Dihydrogen

Step 1: Analyze the properties of CO₂ and H₂ gases
Carbon dioxide (CO₂) is a colourless gas that can dissolve in water to form carbonic acid (H₂CO₃).
Hydrogen gas (H₂) is also colourless and is typically found in its molecular form as a gas.
Step 2: Evaluate the options

• Option (1): Soluble in water: While both gases are soluble in water, this is not their most defining property.
• Option (2): Colourless: Both CO₂ and H₂ gases are colourless, which is the most accurate and defining property in this case.
• Option (3): Acidic: CO₂ is acidic when dissolved in water, but this property does not apply to hydrogen gas, which is neutral.
• Option (4): Heavier than air: CO₂ is heavier than air, but hydrogen is lighter than air, making this option incorrect.

Step 3: Conclusion
The most accurate description of both gases is that they are colourless.
Conclusion:

Both gases are colourless.