Why does sound travel slower through gas?
Because there are less molecules present in gases than, say, a solid. Sound travels through molecules, so if there are more molecules the sound will travel faster. For example, think of a bumpy country road and a smooth tarmac road. You'll travel faster along the tarmac right? This is because there's less bumps and gaps. The same idea works with sound. Less bumps = Smoother transition and therefore higher speed!