Mixing Method: Add ice known T to warm water. Let it melt. Determining L f for Ice mc  (  T) w = mc  T ice + mL f ice + mc  T liq mix. Thermal E lost by water = thermal E gained by ice.

Measuring Latent Heat Vaporization Electric Method –Use electric vaporizer with –heating coil inserted into liquid. –Boil liquid until equilibrium btw vaporization & condensation. – Collect & Mass condensed liquid. E elc supplied = Q absorbed. VIt = mL v +H lost.

Challenge: One student puts his hand in a pot of boiling water at 100 o C, a second places her hand into a container containing steam at 100 o C. Who gets the worse burn? Explain in terms of physics concepts! Caution: Do not try this yourself.

Evaporation Molecules leave surface & enter vapor phase at any T.

Evaporation The molecules in a liquid have a distribution of speeds. The average speed determines the T of the liquid. The fastest molecules have enough energy to overcome the attraction between the molecules in the liquid. They evaporate and become vapor.

Evaporative cooling: as the fastest molecules evaporate, they leave behind the coolest molecules and the average temperature of the liquid decreases. Example: sweat

Volatile liquids = rapid evaporation. Evaporation rate depends on: Surface area. Ambient Temperature. Humidity of Surroundings. Wind over surface.

Boiling Boiling occurs when the average motion of particles is fast enough to overcome the forces holding them close together. This happens evenly throughout a boiling liquid. You will see bubbles form. The temperature is uniform throughout.

Boiling vs. Evaporation In the cases of both boiling and evaporation, intermolecular forces between particles are present. To break the bond between two particles, one particle has to be moving fast enough to overcome the pull of the other, until it gets so far away that pull is diminished.

IB packet

Animation of Molecules