Longitudinal Waves. To see a animations of spherical longitudinal waves check out: In a longitudinal wave the particle displacement is parallel to the direction of wave propagation. The animation at right shows a one-dimensional longitudinal plane wave propagating down a tube.
The boiling point of a substance is the temperature at which the vapor pressure of the liquid equals the pressure surrounding the liquid and the liquid changes into a vapor.
The Macroscopic View. 1  As a liquid is heated, its vapor pressure increases until the vapor pressure equals the pressure of the gas above it. 2  Bubbles of vaporized liquid (i.e., gas) form within the bulk liquid and then rise to the surface where they burst and release the gas.