In the flame test of a mixture of salts, a green flame with blue centre was observed. Which one of the following cations may be present?

A flame test is a qualitative process for determining the particular metal ion, depending on the colour of the produced flame.

When the sample is heated, metal ions gain energy and shift from a lower energy level to a higher energy level. Ions are not stable at a high energy level, and they return to the ground with energy-release. The energy is released in the form of light and it varies from one metal ion to another. Thus, each metal ion gives a characteristic change of colour when it is heated.

Cupric (\(Cu^{2+}\)) salts give green flame with blue centre. The colour of other salts are
\(Sr^{2+} \rightarrow \text{Crimson red}\\
Ca^{2+} \rightarrow \text{Brick red}\\
Ba^{2+} \rightarrow \text{Green}\\\)