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Chemical Tests for Ions and Gases: Question 1

Syllabus 12.5

Multiple choice Core 1 mark

A stained-glass conservator finds a small tin of pale crystals stored beside some old glass-staining chemicals in a workshop archive. She flame-tests a pinch of the crystals on a clean wire loop and observes a lilac flame.

Which ion does this observation most strongly suggest is present in the crystals?

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 conservator observes a lilac flame. Looking down the table, lilac corresponds only to potassium ions, K+\text{K}^+.

Why the other options are wrong

  • A (Li+\text{Li}^+): lithium gives a red flame, not lilac.
  • B (Na+\text{Na}^+): sodium gives a yellow flame, not lilac. This is a common mix-up because sodium’s flame test is the most familiar one.
  • D (Ca2+\text{Ca}^{2+}): calcium gives an orange-red flame, not lilac.

Final answer

The crystals most likely contain potassium ions, K+\boxed{\text{potassium ions, K}^+}, option C.