Why does a sound wave travels faster in a solid than a gas?

Answer 1

Compressibility: solids have less of it while gases have more. This is related to the density, or a measure of how close the particles involved are.

When you put energy into moving a solid, the solid particles adjacent to that canot move very far without either forcing distortion in the particles of the solid or making further movement in the solid. Because distorting a solid is usually a lot harder than simply moving it, the wavefront will move immediately through the solid, and hence travel quicker.

But in a gas, the particles are very far apart and not firmly attached. they can move around and be compressed much, much much more easily than in a solid. This means the particles will "bunch up" in pressure peaks, and these peaks will slowly move onto adjacent particles, where they will be again compressed and the whole process continues. Thus, the wavefront moves much more slowly in a gas.

Note that if the gas is pressurised a lot, the sound will travel quicker, because the particles are much closer and there is less ability for them to move around each other without transmitting the wave onwards.