Which ion is present if a blue-green flame is observed?

Which ion is present if a blue-green flame is observed?

• A. Calcium (Ca2+)
• B. Copper(II) (Cu2+)
• C. Iron(III) (Fe3+)
• D. Magnesium (Mg2+)

Answer: (B)

When metal ions are heated in a flame, their electrons gain energy and then release light as they drop back to lower levels. The color of that light is specific to the metal ion. A blue-green flame points to copper because copper’s electronic transitions emit light in that part of the spectrum. The other ions produce different flame colors—calcium gives a brick-red flame, magnesium a bright white flame, and iron tends toward a yellow-gold color—so they don’t match the observed blue-green. Hence, seeing a blue-green flame indicates copper(II) ions are present.

When metal ions are heated in a flame, their electrons gain energy and then release light as they drop back to lower levels. The color of that light is specific to the metal ion. A blue-green flame points to copper because copper’s electronic transitions emit light in that part of the spectrum. The other ions produce different flame colors—calcium gives a brick-red flame, magnesium a bright white flame, and iron tends toward a yellow-gold color—so they don’t match the observed blue-green. Hence, seeing a blue-green flame indicates copper(II) ions are present.