Find out why white silver chloride turns grey in sunlight.

Correct — C. White silver chloride darkens because sunlight breaks the compound up:

2AgCl → 2Ag + Cl₂ (in sunlight)

The grey is finely divided silver metal left behind on the surface as chlorine gas escapes. Silver bromide behaves the same way, which is what made black-and-white photographic film work.

A single compound splitting into simpler products, with light supplying the energy, is photolytic decomposition — the process the stem is asking you to name.

Why the others are wrong

• (a)
  Due to rusting of silver in presence of oxygen — Rusting describes iron corroding to hydrated iron oxide in moist air. Silver does not rust, and this change needs sunlight rather than oxygen and water.
• (b)
  Due to redox reaction — Electrons do move here — Ag⁺ is reduced and Cl⁻ oxidised — but 'redox' names a category of reaction, not the cause of the darkening. The stem asks what makes the white solid turn grey, and that is the light.
• (d)
  Due to the displacement of silver chloride to silver oxide — No silver oxide is formed and nothing is displaced. A displacement reaction needs a second reactant to take a place in the compound; here one compound falls apart on its own.

Photolytic decomposition is decomposition driven by light, and the silver halides are its stock examples.

Silver chloride is white and silver bromide pale yellow, and both turn grey on exposure as the halide is split and metallic silver is left:

2AgCl → 2Ag + Cl₂
2AgBr → 2Ag + Br₂

That reaction is the basis of black-and-white photography, and the reason silver halides are stored in dark or amber glass.

Key facts

• Sunlight decomposes silver chloride: 2AgCl → 2Ag + Cl₂.
• The grey on exposed silver chloride is finely divided silver metal.
• Silver bromide decomposes in the same way and underlies black-and-white photography.
• Decomposition driven by light is called photolytic or photochemical decomposition.

Common traps

• Redox is a true description of the same reaction, so a candidate who stops at 'electrons moved' has named a category instead of a cause.
• Silver turning black in air is a different change, caused by sulphur compounds rather than by light.

No directly related past PYQ was found.