Metal Reactivity with HCl
When a metal reacts with hydrochloric acid (HCl), it forms a salt and hydrogen gas. The reactivity of different metals with HCl varies, and in this case, the metal that reacts most vigorously with HCl to produce salt and hydrogen is sodium (Na).

Explanation:

1. Reactivity Series:
Metals are arranged in a reactivity series based on their reactivity with other substances, including acids. The reactivity series helps us understand the relative reactivity of different metals with acids. In the reactivity series, potassium (K) and sodium (Na) are the most reactive metals.

2. Sodium (Na):
Sodium is an extremely reactive metal. When sodium reacts with HCl, a vigorous reaction takes place.

- Reaction:
2Na + 2HCl → 2NaCl + H2

- Explanation:
In this reaction, sodium (Na) displaces hydrogen (H) from hydrochloric acid (HCl) to form sodium chloride (NaCl) and hydrogen gas (H2).

- Salt Formation:
The sodium chloride (NaCl) formed during the reaction is the salt. It is a compound that consists of sodium ions (Na+) and chloride ions (Cl-).

- Hydrogen Gas Formation:
Hydrogen gas (H2) is released as a byproduct of the reaction. It is a flammable gas that can be collected and tested by various methods.

3. Zinc (Zn), Tin (Sn), and Lead (Pb):
Zinc (Zn), tin (Sn), and lead (Pb) are also metals, but they are less reactive than sodium (Na). While they can react with hydrochloric acid, the reaction is not as vigorous as with sodium.

- Zinc (Zn):
Zinc reacts with HCl to produce zinc chloride (ZnCl2) and hydrogen gas (H2). However, the reaction is slower and less vigorous compared to sodium.

- Tin (Sn):
Tin reacts with HCl to produce tin chloride (SnCl2) and hydrogen gas (H2). The reaction is even slower than zinc and less vigorous.

- Lead (Pb):
Lead reacts with HCl to produce lead chloride (PbCl2) and hydrogen gas (H2). The reaction is the slowest and least vigorous among these metals.

Conclusion:
In summary, among the given options, sodium (Na) reacts most vigorously with hydrochloric acid (HCl) to produce salt and hydrogen. The reactivity series helps in understanding the relative reactivity of metals with acids, and sodium is one of the most reactive metals.