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?

In the flame test, different metal cations produce characteristic flame colours. We are told that a green flame with blue centre was observed.

The characteristic flame colours of the given cations are:

Option A: Calcium ($$Ca^{2+}$$) - Produces a brick-red (orange-red) flame.

Option B: Copper ($$Cu^{2+}$$) - Produces a green flame with a blue centre. This is the characteristic flame colour of copper salts, especially copper(II) chloride.

Option C: Barium ($$Ba^{2+}$$) - Produces an apple green or yellowish-green flame (without a blue centre).

Option D: Strontium ($$Sr^{2+}$$) - Produces a crimson red flame.

The description of a green flame with a blue centre specifically matches copper.

The correct answer is Option B: Copper.