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  heliumwater vapouroxygenmethanecarbon monoxideneon
• carbon dioxide turns  acidsmetalsthe airmirrorswaterlime water cloudy
•  heliumnitrogencarbon monoxideiodinebrominechlorine 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  magentablackbrowncrimsongreenviolet,while calcium ions burn  goldlilacorange-redoliveindigowhite. Many ions can also be identified from the colour of the  watersolutionflamegascrystalprecipitate which forms when the metal reacts with sodium hydroxide:

• Calcium and magnesium ions:  fawnsilvercolourlessbronzewhiteblack, 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):  pinkgreenburgundypurpleorangemauve
• Iron(III):  magentabrowngreensilverredyellow

There are also qualitative tests for anions:

• Carbonate ions give off carbon dioxide when a dilute acid is added.
•  ChlorideNitrateNegativeLargePhosphateSulfate ions form a white precipitate with barium chloride in hydrochloric acid.
• Halides form coloured precipitates when  calcium sulfatemagnesium chloridepotassium sulfatesilver nitratelead chloridemercury is added. The colour of the precipitate depends on the halide, e.g.  chlorideastatideneoniodidefluoridebromide goes yellow.

 ExperimentalQualitativeInstrumentalVisualAudioOptical 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  chromatographlogarithmscaletable of dataspectrumprecipitate is produced.