The rock pumice often floats on water even though its density is greater than that of water. Which of the following best explains why pumice can float?

• A. Pumice floats due to chemical reactions with water that produce gases, increasing its buoyancy.
• B. Pumice is hydrophobic and repels water, which causes it to float regardless of its density.
• C. Pumice contains many air-filled pores, so its average density $\rho$ (including the air spaces) is less than that of water $\rho \_\mathrm{water}$, allowing it to float.
• D. Pumice floats because its surface tension is higher than that of water.

Answer: (C)

Verified step by step guidance

1. Understand that whether an object floats or sinks in a fluid depends on its average density compared to the density of the fluid. The key principle here is Archimedes' principle, which states that an object will float if its average density is less than the density of the fluid it is placed in.
2. Recognize that pumice is a type of volcanic rock that contains many tiny air-filled pores or cavities within its structure. These pores trap air, which lowers the overall average density of the pumice when considering both the solid material and the air inside it.
3. Express the average density of pumice as a combination of the density of the solid rock material and the density of the air inside the pores. Since air has a much lower density than water, the presence of these air pockets reduces the pumice's average density below that of water.
4. Apply the condition for floating: if the average density of pumice, including the air spaces, is less than the density of water (\(\rho_{pumice} < \rho_{water}\)), then pumice will float on water despite the density of the solid rock material alone being greater than water.
5. Conclude that the reason pumice floats is not due to chemical reactions, hydrophobicity, or surface tension effects, but because its porous structure lowers its average density, allowing it to be buoyant in water.