Evaporation, vapor pressure, & boiling

A guide to WS 7.8 & the boiling home lab

Evaporation & Vapor Pressure

Though water exists mostly as a liquid at room temperature and pressure, water in an open container evaporates over a period of time until it all finally "disappears." The water vapor formed during evaporation is like any other gas: it exerts pressure, and it expands & contracts with temperature changes. You may have noticed that in a closed system, such as in a sealed bottle of water, water does not appear to evaporate.

When placed in a closed container, water does evaporate until the air in the container is saturated with water vapor. When the air is saturated with water vapor, the molecules in the vapor condense to a liquid as fast as the liquid evaporates, and the two processes (evaporation and condensation) continue at equal rates.

This is called an equilibrium.

The evaporation and condensation are proceeding at the same rate, so there is no net change. In a closed container, the pressure due to the water vapor reaches a maximum value (for a given temperature) called vapor pressure. Vapor pressure is the pressure caused by a liquid's own vapor.

Vapor Pressure & Boiling

As the temperature of the water increases, its vapor pressure increases. When the vapor pressure equals the atmospheric pressure on the liquid, the liquid will boil. At high altitudes, the boiling point of liquids is lower than at sea level. In Denver, Colorado, water will boil at about 94°C. Do not confuse boiling with cooking. Cooking pasta in Denver is a slower process because the water is at a lower temperature. Also, realize that water boiling rapidly is no hotter than water boiling slowly. The temperature of the water remains constant during the boiling process. And, the temperature of a boiling liquid never rises above its boiling point. No matter how much heat is applied, the liquid only boils faster, not hotter.

There are fundamental differences between evaporation and boiling. Evaporation occurs at temperatues lower than the boiling point of the liquid. Also, evaporaiton occurs at the surface of the liquid, whereas in boiling, bubbles of vapor arise inside the body of the liquid. For a bubble to form, the pressure of the atmosphere on the surface of the liquid must be overcome.

A liquid boils when the vapor pressure matches the atmospheric pressure. a. At 70° C and 760 mmHg the atmospheric pressure is greater than the vapor pressure of the liquid. b. At the boiling point (100° C for water), the vapor pressure matches the atmospheric pressure. Bubbles of vapor form insude the liquid, and it boils. c. At higher altitudes, the atmospheric pressure is less. Thus the liquid boils at a lower temperature.

High Pressure Boiling

We just saw that in Denver, where atmospheric pressure is less, water will boil at a lower temperature. In contrast, at higher atmospheric pressures, a liquid's boiling point increases. This is because the particles in a liquid need more kinetic energy to escape. Pressure cookers reduce cooking time because at high pressure water boils well above 100° C. An autoclave (see picture) is similar to a pressure cooker. It is commonly used to sterilize medical instruments. Autoclaves are usually set at 120° C and 1520 mmHg (2 atm) for 15 minutes. The high pressure keeps water at a high enough temperature to kill bacteria and their spores.

autoclaves are often used in hospitals to steriize equipment

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