Which of the following substances has the lowest density?

• A. Water (H_2O)
• B. Hydrogen gas (H_2)
• C. Mercury (Hg)
• D. Aluminum (Al)

Answer: (B)

Verified step by step guidance

1. Recall that density is defined as mass per unit volume, given by the formula \(\text{density} = \frac{\text{mass}}{\text{volume}}\).
2. Understand that density depends on both the mass of the particles and how closely they are packed together in a substance.
3. Consider the physical states of the substances: water is a liquid, mercury is a liquid metal, aluminum is a solid metal, and hydrogen is a gas under standard conditions.
4. Recognize that gases generally have much lower densities than liquids and solids because their particles are much farther apart, resulting in a much lower mass per unit volume.
5. Compare the substances by their typical densities: metals like mercury and aluminum have high densities, water has moderate density, and hydrogen gas has the lowest density due to its gaseous state and very low molar mass.