A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.
(a) Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved.
(b) Name the black substance formed and give its chemical formula.

Explanation

### Step-by-Step Solution:

**(a)** Silver articles turn black when kept in the open for a few days due to a chemical reaction with hydrogen sulfide (H₂S) present in the air. This reaction leads to the formation of silver sulfide (Ag₂S), which is a black compound. The phenomenon involved in this process is known as **corrosion**, specifically referred to as **tarnishing of silver**.

**(b)** The black substance formed on the silver article is **silver sulfide**, and its chemical formula is **Ag₂S**.

### Detailed Explanation:

...