Chemguide: Core Chemistry 14 - 16

Flame tests

This page looks at using flame tests to identify the presence of certain metal ions in a compound.

How to do a flame test

A flame test is done by holding a sample of your compound on a piece of wire in a Bunsen flame.

Ideally the wire is made of platinum because that doesn't itself give any colour to the flame. But normally in a school lab, you will probably be using nichrome wire - an alloy of nickel and chromium. This is much cheaper but tends to give a very slight orange tinge to the flame which you have to ignore.

First of all, you have to clean the wire. You do this by dipping it into some concentrated hydrochloric acid, and then putting it into a hot Bunsen flame. You repeat this until the flame doesn't show any colour (apart from any small amount of colour that a nichrome wire produces).

If you are testing a solution, simply dip the wire into the solution and then back into the flame.

If you are using a solid, dip the wire into the concentrated hydrochloric acid again to moisten it, and then dip it into a sample of your solid. The put the wire with some solid stuck to it back into the flame.

Flame colours

The video shows the flame colours for compounds containing Li+, Na+, K+, Ca2+, Sr2+, Ba2+ and Cu2+ ions.

It doesn't matter what the compounds are although chlorides tend to give better results. That's why you pick up bits of solid on a wire moistened with concentrated hydrochloric acid.

Strontium isn't mentioned by any of the current UK GCSE syllabuses, and it isn't easy to distinguish the colours between that and lithium. So I am omitting that.

In summary the flame colours you may need are:

Flame colour
Li+red (sometimes described as crimson)
Na+yellow / orange
K+lilac (sometimes described as lavender)
Ca2+orange / red (sometimes described as brick red)
Ba2+pale green (sometimes described as apple green)
Cu2+blue-green, often with flashes of white

If you are doing this yourself, the calcium flame colour is often the most difficult one to get, and often just looks orange. Look for short-lived flashes of orange / red - often right at the very beginning of the test.

If you are doing this using a solid calcium compound, the best flame colour often comes if you dip the wire back in the concentrated hydrochloric acid and then straight back into the flame. That may well give you a short-lived but accurate colour.

Finally . . .

If you are preparing for an exam, find out exactly which colours you need to know, and exactly how your examiners describe them. Learn those, and forget the rest!

If you are doing a UK-based syllabus, you can find links to the Exam Boards' websites where you can download a copy of your syllabus and other useful stuff on the about this part of Chemguide page.

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© Jim Clark 2020