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1.3.9 Group 7 Elements: Uses and Halide Tests Properties of Group 7 Elements and Compounds Group 7 elements become less reactive down the group They react with metals to form ionic halides (X- ion) Fluorine is too reactive to study (anything in its way is spontaneously burned!) Summarise Table 1 p94 Halide Ions (X- where X is any group 7 ion) When any reagent is reduced, electrons are given to it. ( ). The electrons are given by the reducing agent, which is itself oxidised. ( ) The halides can therefore be classed as reducing agents as they could possibly lose electrons and form ____________ atoms (which in turn pair up to form halogen molecules): Reducing Power of the halides ____________ down the group. (So iodide ions are strongly reducing whereas fluoride ions are non-reducing) Sodium chloride is the most familiar halide Sodium fluoride and tin fluoride are added to toothpaste to prevent tooth decay Crystals of Calcium fluoride are used to make lenses to focus infrared light Identification of halide ions with silver nitrate Silver chloride, silver bromide and silver iodide are all insoluble salts. They are _________________ when an aqueous solution containing the appropriate halide ion is treated with an aqueous solution of _____________________________. The colours of the three silver salts formed with chloride, bromide and iodide ions, and their different solubilities in aqueous ammonia, can be used as a test for the presence of the halide: Halide ion present ClBrI- Addition of AgNO3(aq) Equation Addition of dilute NH3(aq) Additon of conc NH3(aq) Water treatment Chlorine and chlorine compounds are used in water treatment. For many years, small quantities of chlorine have been added to drinking water and to swimming pools in order to kill bacteria. (The concentration of chlorine in drinking water is ~ 0.7mgdm-3, higher concentrations are used in swimming pools.) Key Definitions Precipitation Reaction -