The sulphide that is commonly used in coating electric fluorescent tubes is?

Analyze the question: You're asked which metal sulphide is used to coat the inside of fluorescent tubes (and similar display screens).

Explain the concept: Some compounds are phosphors, meaning they absorb energy (such as ultraviolet light) and re-emit it as visible light, a property called luminescence or fluorescence. Zinc sulphide (ZnS) is one of the best-known phosphors. When it is doped with small amounts of other metals (e.g. copper or silver), it glows brightly. This is why it is used for:

• Coating fluorescent tubes
• Cathode ray tube (TV and oscilloscope) screens
• Glow-in-the-dark paints and luminous dials

Go through the options:

• A. Iron(II) sulphide (FeS): a black solid used in the lab to make H₂S gas. It is not luminescent.
• B. Tin(II) sulphide (SnS): a dark-coloured solid with no use as a phosphor.
• C. Zinc sulphide (ZnS): a white or yellowish solid with strong luminescent properties. This is the correct choice.
• D. Lead(IV) sulphide: it is unstable and is not used for coatings. Lead sulphides are dark and are not phosphors.

Answer: C. Zinc sulphide

Exam tip: Link "Zinc sulphide" with "light" or "glow". Remember "Zinc Zaps light": ZnS is the phosphor for tubes, screens and luminous paint. Also remember that zinc compounds are often white, and ZnS appears as a white pigment when it isn't glowing.