In the flame test of a mixture of salts, a green flame with blue centre was observed. Which one of the following…

Step 1: Recall the standard colours of flame tests Flame tests identify certain metal cations inherently based on the visible light emitted when their thermally excited electrons return to lower energy states.

Let's evaluate the flame colors of the given choices:

• $\ce{Sr^{2+}}$: Imparts a vivid Crimson Red color to the flame.
• $\ce{Ba^{2+}}$: Imparts an Apple Green color to the flame.
• $\ce{Ca^{2+}}$: Imparts a Brick Red color to the flame.
• $\ce{Cu^{2+}}$: Uniquely imparts a distinctive Blue-Green (or green with a blue center) color to the flame, particularly notable if halides are present.

Step 2: Relate to the observation The text explicitly notes a "green flame with blue centre", which is the classic description utilized universally for the Copper(II) ($\ce{Cu^{2+}}$) flame signature.

Conclusion: The cation present is $\ce{Cu^{2+}}$.

Key Points to Remember:

• Flame test for $\ce{Cu^{2+}}$: Bluish-green (or green with a blue center).
• Flame test for $\ce{Ba^{2+}}$: Apple green (strictly green, lacking any blueish component).
• Flame test for $\ce{Sr^{2+}}$: Crimson Red.