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.
As liquid water is heated, the rate of evaporation increases producing more vapor that in turn increases vapor pressure. Air pressure from above slows down evaporation of water by pressing down on the surface of the liquid. As more water vapor forms, it pushes back against atmospheric pressure.