# Phase Changes Our interaction with the world of the small.

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Phase Changes Our interaction with the world of the small

What’s the Point? What happens to all our energy? How does energy convert in freezing, melting, evaporation, and condensation? What is “heat”?

Temperature and Energy Average translational molecular kinetic energy is proportional to a substance’s temperature. Individual molecules can have higher or lower kinetic energies than average.

Demo: Speed of Diffusion in hot vs cold liquid. Which has faster moving particles?

What is heat? Heat is molecular energy transferred from high to low temperature.

Heat transfers from a hotter substance to a colder substance until they are both the same temperature.

What is heat? System vs. Surroundings: –You sit on a cold bench (around 60 0 F). Your body temperature is 98.7 0 F. –You, the system, lose heat. Temperature decreases. Exothermic –The bench, the surrounding, gains that heat. Temperature increases. Endothermic –Heat lost by system = Heat gained by surroundings. –Heat exchange will continue until temps equal.

When a red-hot piece of iron is dropped into a bucket of water, Poll Question A.the water becomes hotter. B.the water’s temperature increases. C.the water’s internal energy increases. D.the water receives heat from the iron. E.all of the above.

What is heat? –You sit on a cold bench (around 60 0 F). Your body temperature is 98.7 0 F. –Heat lost by system = Heat gained by surroundings. –Heat exchange will continue until temps equal. –Final temperature will NOT be half-way in-between the 2 temperatures. Why not?

Specific Heat (Capacity) Heat needed to change the temperature of a unit amount of a substance. – q = heat input – m = mass of sample –  T = temperature change –Different chemicals have different specific heats! c = q mTmT

Specific Heat (Capacity) Heat needed to change the temperature of a unit amount of a substance. –Metal has a very low specific heat –Needs little energy to change temp. –Greater change in temperature. –Water (most of human body) has a very high specific heat –Needs a lot of energy to change temp. –Smaller change in temperature.

You sit on a cold bench (around 60 0 F). Your body temperature begins at 98.7 0 F. Assuming no other heat exchange occurs, when the 2 temperatures become equal, it will be Poll Question A.Below 60 0 F B.Above but close to 60 0 F C.Exactly in-between 60 0 F and 98.7 0 F. D.Below but close to 98.7 0 F E.Above 98.7 0 F

When a red-hot piece of iron is dropped into a bucket of water, the final temperature will be Poll Question A.Closer to water’s initial temperature B.Closer to iron’s initial temperature C.Half-way between water and iron’s initial temperature D.Different for each: water and iron

Phase Changes Endothermic (heat enters) melting boiling Solid Liquid Gas freezing condensing Exothermic (heat exits)

When an ice cube melts in your hand, your hand is __________ heat, going through an _____________ process. At the same time, the ice cube is ______________ the same amount of heat, going through an ___________ process. Exothermic Endothermic Absorbing releasing Poll Question

Phase Changes Melting, boiling, freezing, condensing… Added or removed heat changes the substance’s potential rather than kinetic energy Water freezes at 0 °C, boils at 100 °C (well, about 92 °C in Laramie) Not all heat transfer is expressed as a temperature change.

ice Liquid water steam Heating Curve for Water Water boils Ice melts

Phase Changes Potential energies: Solid < Liquid < Gas During a phase change, potential energy, not kinetic energy (temperature) changes. Heating or cooling a changing phase does not change its temperature!

Latent heat Potential energy of phase change (energy required to change the phase of 1 kg of substance) Water’s latent heat of fusion (melting): 335,000 J/kg Water’s latent heat of vaporization: 2,255,000 J/kg

Evaporation of a Liquid More energetic jostling = higher temperature An especially fast molecule at the surface may detach!

Evaporation of a Liquid More energetic jostling = higher temperature An especially fast molecule at the surface may detach!

Evaporation Evaporating molecules carry away energy KEPE Remaining liquid cools (KE decreases)

Why would this occur?

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