Which gas forms base with water?

Options

A. HCI
B. `SO_2`
C. `NH_3`
D. `H_2 S`

Explanation

To determine which gas forms a base when dissolved in water, we can analyze each option provided:

1. **Hydrochloric Acid (HCl)**:
- When HCl is dissolved in water, it dissociates to form hydronium ions (H₃O⁺). This results in an acidic solution, not a basic one.
- **Reaction**: HCl + H₂O → H₃O⁺ + Cl⁻

2. **Sulfur Dioxide (SO₂)**:
- When sulfur dioxide is dissolved in water, it forms sulfurous acid (H₂SO₃). This is also an acidic solution.
- **Reaction**: SO₂ + H₂O → H₂SO₃

3. **Ammonia (NH₃)**:
- Ammonia reacts with water to form ammonium hydroxide (NH₄OH), which is a base.
- **Reaction**: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻

4. **Hydrogen Sulfide (H₂S)**:
- When hydrogen sulfide is dissolved in water, it forms a weak acid (H₂S), which does not produce a basic solution.
- **Reaction**: H₂S + H₂O ⇌ H₃O⁺ + HS⁻

After analyzing all options, we find that:

- **Correct Answer**: **C. Ammonia (NH₃)** forms a base (ammonium hydroxide) when dissolved in water.