Notes: Expansion, Latent Heat, Phase Changes

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

Expansion joints allow the metal bridge to expand without breaking or buckling.

Bimetallic strips used in thermostats work on the principle that the strip will curl because one metal expands more than the other

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.

Solid Liquid Gas

Melt Vaporize

CondenseFreezePHASE CHANGES

energy

tem

pera

ture

Solid

LiquidGas

Solid + liquid

Liquid + gas

Phase Change diagram

Phase changes occur when temperature stays the same. Every molecule must be converted to the new phase before the temperature will change again.

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” – putting together as in freezing or melting

• 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 mass unit 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 = mcΔTThe heat required during a change of state is

given byQ = mL

where L is the latent heat of either vaporization or fusion

For water:

Lf = 80 cal/g Lv = 540 cal/g c = 1 cal/g Co

Use these values to complete Concept Development Practice Page 23-1The examples on the following slides involve Joules and kilograms.

Examples

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

Cice = 2100 J/kgKQ = 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/kgQ = 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/kgFirst, 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/kgCsteam = 2010 J/kgKFirst, the water must all be boiled, then the

steam must heat up.Q = mLv + mcDTQ = 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!