The correct option is Silver chloride decomposes to silver and chlorine by light..
Explanation
The phenomenon described is an example of a photolytic decomposition reaction (photolysis), where a single reactant breaks down into simpler products upon exposure to light energy.
Detailed Analysis:
- Chemical Process: When white silver chloride ($\text{AgCl}$) is exposed to sunlight, it absorbs light energy. This energy triggers the decomposition of the compound into its constituent elements: silver metal ($\text{Ag}$) and chlorine gas ($\text{Cl}_2$).
- Color Change: Silver chloride is naturally white. The elemental silver produced during the reaction appears as a grey residue. This distinct color change is the visual indicator of the chemical transformation.
- Chemical Equation: The reaction is represented as:
$2\text{AgCl}(s) \xrightarrow{\text{Sunlight}} 2\text{Ag}(s) + \text{Cl}_2(g)$ - Incorrect Options:
- Silver chloride sublimates leaving a residue.: Sublimation involves a phase change from solid directly to gas without a chemical breakdown. $\text{AgCl}$ does not sublime under these conditions.
- Silver chloride converts to silver oxide.: The formation of silver oxide would require a reaction with oxygen, which is not the primary reaction driven by light exposure in this context.
- Silver chloride melts and then solidifies as metal.: Melting is a physical change of state. This process involves a chemical bond breaking, not a phase transition due to heat.
Key Takeaway:
Silver chloride and silver bromide undergo photolytic decomposition in the presence of sunlight to form grey silver metal, a property historically utilized in black-and-white photography.