test for cations

• compounds of some metals produce a certain colour when heated in a flame
• metal ions are tested by putting substances in a flame and recording the colour turned
• lithium - red, sodium - yellow, potassium - lilac, calcium - orange red, copper - blue green
• to carry out this rest we clean the wire loop by dipping it in acid and rinsing with distilled water. dip the wire loop into the metal compound and placing the wire loop in the blue part of the flame and record the colour.
• some metal ions in a solution form a coloured precipitate when reactive with sodium hydroxide. most metal hydroxides are insoluble and precipitate out of solution when formed.
• in the metal hydroxide test you add a few drops of sodium hydroxide solution to the sample. if a hydroxide precipitate forms you can see its colour to determine the metal ion
• aluminium - white at first but redissolves with the hydroxide to form a colourless solution. calcium- white Ca2+(aq) + 2OH-(aq) -- Ca(OH)(s), copper - blue, iron(II) - green, iron(III) brown
• to work out if a sustance as ammonium ions (NH4+) you add sodium hydroxide to a mystery solution and gently heat. if ammonia is given off it contains ammonia ions
• ammonia gas is tested by holding a damp red litmus paper over it and having it turn blue. ammonia also has a strong smell and can be irritant and toxic.
• instrumental methods of analysis include x-ray, mass spectroscopy, gas chromotography and flame photometry