Identification of common gases and ions GapFill

When a gas is produced in a reaction, there are many tests which can be used to find out what the gas is. For example:

• a lit splint gives a squeaky pop with hydrogen
• a glowing splint relights in the presence of  heliumoxygenwater vapourneon
• carbon dioxide turns  lime waterthe airacidsmirrors cloudy
•  brominechlorinenitrogenhelium turns litmus paper white

Metal ions can be identified using flame tests, which involve burning a metal compound in a Bunsen burner and observing the distinctive colour of the flame. For example, lithium ions burn  blackmagentavioletcrimson,while calcium ions burn  orange-redindigoolivelilac. Many ions can also be identified from the colour of the  solutiongasprecipitateflame which forms when the metal reacts with sodium hydroxide:

• Calcium and magnesium ions:  bronzeblackwhitefawn, and does not redissolve when more sodium hydroxide is added
• Aluminium ions: white, and dissolves when more sodium hydroxide is added
• Copper(II): blue
• Iron(II):  mauveburgundygreenpurple
• Iron(III):  silveryellowbrownmagenta

There are also qualitative tests for anions:

• Carbonate ions give off carbon dioxide when a dilute acid is added.
•  PhosphateSulfateLargeNegative ions form a white precipitate with barium chloride in hydrochloric acid.
• Halides form coloured precipitates when  calcium sulfatemercurylead chloridesilver nitrate is added. The colour of the precipitate depends on the halide, e.g.  fluorideastatideiodideneon goes yellow.

 QualitativeExperimentalAudioInstrumental methods are more accurate, more sensitive and quicker than chemical methods. One example is flame emission spectroscopy, where metal ions are put through a flame and a  spectrumchromatographtable of datascale is produced.