Fireworks get their brilliant, vibrant colours from carefully selected metal compounds. In the laboratory, we can observe these same characteristic colours using a flame test. This is a qualitative analytical technique used to detect the presence of specific metal cations based on the colour of light they emit when heated.
To conduct the test accurately, you must use a clean wire loop made of nichrome or platinum. These unreactive metals are chosen because they have very high melting points and do not produce any confusing colours of their own in the flame.
The step-by-step procedure is as follows:
The concentrated serves two vital purposes. It cleans the wire of impurities, and it reacts with the sample to form volatile metal chlorides, which evaporate easily and produce a much brighter, more visible flame colour.
You can identify different metal ions just like you might identify a friend in a crowd by the colour of their coat. The OCR specification requires you to memorise the specific diagnostic flame colours for five metal cations. Using precise colour terminology is essential for gaining marks.
The diagnostic colours are:
When two loud people talk at once, the louder voice completely drowns out the other. Similarly, if a sample contains a mixture of metals, one intense flame colour can completely hide a fainter one. This effect is known as masking.
The brilliant yellow flame of sodium is notorious for masking the much paler lilac flame of potassium. To solve this, chemists look through a piece of blue cobalt glass. The glass acts as a filter that absorbs the yellow sodium light, allowing the lilac potassium flame to be seen.
Sometimes, flame colours can be ambiguous or hard to distinguish. In these cases, flame tests are often paired with sodium hydroxide () precipitation tests to confirm identities. For example, adding to a calcium solution produces a white precipitate, while adding it to a copper solution creates a blue precipitate.
Putting all these clues together turns a simple observation into a solid chemical conclusion.
A student tests an unknown solid mixture. When tested in a roaring blue flame, a bright yellow colour is seen. When viewed through blue cobalt glass, a faint lilac colour is visible. Identify the metal cations present in the mixture.
Step 1: Analyse the initial observation.
Step 2: Analyse the secondary observation.
Step 3: State the final conclusion.
Students often state they would use a yellow 'safety flame'. This will lead to a false positive for sodium, as the yellow flame completely masks the true colour of the ion.
In 6-mark questions asking you to describe the procedure, examiners always look for you to explicitly mention cleaning the wire loop with concentrated hydrochloric acid.
Be incredibly precise with your colour terms for OCR: write 'crimson' for lithium (not just 'red') and 'orange-red' for calcium.
If asked why a nichrome wire is used instead of a standard iron or copper wire, always state that it is unreactive, has a high melting point, and does not colour the flame.
Flame test
A qualitative analytical technique used to identify the presence of specific metal cations based on the characteristic colour of light emitted when heated.
Cation
A positively charged ion, such as the metal ions tested in a flame test (e.g., Li⁺, Na⁺, K⁺).
Nichrome
An unreactive nickel-chromium alloy used for flame test wires because it has a high melting point and produces no flame colour of its own.
Hydrochloric acid
A strong acid used in flame tests to clean the wire and convert samples into volatile metal chlorides for a brighter flame.
Non-luminous flame
The hot, blue flame produced by a Bunsen burner when the air hole is fully open, making it transparent enough to see the test colours.
Contamination
The presence of unwanted substances, such as sodium from sweat or previous samples, that can hide the true diagnostic colour of a test.
Masking
When the high intensity of one emission colour (like sodium's yellow) prevents the observer from seeing a second, less intense colour.
Precipitate
An insoluble solid that emerges from a liquid solution, often used as a secondary test to confirm the identity of a metal ion.
Put your knowledge into practice — try past paper questions for Chemistry B
Flame test
A qualitative analytical technique used to identify the presence of specific metal cations based on the characteristic colour of light emitted when heated.
Cation
A positively charged ion, such as the metal ions tested in a flame test (e.g., Li⁺, Na⁺, K⁺).
Nichrome
An unreactive nickel-chromium alloy used for flame test wires because it has a high melting point and produces no flame colour of its own.
Hydrochloric acid
A strong acid used in flame tests to clean the wire and convert samples into volatile metal chlorides for a brighter flame.
Non-luminous flame
The hot, blue flame produced by a Bunsen burner when the air hole is fully open, making it transparent enough to see the test colours.
Contamination
The presence of unwanted substances, such as sodium from sweat or previous samples, that can hide the true diagnostic colour of a test.
Masking
When the high intensity of one emission colour (like sodium's yellow) prevents the observer from seeing a second, less intense colour.
Precipitate
An insoluble solid that emerges from a liquid solution, often used as a secondary test to confirm the identity of a metal ion.