Which physical state of matter is generally less dense than liquid water?

The physical state of matter that is generally less dense than liquid water is gas. This is because gases typically have a much lower density compared to liquids due to the significant amount of space between their molecules. In gases, the particles are far apart and move freely, which contributes to their lower density.

When considering solids, it's important to note that while many solids can be less dense than liquids, most common solids (like ice) are actually less dense than liquid water. Ice, for instance, is a solid form of water that is less dense than liquid water, which is a unique property due to hydrogen bonding; however, in general, most solids tend to be denser than liquids.

Plasma, a state of matter where gases are ionized and carry electric currents, typically has varying densities depending on conditions but is generally not relevant when discussing density compared to water.

Overall, the density of gases is consistently less than that of liquids, making gas the correct choice.