Change of Phase Chapter 23

Topics  Phases  Evaporation  Condensation  Evaporation & condensation rates  Boiling  Freezing  Energy & changes of phase

Phases  Matter around us exists in 3 common phases-solids, liquids, gases  The phase of matter depends upon its temperature and the pressure that is exerted upon it  Changes of phase usually involve a transfer of energy

Evaporation  During evaporation, a liquid changes phase at its surface and becomes a gas  Evaporation is a cooling process  When our bodies overheat, sweat glands produce perspiration which gets evaporated and helps maintain a stable body temperature  Animals w/o sweat glands wallow, pant

Condensation  During condensation, a gas changes phase and becomes a liquid  Condensation is a warming process

Which would hurt more?  A steam burn  OR  A boiling water burn  A steam burn because steam give up energy when it condenses to the liquid that wets the skin

Evaporation and Condensation Rates  When evaporation and condensation occur at the same rate, the liquid is in equilibrium and there is no change in the liquid’s volume  A liquid is in equilibrium when the surrounding air is saturated with its vapor  In dry air, water evaporates much faster than it condenses; in humid air, it evaporates only slightly faster than it condenses

Chilly outside the shower  If you feel chilly outside the shower, step back inside and be warmed by the condensation of the excess water vapor

Boiling  During boiling, a liquid changes phase at any place within the liquid, and gas bubbles form  The boiling temperature of a liquid depends on the pressure on its surface  Boiling, like evaporation, is a cooling process  Heating water is one thing, boiling is another

Freezing  During freezing, a liquid changes phase and becomes a solid  The freezing temperature of a liquid is lowered by adding other substances to it

Energy and Changes of Phase  During phase changes, energy is given off or taken in  While a substance changes phase, its temperature does not change  Much more energy is given off when water vapor condenses than when an equal mass of water freezes

Energy Changes with the Change of Phase