Dip a clean wire loop into a lithium chloride solution, hold it in a Bunsen flame, and the flame turns crimson red. Flame tests exploit the fact that different metal ions release energy as specific colours of light. In seconds you can identify an unknown metal ion in a sample — here is exactly how.
What is a flame test and what does it tell you?
A flame test is a qualitative analytical technique used to identify which metal ion is present in a sample. It cannot tell you how much of the metal is present (that would require quantitative analysis), but it can identify which metal from the characteristic colour its ions produce in a hot flame.
Flame tests are particularly useful for identifying:
| Metal ion | Symbol | Flame colour |
|---|---|---|
| Lithium | Li⁺ | Crimson red |
| Sodium | Na⁺ | Bright, persistent yellow-orange |
| Potassium | K⁺ | Lilac (pale violet) |
| Calcium | Ca²⁺ | Orange-red (brick red) |
| Barium | Ba²⁺ | Green (pale green) |
| Copper | Cu²⁺ | Blue-green (turquoise) |
A memory aid: Lizards Crawl Below Sheds Peacefully Kiting = Lithium (Crimson), Calcium (brick red), Barium (green), Sodium (yellow), Potassium (lilac) — or simply learn the table above.
Why do metal ions produce different colours?
The explanation involves electron energy levels — a concept you will explore in more depth at GCSE and A-level. Here is the KS3 version:
- Metal ions in the flame absorb energy from the heat.
- Electrons in the metal ions are temporarily promoted to higher energy levels (they move further from the nucleus into a higher-energy shell).
- The electrons are unstable at these higher levels. They fall back to their original (ground state) energy levels almost instantly.
- When they fall back, the extra energy is released as a packet of light (a photon).
- Different metal ions have different spacing between their energy levels. Different energy gaps produce different colours of light.
- Smaller energy gap → longer wavelength → red/orange light (lithium)
- Larger energy gap → shorter wavelength → blue/violet light (potassium)
Each metal ion has a unique pattern of energy levels, so each produces a characteristic colour — a kind of fingerprint. This is the same principle used in spectroscopy and in identifying the composition of distant stars from the light they emit.
How do you carry out a flame test safely?
Equipment needed
- Bunsen burner
- Nichrome or platinum wire loop (mounted in a glass rod or wooden handle)
- Dilute hydrochloric acid (for cleaning the wire)
- Sample to be tested (solid or solution)
- Eye protection
Step-by-step procedure
- Put on eye protection. Some flame colours are bright enough to strain the eyes.
- Set the Bunsen burner to a roaring blue flame (non-luminous flame, hottest and cleanest).
- Clean the wire loop: dip it into dilute hydrochloric acid, then hold it in the blue flame. Repeat until the wire produces no colour in the flame. This removes any contaminating metal ions from previous tests. A clean wire produces no colour.
- Collect the sample: dip the clean wire loop into the sample (if a solid, moisten the wire with acid first to pick up a small amount).
- Hold the wire in the edge of the blue flame (not the middle — the edge is cooler and will not burn the wire rapidly).
- Observe the flame colour and record it immediately — many colours are fleeting (potassium's lilac colour, in particular, can be very brief).
- Clean the wire again before testing a different sample.
Safety precautions:
- Hydrochloric acid is corrosive — handle carefully and wash off skin immediately.
- Never leave a Bunsen burner unattended.
- Barium compounds are toxic — wash hands after use.
- Tie back hair and loose clothing near flames.
What are the common mistakes in flame tests?
Several things can give misleading results:
Sodium contamination is the biggest problem. Sodium produces an intensely bright yellow-orange colour that can mask other, subtler colours. Common sources of contamination include fingerprints (skin contains sodium), impure chemicals, and an unclean wire. If a sodium yellow appears when it is not expected, clean the wire more thoroughly and repeat.
Potassium's lilac colour can be difficult to see. It appears pale violet and lasts only a moment. Viewing through blue cobalt glass filters out sodium's yellow and makes potassium's colour more visible — this is a common technique at GCSE and A-level.
Calcium and sodium can be confused. Calcium gives a brick-red to orange-red colour; sodium gives a bright yellow-orange. The key differences are that calcium's red has more of a brick hue, and sodium's yellow is more persistent and more intensely bright. Clean the wire well between tests.
Not cleaning the wire between samples gives a mixture of colours from different tests — entirely unreliable results.
How is the flame test used beyond the school laboratory?
The principle behind the flame test — that specific elements emit specific wavelengths of light when energised — underlies some of the most powerful analytical techniques in modern science:
Atomic emission spectroscopy (AES) passes a sample's light through a prism or diffraction grating to separate it into component wavelengths. Each element produces a unique spectral "barcode" of emission lines, identifying elements with far greater precision than a simple colour observation.
Fireworks exploit flame test chemistry directly. The colours come from metal salts: strontium salts for red, barium for green, copper for blue, sodium for yellow, and magnesium for white/silver.
Astronomical spectroscopy applies the same principle to starlight. Emission lines in a star's spectrum reveal which elements are present in its outer layers — this is how we know the Sun contains hydrogen, helium, sodium, and calcium, without ever visiting it.
How do you answer flame test questions in the exam?
For a question such as "A student tests an unknown solution and observes a lilac flame colour. Identify the metal ion present and explain why this colour is seen":
- Identify the ion: potassium (K⁺).
- Explain the mechanism: electrons absorb energy from the flame and are promoted to a higher energy level; they then fall back to the ground state, releasing energy as light; the energy gap in potassium ions corresponds to the wavelength of lilac/violet light.
- Mention a limitation: the lilac colour is fleeting and may be masked if sodium contamination is present; blue cobalt glass can be used to filter out sodium yellow.
Frequently asked questions
Why must the wire be cleaned with hydrochloric acid before each test?
Even a tiny trace of a different metal ion left on the wire will contaminate the next test and produce a mixed or misleading flame colour. Dipping the wire in dilute hydrochloric acid converts any metal residue into its chloride salt, which is more volatile and burns away easily in the flame. Heating the clean acid-dipped wire until it produces no colour confirms that no contaminants remain. Platinum wire is preferred because it does not itself produce a flame colour and can be used repeatedly without degradation.
What would happen if two metal ions were mixed in the same sample?
The flame test would show a mixture of colours, making it unreliable for identification. For example, a solution containing both copper and sodium would show a persistent yellow (sodium) that might completely mask the blue-green (copper). This is a significant limitation of the flame test — it cannot easily separate overlapping colours. More sophisticated spectroscopic techniques can resolve this by separating the different wavelengths mathematically, rather than relying on human colour perception.
Are flame test colours the same as emission spectra?
The simple colour seen in a flame test is a crude version of what an emission spectrum reveals. A flame test shows the dominant visible colour produced by the metal ion. A spectroscope or spectrometer spreads that light into individual wavelengths — and each element produces a unique pattern of very specific bright lines (not a continuous rainbow). Sodium, for example, produces a famous pair of closely-spaced yellow lines at 589 and 589.6 nm. These individual wavelengths are far more precise identifiers than a rough colour.
Why is sodium's flame colour so dominant?
Sodium produces an exceptionally intense yellow-orange emission because the energy gap between two specific electron energy levels in sodium atoms corresponds exactly to the wavelength of yellow-orange light (~589 nm), and this transition is extremely probable (chemists call it "allowed"). Sodium is also present almost everywhere as a trace contaminant — in dust, fingerprints, most reagents — so even a tiny amount produces a striking colour. This makes sodium contamination the most common problem in flame test experiments.
Unsure about why different metals produce different colours, or which ions to learn for your exam? Professor Curie at aitutors.me will quiz you on every flame colour and help you build the particle-level explanation from scratch.