Which of the following substances has the lowest density?

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

Answer: (A)

Verified step by step guidance

1. Understand that density is defined as mass per unit volume, given by the formula \(\rho = \frac{m}{V}\), where \(\rho\) is density, \(m\) is mass, and \(V\) is volume.
2. Recognize that the substances listed are in different states of matter: hydrogen gas (H\(_2\)) is a gas, mercury (Hg) is a liquid metal, aluminum (Al) is a solid metal, and liquid water (H\(_2\)O) is a liquid.
3. Recall typical density values or relative densities for these substances: gases generally have much lower densities than liquids and solids because their molecules are far apart.
4. Compare the densities qualitatively: hydrogen gas, being a light gas, has a much lower density than liquid water, mercury (which is very dense), and solid aluminum.
5. Conclude that hydrogen gas (H\(_2\)) has the lowest density among the given substances because gases have lower mass per volume compared to liquids and solids.