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.
Heat increases the kinetic energy of the liquid particles and therefore the vapor pressure. When the vapor pressure of the liquid equals the external pressure, the particles beneath the surface have enough kinetic energy to separate into a vapor. At this point, the liquid begins to boil.