Chemists are sort of like detectives, they take small clues that allow them to make an analysis about a particular chemical. There are a variety of tests designed to give one more clue to identify a compound. Flame tests can help distinguish certain metal ions by observing the colour of the flame when the chemical is burnt. There may be more than one metal ion in a mixture and the flame test may not be able to give a definite answer to which ion is present, but it can give a hint into which direction the chemist should look.
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Flame Test Basics
In a flame test, a small bit of a sample is ignited over a flame to observe a change in colour in the flame. It is very basic in nature, but can take a little bit of practice to use it correctly. Two different types of wires can be used, platinum and nichrome. There will be a little bit of orange in the flame due to the nichrome which platinum does not do. Platinum is better to reduce the interference, but is very expensive in comparison.
Performing a Flame Test Experiment
You will need a Bunsen burner, nichrome wire, concentrated hydrochloric acid and a sample of interest. Clean the wire by dipping it into the acid and holding it in the flame repeatedly until the wire doesn't produce a colour in the flame. There will be a small tinge of orange when using the nichrome wire, which is normal. Moisten the tip of the wire with more acid then dip into a sold sample, hold in the flame and observe the colour change. The results can be compared with a table of standard flame test outcomes to help determine what is in the compound. Liquid samples can by making a small loop in the end of the wire and dipping it directly into the liquid after the wire has been cleaned.
Why Are There Different Colors?
Different wavelengths of light produce various colours that can be observed. This done by a process that involves an electron becoming excited into a higher energy state. When an electron is in this higher energy state, it is very unstable and will tend to fall back into the lower energy state. This can be done in a single step or in several to get the electron back to the ground state in which it is more stable. The falling into the lower energy produces energy to be released in the form of light. The various levels that an electron falls from will emit a different amount of energy and a corresponding colour. Certain ions will undergo a predictable excitability state and thus a predictable colour will be given to compare.
Common Color Changes
As discussed before, there are predictable colour changes that happen with certain metal ions. A purple-pink colour, for instance, is found in the presence of potassium. Red colours are found in lithium, rubidium and strontium. Barium gives a pale green flame while copper gives a blue-green colour, often with white flashes. A practical application of different flame colours in the presence of metals comes to play in fireworks. As different metals burn they produce the different colours, as mentioned, we see in a fireworks display.
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