# Unit 4: Energy of Phase Changes

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Unit 4: Energy of Phase Changes

Bell Ringer What are the three phases (states) of matter?

Kinetic Molecular Theory of Matter
Particles of matter are always in motion. The kinetic energy (speed) of these particles increases as temperature ncreases.

Q: What physical arrangement of the molecules of water could describe the differences in the states of matter seen below? Remove Heat Remove Heat Solid Liquid Gas Molecules held tightly together but can still vibrate Has definite shape Molecules vibrate more rapidly enough to overcome the forces that hold them together Can slide past one another Has the shape of container Molecules have overcome the forces that keep them near their neighbors in a liquid and they fly apart Add Heat Add Heat

Solid Liquid Gas Tightly Vibrating Molecules
Has definite shape Molecules vibrate more rapidly Can slide past one another Very Rapid Vibrations they fly apart

Heating vs. Cooling Heating Cooling
Increases the vibration of the molecules Cooling Slows down the motion of the molecules

How does a Thermometer work?
Red Tip Reservoir of colored alcohol with really thin glass walls. When molecules surrounding the thermometer strike the bulb..... They transfer some of their energy to the alcohol inside Alcohol expands Height of liquid can be used to measure the “hotness” of the surroundings

“Cold” Surrounds?? What happens when the thermometer is placed in surroundings that are “colder” than the alcohol in the reservoir? Energy flows from the hotter alcohol to the surroundings Molecules in the alcohol slow down Level of the alcohol falls

How to read a thermometer “hotness meter”……
Temperature A measure of the “hotness” of a substance or how fast the molecules are moving. 3 Temperature Scales Celsius Most Common: Water boils at 100° C, freezes at 0° C Fahrenheit Mostly in the U.S.: Water boils at 212° F, freezes at 32° F Kelvin More scientific: Absolute zero is 0 K

Temperature vs. Heat Temperature Heat
A measure of the “hotness” of a substance or how fast the molecules are moving. Heat A measure of the energy transferred into or out of a system. Always flows from hot  cold

Consider the following situation….
Metal plate and a wooden dish left in the refrigerator over night are now placed on the counter. Which would feel colder? Metal plate because it transfers energy more rapidly from our fingers than wood VS.

Consider the following situation….
If you wanted to warm up the water in your wading pool, would it be better to use a teacup full of water at 100° C or a bucket full of water at 50° C? VS. https://www.youtube.com/watch?v=jb5Tv3G1vSc (4 mins 51 seconds)

Consider the following situation….
If you wanted to warm up the water in your wading pool, would it be better to use a teacup full of water at 100° C or a bucket full of water at 50° C? VS. https://www.youtube.com/watch?v=jb5Tv3G1vSc (4 mins 51 seconds)

Thermal Expansion of a Gas
If you blow up a balloon and place it in the sun, what will happen to it? WHY??

Work on WS #1 & WS #2

Bell Ringer Describe in steps how a thermometer works when placed in a hot water bath… If an ice cube is placed in a hot cup of soup, describe the energy transfer and the motion of the molecules.

Go over WS #1 & WS #2

WS #1: Problem #2 Suppose someone in your class said that when water freezes, it is because the molecules get cold and turn hard. Provide a better explanation for this in terms of the motion of molecules and the attractions between them.

WS #1: Problem #3 When asked, "What is between the molecules in liquid water?", a student replied that water filled in the spaces. Is this answer correct? Explain.

WS #2: Problem #1 You decide to boil water to cook noodles. You place the pan of water on the stove and turn on the burner. How does the behavior of the water molecules change as the pan of water is heated? What about your answer to (a) would change if there were more water in the pan?

WS #2: Problem # 2 What property of matter best describes the way a typical alcohol thermometer works? Explain (in terms of energy transfer) why the alcohol level in the thermometer rises (or falls) when you place the thermometer in contact with both warmer (or colder) objects.

WS #2: Problem #3 If you feel feverish, why can't you take your own temperature with your hand?

WS #2: Problem #4 Your older brother announces that the lid to a jar of pickles from the refrigerator is “impossible” to loosen. You take the jar, hold the lid under the hot water from your sink’s faucet for a few seconds, and calmly open the jar. Your brother, when faced with this blow to his pride, claims that he loosened it for you. What knowledge of materials have you applied in this situation that really explains how you were able to open the lid?

Phase Changes Sublimation: solid  gas. Deposition: gas → solid.
Vaporization/Evaporation: liquid  gas. Condensation: gas → liquid. Melting (Fusion): solid  liquid. Freezing: liquid  solid.

Phase Diagrams Show the phases of a substance at different temperatures and pressures. Triple point: temperature and pressure at which all three phases are in equilibrium. Critical point: above this point, you cannot distinguish between liquids and gases

Melting/Freezing Point Lab

Heating and Cooling Curves

Bell Ringer What process occurs when we add heat energy to a solid and it turns into a liquid? Describe what happens to the molecules of a solid when we add heat energy. What is the melting point of water? In Celsius? In Fahrenheit?

Review What happens to molecules as the temperature increases? What are on the axis of a phase change diagram? What are the three phases that a substance passes through? Do all phase change diagrams look the same, why?

What is a plateau? A high, flat area of land where the elevation doesn’t change

What does a plateau mean on a heating curve?
During the Plateau for Melting: While the energy of the contents of the beaker was continually increasing…. the temperature (or “hotness”) of the solid-liquid mixture was relatively constant KE used to separate molecules, PE is being stored Once the beaker contained only liquid: The temperature rose steadily. The molecules are free to move around more rapidly.

Heating Curve The purpose of a heating curve is to see the temperature change as heat (energy) is added to a system. From a heating curve you can get a substances melting/freezing point and boiling point

Heating Curve As heat is added to the solid the temperature increases.
When it reaches the melting point the temperature remains constant until all of the solid has become liquid. Temperature is constant

Heating Curve Once the substance has all become liquid the temperature will begin to rise again. The temperature rises until it reaches the boiling point. Again, the temperature will remain constant until all of the liquid has become vapor/gas

Heating Curve Gas - KE  Boiling - PE  Liquid - KE  Melting - PE 
Solid - KE 

What does a plateau mean on a cooling curve?
During the Plateau for Freezing: While the energy of the contents of the beaker was continually decreasing…. the temperature (or “hotness”) of the solid-liquid mixture was relatively constant PE Stored is “squeezed out” as thermal energy Keeps substance at a constant temp Once the beaker contained only solid: The temperature decreased steadily. The molecules are slowing down

Cooling Curve Cooling curve is the same thing as a heating curve, just in the opposite direction. In this graph we start at high temperatures and get lower. Remember that melting point occurs at the same temperature as freezing point.

Freezing and Melting “TOP 5”
1. MELTING: A substance melts by changing from a solid to a liquid FREEZING: A substance freezes by changing from a liquid to a solid. 2. Substances freeze at the SAME temperature that they melt. 3. The amount doesn’t make a difference – subs. will melt or freeze at the same temp. 4. A plateau is a flat section on a graph where the temps. stay the same and the substance is changing phase. 5. Heat Energy is either absorbed or released during a phase change.

Bell Ringer Is this a heating or cooling graph?
During what time/stage did the substance freeze? Label what stage was it a Liquid? Solid? Both?