Thermodynamics Ch 23

Changes of State involve the transfer of energy to or from the environment.

This energy affects the vibrational motion of the atoms or molecules.

Transitions between states of matter: MeltEvaporate or Boil Solid← Liquid←Gas →→ FreezeCondensate

Evaporation is different from Boiling Evaporation takes place at the surface of a liquid Particles within the liq move about with random motion Particles within the liq move about with random motion When a molecule or atom near the surface is “bumped” by atoms below, they transfer some of their KE to the surface particle giving it enough energy to escape the liquid as a gas. What happens to the “left behind” atoms or molecules- they have given some of their KE to the “escapee” ? What happens to the “left behind” atoms or molecules- they have given some of their KE to the “escapee” ?. They become slower and colder. Evaporation is a cooling process

. Boiling takes place throughout the liquid. Most people think you have to add heat to make something boil – but you know better – remember the demo in chem. where we turned the flask upside down and made it boil by pouring ice water over it????). Boiling happens when molecules within the liquid gain enough KE to become gas. Since the gas is less dense it raises to the surface of the liquid, BUT, before it can escape it must have enough KE to overcome the pressure of the Earth’s atmosphere pushing down on it from above.. When this high energy (and high temp) molecules escape they leave behind a liquid with a lower/cooler average temperate. Boiling is also a cooling process.

Condensation is the opposite of Evaporation and Boiling In this case randomly moving gas molecules collide. Those that are already slow (have low KE) are much more likely to stick together and form a liquid. Condensation in the atmosphere is limited by how much water the air will hold. Condensation in the atmosphere is limited by how much water the air will hold. Air will hold more moisture when warm than cold. When the air is “full” of water vapor, it is saturated and the relative humidity is 100% When the air is “full” of water vapor, it is saturated and the relative humidity is 100% What happens to the temperature of the surrounding air as water condenses? It warms up ! Why? Because as the molecules collide and stick together they give away some of their KE which goes into warming the air. So condensation is a warming process.

Freezing organizes matter Atoms or molecules in a liquid zig about past one another, but when matter cools and the molecules slow they can form organized crystal structures (solids). THINK ! Are all frozen things cold? NO - The metal in your chair is frozen into a solid. In fact, it freezes at about 2500 °F, hardly cold.

Freezing like boiling responds to pressure. Regelation occurs when pressure is applied to a solid causing it to melt at a lower than normal temp. This is most noticeable in ice. An ice skater puts pressure on the ice causing it to melt even though the temp is below freezing. This allows the skater to glide across the ice on a thin sheet of water that freezes right behind him or her as soon as the pressure is gone.

Go to this link and take the interactive quiz sics_ThermometricProperties.xml