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Chemical Tests for Ions and Gases: Question 6

Syllabus 12.5

Multiple choice Core 1 mark

A small pottery workshop colours its glazes with metal carbonate powders. A technician finds an unlabelled tub of pale powder that workshop records say should contain either barium carbonate or copper carbonate. She places a small sample on a clean flame-test wire in a blue Bunsen flame and observes a light green colour in the flame.

Which ion does this observation confirm is present in the powder?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall the flame-test colours

A flame test identifies certain metal cations by the colour they produce when a sample is heated in a clean flame:

CationFlame colour
lithium, Li+\text{Li}^+red
sodium, Na+\text{Na}^+yellow
potassium, K+\text{K}^+lilac
calcium, Ca2+\text{Ca}^{2+}orange-red
barium, Ba2+\text{Ba}^{2+}light green
copper(II), Cu2+\text{Cu}^{2+}blue-green

Step 2: Match the observed colour to the table

The technician observes a light green flame. Looking down the table, light green corresponds only to barium ions, Ba2+\text{Ba}^{2+}.

Why the other options are wrong

  • A (Cu2+\text{Cu}^{2+}): copper(II) gives a blue-green flame, a different shade from the light green described here.
  • B (Ca2+\text{Ca}^{2+}): calcium gives an orange-red flame, not light green.
  • D (Na+\text{Na}^+): sodium gives a yellow flame, not light green. This is a common mix-up because sodium’s flame test is the most familiar one.

Final answer

The powder most likely contains barium ions, Ba2+\boxed{\text{barium ions, Ba}^{2+}}, option C.