are mixed together, though not at the chemical level.

Formulations may be defined as mixtures that have designed with a utility in mind. In complex mixtures, each chemical serves a given purpose. Fertilizers are a good example of a mixture in action.

Formulations Formulation chemistry is the mixing of compounds/substances that do not react with each other but produce a mixture with the desired characteristics/properties to suit a particular application/use.   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: Fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.

Chromatography is a technique used by chemists. It can be used to identify and separate substances that are contained in a mixed solution. There are several different types including paper and thin layer chromatography.

This illustration demonstrates paper chromatography in action. This experiment can be performed in a laboratory using a simple solvent such as water. The paper is submerged and the resultant spots show the chemical composition.

There are a number of ways to identify common gases. Testing for chlorine gas, for instance, uses damp litmus paper. In the presence of chlorine the paper will bleach.

Identification of Common Gases Hydrogen 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.   Oxygen The test for oxygen uses a glowing splint inserted into a test tube of the gas. The splint relights in oxygen.   Carbon dioxide 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).   Chlorine The test for chlorine uses litmus paper. When damp litmus paper is put into chlorine gas the litmus paper is bleached and turns white.