A colourless pungent smelling gas, which easily liquefies to a colourless liquid and freezes to a white crystalline solid, gives dense white fumes with ammonia. Identify the gas and write the chemical equation for its laboratory preparation.

Explanation

**Step-by-Step Solution:**

1. **Identify the Gas:**
The gas described is colorless, has a pungent smell, easily liquefies, freezes to a white crystalline solid, and produces dense white fumes with ammonia. The gas that fits this description is Hydrochloric acid (HCl) gas.

2. **Chemical Properties:**
- HCl is a colorless gas with a strong, pungent odor.
- It can be liquefied into a colorless liquid and can freeze to form a white crystalline solid (solid HCl).
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