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1 A Phase Change occurs when matter changes its state. Recall that “state” refers to physical form only: gas or liquid or solid. No chemical reaction is.

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Presentation on theme: "1 A Phase Change occurs when matter changes its state. Recall that “state” refers to physical form only: gas or liquid or solid. No chemical reaction is."— Presentation transcript:

1 1 A Phase Change occurs when matter changes its state. Recall that “state” refers to physical form only: gas or liquid or solid. No chemical reaction is taking place. For example, H 2 O (s)  H 2 O (l) describes the change ice  water.

2 2 Reminder: the subscripts (s), (l), and (g) after a formula tell the state of the substance. Solid = (s), Liquid = (l), and gas = (g). Phase changes are caused by changes in the environmental pressure and/or temperature of a substance. Most changes occur at 1 atmosphere, so we most commonly see temperature driving the change.

3 3 What happens when a solid is heated at constant pressure? melting point or freezing point At first the solid gets warmer. After a while, adding more heat does not raise the temperature. Instead, both the solid and liquid state of the substance are present at the same temperature. This is called the melting point or freezing point.

4 4 If heating continues, eventually only liquid is present. boiling point or condensation point Adding more heat makes the liquid get warmer until it rapidly begins to pass into the gas state with a pressure equal to the atmospheric pressure. Once again, the temperature does not rise. This temperature is called the boiling point or condensation point.

5 5 To summarize: Heating Time  TemperatureTemperature Solid + Liquid + Gas Gas

6 6 Heat is added to drive the phase changes solid  liquid and liquid  gas. Heat energy overcomes the intermolecular forces and makes the molecules move faster.

7 7 heat of fusion The heat needed to melt a solid is called the heat of fusion. “Fusion” is an older term for melt. The same amount of heat is released when a liquid returns to the solid state: freezing. Melting and freezing are different only in the direction heat flows (in or out) of the substance changing phase.

8 8 heat of vaporization Similarly, the heat input to make a liquid boil is called the heat of vaporization. When a gas returns to the liquid state (= condensation) the same amount of heat is released from the liquid. Boiling and condensation involve the same heat change – only the direction of heat flow is different.

9 9 Practical Applications The air temperature in Florida rarely reaches the freezing point. When a freeze is predicted, fruit trees are sprayed with water. As the air temperature falls to the freezing point, the layer of water coating the plants starts to freeze….

10 10 releasing heat into the fruit, which does not freeze until the air temperature falls to about 28° F (which rarely happens). When it does, orange juice becomes real cheap! On the other hand, fresh oranges become expensive.

11 11 Refrigerators Refrigerators, air conditioners, dehumidifiers, and heat pumps all make use of the condensation of gases like “Freon” to move heat around. A gas can be caused to condense or turn to liquid by applying pressure. Refrigerant gases usually condense at temperatures around -30° C.

12 12 When the gas condenses, it releases its heat of vaporization. Refrigerators use this principle to move heat out of the storage box (heavily insulated). Pipes in the walls of the box contain refrigerant, originally liquefied under pressure by the “compressor.” As the liquid moves through the pipes heat migrates from the food, etc. stored in the box…..

13 13 The transfer of heat causes the refrigerant to vaporize (convert to gas). When the gas reaches the compressor unit, it is liquefied and the heat released is blown out into the environment outside the box. So: refrigerators do not make “cold” – there is no such thing – but rather transfer heat to create a difference in temperature.

14 14 Vapor Pressure of Liquids Molecules of a liquid are moving about; some molecules near the surface of the liquid have enough energy to escape into the gas (vapor) state. The energy of the molecules increases as temperature increases. Not surprisingly, the number of molecules that pass into the vapor...

15 15 also increases with temperature. The vapor above the liquid surface has a partial pressure in the atmosphere like any other gas. So: vapor pressure of a liquid The vapor pressure of a liquid is the partial pressure of its vapor above the liquid at a particular temperature.

16 16 Liquid Vapor Partial pressure of the vapor above the liquid = Vapor Pressure of the liquid. Partial Pressure of vapor

17 17 Boiling Boiling occurs when the vapor pressure of the liquid equals the atmospheric pressure. The “bubbles” that can be seen in the boiling liquid are pockets of gas rising to the liquid surface to escape. The boiling point of a liquid falls at higher altitudes because the atmospheric pressure is less.

18 18 temperature Vapor pressure Vapor pressure = atmospheric P Boiling Point

19 19 Comparing Liquids’ Vapor Pressures Some liquids’ vapor pressures change more slowly over a range of temperatures than others. So, the liquid with the lower vapor pressure will need to be heated to a higher temperature to reach the boiling point …

20 20 temperature Vapor pressure Liquid 1 Liquid 2 BP of 2 BP of 1 P = atmospheric

21 21 Evaporation Evaporation occurs when the container holding the liquid is left open to the atmosphere. The vapor above the liquid is free to mix with the atmospheric gases and move away. As a result, more of the liquid is converted to vapor. Eventually, all the liquid can dissipate this way.

22 22 Distillation Distillation is used to separate a mixture of materials having different boiling points. When the mixture is heated to boiling in a container, the material which has the greatest vapor pressure at a particular temperature will make up most of the vapor.

23 23 If the vapor is collected and condensed, a liquid containing mostly one substance is obtained. The collected liquid is called the distillate. See sketches next 2 slides…

24 24 Distillation flask (“pot”) heater column thermometer Cooling coil/condenser distillate Collection flask

25 25 Mixture boiling * * * * * * * * * * * Vapor 1 * Vapor 2 * * Liquid 1 has lower BP than 2 * Thermometer reads BP of liquid 1 Most of liquid 2 stays in boiling flask because it has higher boiling point

26 26 sublimation Solids also have a small amount of vapor over their surface => Solids usually have very low vapor pressures. If the vapor pressure is significant and the vapors allowed to dissipate, the solid can pass directly into the gas state without forming the liquid first. This process is called sublimation.

27 27 For example, ice has enough vapor pressure to slowly evaporate without melting. As a result, ice cubes can “disappear” from the freezer tray and wet laundry can be hung outside in the winter to dry.

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