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.
1 Explain that the vapor pressure of a liquid increases as its temperature increases. 2  Explain that boiling point increases as external pressure increases.  Explain that vapor pressure is unaffected by changes in external pressure.