Notes: Expansion, Latent Heat, Phase Changes

Other changes due to heat flow…Gases:If the volume of a gas is held constant, as the temperature goes up, the gas pressure goes up as well.

Thermal ExpansionGenerally, as

temperature increases, the lengths and volumes of substances also increase.

Examples: concrete sidewalks, metals, gases

Bimetallic strips: used in thermostats

One exception to the rule of thermal expansion is water

As the temperature of water drops its volume decreases until 4°C is reached.

Then its volume expands as the temperature continues to drop to 0°C.

Volu

me

Temperature

Result: ice is less dense than water so that ice floats. This protects underwater life in cold climates

Another consequence of the structure of water when frozen:

• Have you ever put a canned soda in the freezer? There is a large part of the soda that is water. What happens to the can?

Phase Changes

Evaporation: A change of phase from liquid to gas that takes place at the surface of a liquid.

Boiling: a change of phase from liquid to gas that takes place beneath the surface of a liquid.

Condensation: The change of phase from gas to liquid.Freezing: the change of phase from liquid to solid.Sublimation: the process in which a substance goes

from solid to gas, bypassing the liquid state.Precipitation: rain, sleet, snow, hail that falls to Earth

when cloud particles become too heavy

Solid Liquid Gas

Melt Vaporize

CondenseFreezePHASE CHANGES

energy

tem

pera

ture

Solid

Liquid

Gas

Solid + liquid

Liquid + gas

Phase Change diagram

Changes of State

– solid to liquid OR liquid to solid• melting point: the temperature at

which a substance changes from solid to liquid OR liquid to solid“fusion” -- melt

• latent heat of fusion, Lf - the amount of heat per kilogram that is required to be transferred to melt or freeze a substance

– Liquid to vapor OR vapor to liquid• Boiling point: the temperature at

which a substance changes from liquid to vapor OR vapor to liquid.

• Latent heat of vaporization, Lv- the amount of heat per kilogram that is required to be transferred to boil or cause condensation

There is NO change in temperature during a change of state.

The heat required when the temperature changes is given by

Q = mCDTThe heat required during a change of state is

given byQ = mL

where L is the latent heat of either vaporization or fusion

Examples

How much heat must be added to 3 kg ice at -7º C till it just begins to melt?

Cice = 2100 J/kgK

Q = mcDT Q = 3 (2100) (7)Q = 44100 J

How much heat must be added to 3 kg of ice at 0º C to melt all of it?

Lf ice = 3.33 x 105 J/kg

Q = mLf

Q = 3 (3.33 x 105 ) Q = 999000 J

How much heat must be added to 3 kg of water at 30º C so that all of it becomes steam at 100 º C ?

Lv water = 22.6 x 105 J/kg

First, the water must reach its boiling temperature, then it must all be boiled.

Q = mcDT + mLv

Q = 3 (4186)(70) + 3(22.6 x 105 )Q = 7659060 J

How much heat must be added to 4 kg of water at 100º C so that all of it becomes steam at 120 º C ?

Lv water = 22.6 x 105 J/kg

Csteam = 2010 J/kgK

First, the water must all be boiled, then the steam must heat up.

Q = mLv + mcDT

Q = 4(22.6 x 105 ) + 4(2010)(20)Q = 9200800 J

Water Plasma????

For water vapor to change into “water plasma” would require a temperature of approximately 12,000 degrees Celsius!

By FAR, the most common form of matter in the universe both by mass and volume is......

PLASMA!