Aim
To identify the metal ion (cation) present in a salt from the colour it gives to a non-luminous Bunsen flame, and to see how cobalt blue glass removes the interference of sodium.
Apparatus and materials
- Platinum or nichrome wire fused into a glass rod (or a wire loop with a handle)
- Bunsen burner adjusted to a non-luminous (blue) flame
- Concentrated hydrochloric acid in a test tube or watch glass
- Watch glasses with small amounts of the salts: LiCl, NaCl, KCl, CaCl₂, SrCl₂, BaCl₂, CuCl₂ (optionally RbCl, CsCl)
- Two pieces of cobalt blue glass
Theory: why metal ions colour a flame
When a volatile salt is heated in the flame, some of the metal ions are reduced to free atoms in the hot gas. The flame gives these atoms enough energy to push an outer electron into a higher energy level. That excited state lasts only a tiny fraction of a second; when the electron drops back, the extra energy leaves as a photon whose energy is exactly the gap between the two levels, ΔE = hν = hc/λ.
Each element has its own set of energy levels, so each emits its own wavelengths: its line emission spectrum. If the strongest lines fall in the visible region (about 400–700 nm) the flame takes on a colour our eye can recognise. Sodium's famous yellow comes from the 589 nm doublet; potassium's faint lilac is a mix of a deep-red line at 766 nm and a violet line at 404 nm. This is the same physics behind the Bohr model and atomic spectra in Class 11 Structure of Atom, which makes the flame test a favourite JEE and NEET concept question.
Chlorides are used because they are the most volatile salts. A paste with concentrated HCl turns any nitrate, sulphate or carbonate into a chloride and also gives a brighter, longer flash. Magnesium does not colour the flame because its electrons need more energy than a Bunsen flame can supply.
Procedure
- Adjust the burner so the air hole is open and the flame is blue and non-luminous.
- Dip the wire in concentrated HCl and hold it in the hottest part of the flame (just above the inner blue cone). Repeat until the wire gives no colour. This is the cleaning step.
- Dip the clean, moist loop into the salt so that a little paste sticks to it.
- Hold the loop at the edge of the flame and note the colour seen with the naked eye.
- Look at the flame again through two layers of cobalt blue glass and note the colour.
- Clean the wire again in HCl before testing the next salt.
Observations
| Salt | Colour in flame | Through cobalt glass | Ion |
|---|---|---|---|
| LiCl | crimson red | violet-purple | Li⁺ |
| NaCl | intense golden yellow | not visible (yellow absorbed) | Na⁺ |
| KCl | lilac (pale violet) | crimson-pink | K⁺ |
| CaCl₂ | brick red (orange-red) | light green | Ca²⁺ |
| SrCl₂ | crimson red | purple | Sr²⁺ |
| BaCl₂ | apple green (pale green) | bluish green | Ba²⁺ |
| CuCl₂ | bluish green | light blue | Cu²⁺ |
| RbCl | red-violet | violet-red | Rb⁺ |
| CsCl | blue-violet | blue-violet | Cs⁺ |
Copper(II) chloride gives a bluish-green flame; other copper salts can look more emerald green. Sodium's yellow lingers far longer than any other colour, which is why even a trace of it matters.
Result
The flame colour identifies the cation: a persistent golden-yellow flame shows Na⁺, lilac shows K⁺, brick red shows Ca²⁺, crimson shows Sr²⁺ (or Li⁺), apple green shows Ba²⁺ and bluish green shows Cu²⁺.
Precautions
- Always clean the wire with conc. HCl until it imparts no colour; otherwise colours from the previous salt and sodium contamination will mislead you.
- Use a non-luminous flame. A yellow, sooty flame hides the colours.
- Hold the loop in the outer, hottest zone, not in the cool inner cone.
- Handle conc. HCl carefully and keep your face away from the fumes.
- Use cobalt glass to check for potassium whenever the flame looks yellow.
Viva questions with answers
- 1. What is a non-luminous flame and why is it used?
- It is the blue flame obtained with the air hole open. It burns completely, has no glowing soot particles of its own and is hotter, so the colour from the salt is easy to see.
- 2. Which part of the flame is the hottest?
- The region just above the tip of the inner blue cone, in the outer (oxidising) zone.
- 3. Why does sodium give a persistent colour?
- Sodium compounds are present as impurities almost everywhere, even in tap water and on fingers, and its 589 nm emission is very intense, so it lasts and swamps other colours.
- 4. What colour does potassium show through blue glass?
- Crimson or pinkish-violet. The glass removes the sodium yellow and lets the red and violet lines of potassium through.
- 5. Why does Mg²⁺ not give a flame test?
- The energy needed to excite its electrons is too high for a Bunsen flame because magnesium is small and holds its electrons tightly.
- 6. Name a cation from the salt analysis scheme that is confirmed by flame test.
- Ca²⁺ (brick red), Sr²⁺ (crimson) and Ba²⁺ (apple green) in Group V, and Cu²⁺ (bluish green). See the salt analysis lab.
Related practice
Continue with the full salt analysis virtual lab, where the flame test is one of the preliminary dry tests, or try the acid–base titration lab. For exam practice, solve previous-year JEE Main questions on atomic structure and the s-block, or browse the question bank.
Frequently asked questions
Why is cobalt blue glass used in the flame test?
Sodium is present as an impurity in almost every salt and its intense yellow light hides weaker colours. Cobalt glass absorbs yellow light, so the sodium colour disappears and the lilac of potassium (seen as crimson-pink through the glass) becomes visible.
Why is concentrated HCl used to clean the wire and make the paste?
HCl converts metal impurities and the salt into chlorides. Metal chlorides are the most volatile salts, so they vaporise quickly in the flame and give a bright colour; leftover traces on the wire burn off when the wire is reheated until it gives no colour.
Why is a platinum or nichrome wire used and not a glass rod?
Platinum and nichrome do not melt or colour the flame at Bunsen temperatures and do not react with HCl. Glass contains sodium, so a glass rod would always give a yellow flame.
Why do lithium and strontium both give a red flame? How do you tell them apart?
Both atoms have strong emission lines in the red part of the spectrum (Li at 671 nm, Sr around 640–680 nm). Lithium is a deeper carmine red and is short-lived; strontium is crimson and persists. Through cobalt glass lithium looks violet and strontium purple, and a spectroscope separates their lines easily.
Do magnesium and beryllium salts give a flame colour?
No. Their electrons are held so tightly that the Bunsen flame cannot excite them, so Mg²⁺ and Be²⁺ give no characteristic flame colour.
Is this flame test simulator useful for CBSE practical exams?
Yes. It follows the procedure expected in the Class 11 and 12 salt-analysis practical: clean the wire, make a paste with conc. HCl, heat in the non-luminous flame and report the colour, including the use of cobalt glass.