Flame Tests and the Test for Ammonium Ions Flashcards
what do flame tests basically do
- they indicate the presence of group 1 or 2 metals
- in the form of ions
after you have worn safety gear and have lit the bunsen burner in a fume cupboard, what are the steps for how to do a flame test for a small quantity of a solid
- using a dropper add a few drops of concentrated HCl to the solid
- mix them so the metal begins to dissolve
- dip a clean nichrome wire into the mixture to obtain a sample
- hold the end of the wire in the flame and observe the colour
why is HCl used to dissolve the metal compound
- it converts and metal compound to a chloride
- chlorides are more volatile than other salts
- so they are more likely to give better results
what other alternatives are there other than nichrome wire
- platinum wire
- silica rod
what is one problem with flame tests regarding the metals purity
- many metal compounds contain small traces of sodium compounds as impurities
- so the intense colour of it can mask other colours
what is another problem with flame tests regarding their manner of obervation
- describing the colours is subjective
- people have different levels of colour vision
- so a word description may mean different colours according to the person
what is a simple example of how colour description can become subjective easily
- if someone describes a colour as brick red
- it not only relies on the subjective recollection of what a brick looks like
- but it also assumes everyone has seen the same shade of red on a brick
- some people have seen multiple shades or brick red
regarding the flame tests, what is the colour of lithium ions
red
what is the colour of sodium ions
yellow-orange
what is the colour of potassium ions
lilac
what is the colour of rubidium ions
red-purple
what is the colour of caesium ions
blue-violet
what is the colour of beryllium ions
no colour
what is the colour of magnesium ions
no colour
what is the colour of calcium ions
brick red
what is the colour of strontium ions
crimson red
what is the colour of barium ions
apple green
what is the simple reason for these colours appearing during flame tests
electron transitions
how do electrons result in these colours appearing when the metal compound is being heated
- the electrons absorb energy from the flame and quantum leap to higher energy levels
- from this excited state, the electron quickly loses its energy and leaps back to its original quantum shell
- this results in the emission of electromagnetic radiation
- if it has the right wavelength it will fall within the range of visible light which we can see
therefore why does magnesium have no colour in the flame test
- because the electromagnetic radiation emitted doesnt have a wavelength that falls within the range of the visible spectrum
- therefore we cant see it
- but it would still be infrared or ultraviolet
what is the process for the test for ammonium ions
- add sodium hydroxide solution
- warm the mixture
- the warming releases ammonia gas
what is the reaction occurring between the ammonium ions and sodium hydroxide solution
NH4+ + OH- = NH3 + H2O
how do you know if ammonium ions were present from this test
- it can be recognised from its smell
- but its better to use damp litmus paper
- if it turns blue ammonia gas is present
- therefore the ions were present
what is a further reaction you could carry out to prove the ammonia ions presence
- react the ammonia gas with HCl
- it forms white fumes of ammonium chloride
- NH3 + HCl = NH4Cl
