AN607

Rev. 1.7

17

Figure 12. Water in Closed Container

Figure 13. Water Boiling

Boiling starts at the liquid surface when the vapor pressure equals the atmospheric pressure. As temperature

increases, bubbles form below the surface as the vapor pressure increases to equal the atmospheric pressure plus

the additional pressure due to the weight of the column of liquid above the bubble. As temperature and vapor

pressure continue to increase, bubbles will form throughout the entire volume of liquid as shown in Figure 13.

Water vapor is completely absorbed into the air which can cause the air in close proximity to the boiling water to

become locally saturated. Visible steam consists of very small water droplets which have condensed out of this

localized super-saturated air.

Evaporation = Condensation

Saturated vapor

p’>P

Evaporation >>> Condensation

Humidity in the open air is increasing

Steam