All you wanted to know about phase changes

*But were afraid to ask!

How do liquids behave?• REVIEW:

– According to the kinetic theory, particles of all substances are in constant motion. So, that includes liquids!

– Liquid particles move from one place to another. This is why a liquid can flow.

• However, unlike gases, the particles in a liquid are attracted to each other, which is why a liquid is a liquid and not a gas.

• So, it’s all due to ATTRACTIONS BETWEEN PARTICLES!!

Let’s talk kinetic energy now…

• Since the particles in a liquid are moving, they all have kinetic energy (energy of MOTION).

• While the forces of attraction between liquid particles keep most of the particles in liquid state, some of the particles have enough energy to ESCAPE!

• When a liquid particle escapes, what phase of matter does it enter?

• THE GASEOUS PHASE! That’s right – the liquid particle becomes a gas particle once it escapes. AMAZING!!

Bye-bye, fellow liquid particles, I’m

off to vacation in another “state”!

When a liquid particle enters the gaseous state, what is this process

called?• Yup – you guessed it – VAPORIZATION! A

liquid particle turns into VAPOR!• So anytime a liquid enters the gaseous

state, we call it vaporization. – Yes, this means that we can also call

evaporation and boiling by the same term – vaporization!

So, if evaporation and boiling are the same process, what’s

the big difference?

• The difference has to do with what we call VAPOR PRESSURE (remember this term?).

• Review: vapor pressure is the force caused by the collisions of the vapor particles of a liquid with another surface.

Visualizing Vapor Pressure• The diagram on the left

shows ethanol in a flask at 0oC.

• Some of the particles have vaporized, and those particles are colliding with the mercury in the tube to the right of the flask.

• Since they are colliding with the mercury, the vapor particles are “pushing” down on the mercury, creating what we call vapor pressure.

• The diagram on the right shows ethanol in a flask at 20oC.

• Again, some of the particles have vaporized, and those particles are also colliding with the mercury in the tube to the right of the flask.

• What difference do you see in this diagram?

Aha! So, we can say that when temperature goes up, so does the vapor pressure! (didn’t we already learn this

concept??)• If we can increase the

temperature enough that the vapor pressure of the liquid is equal to the atmospheric pressure, we have the phenomena called BOILING!!

• The normal boiling point of a liquid is the temperature at which the vapor pressure of the liquid is equal to the atmospheric pressure at sea level (1 atm or 101.3 kPa or 760 mm Hg)

Determination of boiling point

• Since boiling occurs when vapor pressure = atmospheric pressure…

• What’s the normal BP of chloroform?

• About 60oC!• What about ethanol?• About 78oC!

What happens to the boiling point of a substance at different

elevations? • Since the boiling point is determined by

the value of atmospheric pressure, boiling point can change due to atmospheric pressure changes.

• As we go higher in elevation, the atmospheric pressure decreases since the amount of air particles also decreases.

Test what you have learned! Predict what the boiling point of

water will be if…

You are vacationing in the Andes mountains

You are stranded in Death Valley

•You are in Palatine, Illinois

Below 100oC

You got it! Above 100oC

Just a little below 100oC

Now, finally… the answer to the question – what is the difference

between evaporation and boiling?• Evaporation occurs when a

liquid particle enters the gaseous state, BUT the vapor pressure of the liquid IS NOT equal to the atmospheric pressure.

• Boiling occurs when a liquid particle enters the gaseous state AND when the vapor pressure of the liquid IS equal to the atmospheric pressure.

Gee, that’s all?

The last bit about liquids…• Just like a liquid can enter gaseous

phase, a gas can enter liquid phase.• When gas particles lose kinetic

energy, they slow down, and become attracted to one another.

• When this occurs, the gas particles become liquid.

• This is called CONDENSATION, and occurs when you breathe on a cold window. The gas you breathe out all of a sudden becomes liquid.

Now onto SOLIDS!

• The particles in a solid are also in constant motion, but instead of moving from place to place, they stay fixed in one spot, and they rotate in this one spot.

• The attractions between solid particles are VERY strong. Since they are very strong, the particles stay together, which is what makes a solid a solid!

• The two types we will define are crystalline solids and amorphous solids

Crystalline Solids• Particles in

solids of this type are arranged in an orderly pattern.

• When broken, crystalline solids will have very defined edges that are consistent.

• You will see that many of the edges are parallel to one another

Amorphous Solids• The particles in amorphous solids are

not arranged in any sort of order. The particles are randomly arranged. If you were to tear or break an amorphous solid, there would not be any defined/parallel edges.

• Examples of amorphous solids are…

Plastic

Glass

Rubber

Solids go through changes just like liquids

• While liquids enter the gas phase with increased temperature, solids enter the liquid phase with increased temperature.

• When heat is applied to a solid, the heat supplies energy needed by the solid particles to move around and overcome the attractive forces between them.

• What is this process called?• MELTING!!

I’m changing

from solid to

liquid phase!

The opposite of melting is called…

• Freezing! • This is when a liquid does not have

enough energy to stay in the liquid phase.

• The particles’ kinetic energy decreases, and they slow down. Eventually they become attracted to one another, and re-form a solid.

• Freezing and melting occur at the same temperature. At a certain temperature, some particles have enough energy to leave the solid phase, to become liquid. Then, there are some particles that do not have enough energy to stay in the liquid phase. So, they enter the solid phase.

All phase changes can be summed up in a Phase

Diagram • Phase Diagrams show us the various states of matter in which a substance can exist under certain pressures and temperatures.

How do you read a phase diagram?

• Let’s try some examples...

• The diagram to the right is for H2O.

• What phase will H2O exist at 50oC and 95 kPa?

• Liquid!• What about at…50oC

and 0.25 kPa?• Vapor (gas)!• And at 0.016oC and

0.61kPa? • All three! This is called

the triple point, when a substance can exist in all three phases at once.

So, basically the reason why the three phases of matter are the way

they are, are due to…

• Kinetic Energy of the particles

• Attractions between the particles.

• So, you can say that kinetic and energy and attractions are what makes the world go round…

And that’s it!• Any questions about

phase changes??Yee-Haw! I’ve learned so much about phase

changes that I know this is SOLID gold!