Exambusters Study Cards 12 Phase Changes

How much heat is required to change 10 grams of water to steam at 100 ºC?
Heat of vaporization of water is 540 cal/g (10 g) (540 cal/g) = 5400 cal
How much heat is required to melt 0.5 grams of ice at 0 ºC?
Heat of fusion of water is 80 cal/g (0.5)(80 cal/g) = 40 cal
Define boiling point in terms of vapor pressure.
The temperature at which the vapor pressure over a liquid reaches atmospheric pressure. The substance boils (liquid to gas).
Triple Point
The only temperature and pressure combination at which the three phases of a substance (solid, liquid, gas) can co-exist at equilibrium.
Vapor Pressure
Pressure the gas phase exerts on its liquid phase in a closed container. This pressure varies with temperature.
Heat of Fusion
The amount of energy required to change a gram of substance from solid to liquid at its melting point.
Heat of Vaporization
The amount of energy required per gram to change a liquid to a gas at its boiling point.
Heat of Fusion for water
80 calories/gram
Heat of Vaporization for water
540 calories/gram
Melting Point
The temperature at which solid and liquid phases co-exist.
Critical Temperature
The temperature above which the liquid phase of a substance can no longer exist.
Critical Pressure
Minimum pressure required to liquify a gas at its critical temperature.
Normal Boiling Point
When vapor pressure reaches atmospheric pressure, the temperature at which a liquid phase becomes a gas phase. assume pressure = 1 atm
Phase equilibrium
For a liquid in a closed container: when the rates of evaporation (liquid to gas) and condensation (gas to liquid) equalize, the concentration of each is stable.
Dynamic Equilibrium
In a closed container where opposing changes are taking place at equal rates, the concentration of all components remains constant.
Describe a phase change from liquid to gas.
When a molecule gains enough kinetic energy to overcome the intermolecular attractive forces within the liquid, it escapes the surface and becomes gas.
A change from solid to vapor without the formation of liquid. Compounds which show sublimation: CO2, I2

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