Change of Phase

Change of Phase:

When a substance goes from one form to

another, (almost) always in the same order.

solid liquid gas

This goes for all substances, remember.

Now, some terms.

HEAT HEAT

- HEAT - HEAT

Change of Phase (for water):

solid liquid gasMELTING BOILING

FREEZING CONDENSING

0ºC 100ºC

melting/freezing point boiling/condensation point

(ice) (steam)(water)

3 Types of Heat TransferWhen we say “heat transfer”, we mean, of course, when heat

travels from one place to another (the Q in lab). There are

3 types: conduction, convection, and radiation.

1) Conduction: heat transfer that occurs by means of particles in a material, mainly electrons. Conduction occurs by electrons colliding and atoms colliding inside the object being heated.

The electrons in the metal atoms

and the atoms themselves are

transferring the heat down the rod.

Let’s talk terms for a second:

An insulator is a substance that does not conduct heat well.

examples of good insulators: air (blankets), glass, snow

examples of bad insulators: metals

A conductor is a substance that does conduct heat well.

examples of good conductors: metals

examples of bad conductors: air, glass, snow

Important point: good conductors = bad insulators

bad conductors = good insulators

Look at the following picture, and tell me what you notice about good and bad insulation:

3 Types of Heat Transfer (cont’d)(BTW: “Fluid” = gas or liquid)

2) Convection: transfer of heat by motion of a fluid through currents. As a fluid is heated from below, the molecules at the bottom begin heating faster. They spread apart and become less dense. They are then buoyed upward, only to be replaced by cooler, denser fluid. Some examples:

3 Types of Heat Transfer (cont’d)

2) Convection(cont’d): the beach is an excellent illustration of the concept of convection currents.

During the day, you get wind

towards the beach.

During the night, you get wind

away from the beach.

65º

65º

75º

50º

3 Types of Heat Transfer (cont’d)

3) Radiation: energy transferred by radiant energy in the form of electromagnectic waves. Here is the electromagnetic spectrum. Let’s discuss:

The electromagnetic spectrum describes all forms of radiant energy. The wavelength, or distance from one crest to the

next, determines the energy and type of wave.

3 Types of Heat Transfer (cont’d)

3) Radiation (cont’d): we are most familiar with visible light because our eyes are designed to detect it. We cannot see x-rays, for example, because our eyes are insensitive to it.

shortwavelength

high energy

longwavelength

low energy

RY OB GV I

Emission of Radiant EnergyAll substances at any temperature above absolute zero (-273ºC) emit

radiant energy. The average frequency ƒ of the radiant energy is directly

proportional to the absolute temperature T of the emitter:

ƒ ~ TThe sun has a high temperature and therefore emits radiation at a high

frequency- high enough on the electromagnetic scale to be visible light.

High frequency ƒ = many types of radiation,

including visible light

Emission of Radiant EnergyThe earth, in comparison, is relatively cool. So the radiant energy it emits

has a frequency lower than that of visible light. The radiation emitted by

the earth is in the form of infrared or heat.

Low frequency ƒ = infrared radiation, aka

heat

Evaporation = A Cooling Process

Liquid water changes to the gaseous

phase by the process of evaporation.

The molecules in a liquid move

randomly and at a wide variety of

speeds all the while bumping into one

another and transferring KE. Some

gain enough to break free of the

liquid state and become water vapor,

the gaseous form of water. So

when a water molecule leaves a

liquid, it takes some of the KE, i.e.,

heat from the sample, thereby cooling

the sample.

A change of phase from liquid to gas

So why do we sweat? To remove

heat from our bodies. Evaporation is

a cooling process, drawing heat as

the water goes into gaseous form.

Just like stepping out of the shower

on a cold day, sweating regulates

body temperature by getting rid of

excess heat.

Dogs, on the other hand, do not have sweatglands and must get rid of heat throughpanting. Dogs have wide tongues, which provide a large surface area for evaporation,allowing them to cool off.

Condensation = a warming process

When gas molecules near the surface of a liquid are attracted to the liquid,

the strike the surface with increased KE, adding to the overall heat of that

liquid. Let’s take two examples:

1) When a gaseous water molecule in warm, moist

air strikes a cold soda can, it

loses it’s energy and transfers

the heat it has to the can of coke, warming the

can and losing the heat it needs to remain a gas.

This happens a few billion times, and the can

starts to “sweat”, or collect liquid water from the

air. Also, the can of coke gradually warms as

these billions of molecules transfer their heat

into it.

A change of phase from gas to liquid

Condensation = a warming process

2) When you take a hot shower, the water vapor (steam) in the air makes

evaporation difficult, and you feel much warmer inside the shower than

outside. This is because the steam is

condensing on your skin and transferring

heat into your body.

But this also explains why humidity during

the summer is so miserable: your body

cannot evaporate water from the skin and

it is very hard to cool off.

A change of phase from gas to liquid

Evaporation: Salt Ponds

• Loose outer electrons

• Liquids and gases?

Evaporation: Lake Powell

Condensation

Condensation: Atmosphere

Condensation: Atmosphere

• Saturation– The faster molecules move the less able they

are to condense

• Relative Humidity– Higher temp = more vapor needed to saturate

Relative humidity--the measure of how much water vapor is held in the air compared to how much could be held (at a given temperature)

Boiling = evaporation within a liquid

The bubbles that form in a boilingliquid are not air, but the liquid becoming a gas within the liquid.So in boiling water, steam formsbubbles under water and thenrises to the top to escape into theatmosphere.

Now, the reason that water cannot become hotterThan 100ºC is because that at that point, any heat you add to it is used to evaporate water, which, asyou know, is a cooling process. Therefore, the wateris constantly cooled by the steam that is leaving from the surface. So the more you heat it, the morethe water evaporates, cooling the water.

Energy is needed for change of phase!

solid liquid gasMELTING BOILING

FREEZING CONDENSING(ice) (steam)(water)

Energy is absorbed when change of phase is in this direction

Energy is released when change of phase is in this direction

Energy is needed for change of phase! 1) The amount of energy needed to change any substance from

solid to liquid is called the heat of fusion.

* The heat of fusion for water is: 335 joules/gram

2) The amount of energy needed to change any substance from

liquid to gas is called the heat of vaporization.

* The heat of vaporization for water is: 2255 joules/gram

solid liquid gas

MELTING BOILING

FREEZING CONDENSING(ice) (steam)(water)

Energy is needed for change of phase!

solid liquid gasMELTING BOILING

FREEZING CONDENSING(1 g ice) (1 g steam)(1 g water)

335 j / 80 calories 2255 j / 540 calories

335 j / 80 calories 2255 j / 540 calories

Requires Energy

Gives Off Energy

Freezing

Regelation

Regelation

Regelation

Energy and Change of Phase

Energy and Change of Phase