Flame Tests for Metal Cations for the ESAT

Introduction to Flame Tests

Analytical chemistry involves identifying the components of a substance. Flame tests are a simple yet effective way to detect the presence of certain metal cations in a compound. When metal ions are heated, they absorb energy, which causes their electrons to be promoted to higher energy levels. As these electrons fall back to their original levels, they release energy in the form of light. The wavelength of this light corresponds to a specific colour that is characteristic of the element.

The Experimental Method

To ensure accurate results, the flame test must be conducted with care to avoid contamination. The standard procedure is as follows:

1. Use a clean wire loop made of a high melting point metal, such as platinum or nichrome.

2. Clean the wire by dipping it into concentrated hydrochloric acid, $HCl$, and then holding it in a hot Bunsen burner flame. This process is repeated until the wire no longer produces a colour in the flame.

3. Once cleaned, dip the wire into the concentrated hydrochloric acid again before touching it to a small amount of the solid metal compound so that the sample sticks to the loop.

4. Place the loop into the edge of a non-luminous, blue Bunsen flame. The blue flame is used because its own colour is minimal, allowing the characteristic colour of the metal cation to be seen clearly.

5. Observe and record the colour produced immediately, as some colours may be fleeting.

Characteristic Flame Colours

You must be able to recall and recognise the specific colours associated with the following five metal cations:

• Lithium ($Li^{+}$): Produces a crimson red flame.
• Sodium ($Na^{+}$): Produces a yellow-orange flame. This colour is very intense and can often mask other colours if the sample is contaminated.
• Potassium ($K^{+}$): Produces a lilac flame. This can sometimes be difficult to see if sodium is present.
• Calcium ($Ca^{2+}$): Produces a red-orange flame (often referred to as brick red in other contexts, but specified as red-orange for the ESAT).
• Copper ($Cu^{2+}$): Produces a green flame.

The Science of Electron Excitation

The colours observed in a flame test are the result of electronic transitions. In an atom, electrons exist in specific energy levels or shells. When the metal ions are placed in the heat of a Bunsen flame, they absorb thermal energy. This energy provides the 'excitation energy' required to move an electron from its lowest possible energy level (the ground state) to a higher energy level (an excited state).

The excited state is unstable. Consequently, the electron quickly falls back down to a lower energy level. As it does so, it must release the energy it absorbed. This energy is emitted as a photon of light. Because each element has a unique arrangement of energy levels, the energy difference between levels is unique to that element. Therefore, each metal emits light of a specific frequency and wavelength, resulting in the characteristic colours we observe.

Worked Examples

Example 1

A student is given an unknown salt and performs a flame test. The resulting flame is lilac. Which metal cation is present in the salt?

Solution: Lilac is the characteristic flame colour for the potassium ion, $K^{+}$. Therefore, the salt contains potassium.

Example 2

During a lab practical, a sample produces a green flame when heated. Identify the cation.

Solution: A green flame is the result of the copper ion, $Cu^{2+}$, being present in the sample.