Question

The metal whose salts are sensitive to light is?
A. Zinc
B. Silver
C. Copper
D. Aluminium

Answer 1

An amount to which an element reacts on receiving photons, specifically visible light is known as photosensitivity. In simple words, the elements or molecules which have a tendency to undergo a photochemical reaction are known as photosensitive elements or molecules.

Complete answer:
A photochemical reaction can be divided into three categories as follows:
1. Reactions in which quantum yield is small integers such as 1, 2 or 3. For example, dissociation reactions of hydrogen iodide and hydrogen bromide.
2. Reactions in which quantum yield is less than 1. For example, dissociation reactions of ammonia and nitrogen dioxide.
3. Reactions in which the quantum yield is extremely high. For example, combination reactions of hydrogen and chlorine, etc.
One of the important examples of photochemical reactions are the reactions involving the salts of silver metal, specifically silver halides which are light sensitive and thus, undergo a photographic process. When these halides are exposed to light, a chemical reaction occurs which darkens the film to produce an image. Hence, these compounds are generally used in making photographic films.
For example, silver chloride crystals consist of tightly packed ions which are denoted by $A{g^ + }$ and $C{l^ - }$. When a film containing silver chloride crystals is exposed to light, the extra electrons of chlorine ions get excited and ejected which are captured by silver ions to get converted into silver metal.
The reactions are as follows:
Ejection of electron from the chlorine:
$A{g^ + } + C{l^ - } + {\text{light energy}} \to A{g^ + } + Cl + {e^ - }$
Electron capture by silver:
$A{g^ + } + {e^ - } \to Ag({\text{metal)}}$
When silver metal is formed as a product in the chemical reaction, the entire crystal is converted into metallic silver and results in formation of a dark image on the film.
Therefore, the metal whose salts are sensitive to light is silver.
So, option (B) is the correct answer.

Note:
It is important to note that within the reaction process, the silver ion is reduced to silver metal and chlorine ions get oxidized to elemental chlorine at the same time, so a redox reaction occurs within the process. Also, except silver fluoride, all halides of silver are used in photography.