Thermal Properties of Matter (Part I)Textbook Chp 11, Pg 185 - 206

Heating and Cooling Curves Freezing / Melting Boiling / Condensation Boiling vs Evaporation

Topics

If I heat a beaker of ice and plot the graph of its temperature against time, what would I see?

Heating Curve

Heating Curve

Melting

Boiling

Boiling Point

Melting Point

What is the state at:

AB? BC? CD? DE? EF?

Heating Curve

Solid

Mixture of Solid and LiquidLiquid

Mixture of Liquid and GasGas

Cooling Curve

Condensation

Freezing

Gas

Mixture of Gas and Liquid

Liquid Mixture

of Liquid and Solid

Solid

Note that the rising or falling sections may not be a straight line and may be a curve (you will not be marked for this)

Pay attention to the question, sometimes you may only be required to draw one plateau, not two.

You will be plotting a cooling curve for your next practical session

Heating/Cooling Curve

Note: some textbooks use “vaporisation” instead of “boiling”

You are required to describe the microscopic process of melting / freezing / boiling / condensation

For melting or boiling, heat transferred to the substance is used to break bonds between particles, without changing the temperature (average KE of particles)

For condensation or freezing, heat is released to form bonds without changing the temperature (average KE of particles)

At Microscopic Level

I do not have a dryer at home, so I have to hang wet laundry out to dry.

No matter how hot the day is, it will never be 100°C, and water will not be boiling. How then does the water turn from liquid to gas?

Evaporation

Recall what temperature is – the average kinetic energy of molecules

“Average” means that not all molecules have this KE. Some have more (i.e. move faster) and some have less (i.e move slower)

At the surface of the liquid, molecules which have higher KE are able to break free from inter-molecular bonds to form free molecules (vapour) while lower KE molecules cannot break free

Evaporation

Cooling Effect of Evaporation

https://www.youtube.com/watch?v=Ld9tBUPmFGI

Blowing bubbles into liquid speeds up its rate of evaporation

Evaporating Ether is so cold that its temperature drops below 0°C!

Note: do NOT try this experiment on your own. You may end up breathing in ether vapour, which is bad for your health

Ether demo

Why does evaporation have a “cooling effect”?

During evaporation, the higher KE molecules have escaped while only the lower KE molecules are left

Recall temperature is the measure of the average KE of the molecules

Since only lower KE molecules are left, the average KE has been reduced, hence the temperature has been reduced

Cooling Effect of Evaporation

Boiling EvaporationRequires heat

suppliedDoes not require

heat suppliedMust take place at

boiling pointCan take place at any temperature

Takes place throughout whole

liquid

Only takes place at surface of liquid

Boiling vs Evaporation

Heating & Cooling Curves Microscopic explanation for boiling /

melting / freezing / condensation Microscopic explanation for evaporation Evaporation vs Boiling

Summary