**Core Concept**
Common salt (sodium chloride) is used as a chemical antidote to neutralize certain heavy metal poisonings. In the context of oral poisoning, it is employed to treat silver nitrate ingestion by forming a soluble complex that reduces the toxicity of the metal.

**Why the Correct Answer is Right**
Silver nitrate is a potent irritant and corrosive agent that can cause severe gastrointestinal damage upon ingestion. When common salt (sodium chloride) is used as a chemical antidote, it reacts with silver nitrate to form a soluble silver chloride complex, which is less toxic and can be easily eliminated from the body. This reaction is based on the principle of precipitation, where the addition of a salt (sodium chloride) helps to stabilize and remove the toxic metal ions from the body.

**Why Each Wrong Option is Incorrect**
**Option A:** Mercuric sulphide is not typically treated with common salt, as its toxicity is not effectively neutralized by this method.
**Option C:** Copper sulphate poisoning is usually managed with supportive care and administration of penicillamine, not common salt.
**Option D:** Lead bicarbonate is not a common presentation of lead poisoning; rather, lead is often ingested in the form of lead salts (e.g., lead acetate or lead carbonate), which are not effectively neutralized by common salt.

**Clinical Pearl / High-Yield Fact**
When treating oral poisoning with common salt, it is essential to consider the specific type of toxin ingested and the underlying mechanism of action. In the case of silver nitrate, the use of common salt as a chemical antidote can be life-saving by reducing the risk of severe gastrointestinal damage and promoting the formation of a less toxic silver complex.

**✓ Correct Answer: B. Silver nitrate**