Liquid carbon dioxide fire extingushers are also a good example of a change in state brought on by pressure. This topic will consider changes of state between solid, liquid and gas brought about by changes in temperature only, as these are the most commonly experienced changes. Remember changes in state are physical changes, not chemical. The substance itself remains chemically the same, i. See The water cycle for more on the changes of the state of water. Energy and changes in state Changes in state occur because energy is either added or removed from a substance, affecting the way the particles interact with each other.
If a substance is heated, energy is added and the particles will become more active; vibrating, rotating and even moving about faster. If the substance has enough energy, it can overcome the bonding forces holding the particles together and, in doing so, undergo a change in state.
Cooling, on the other hand, removes energy thus making the particles less active and allowing the bonding forces to take hold within the substance.
Melting - solid to liquid Melting occurs when a solid is heated and turns to liquid. The particles in a solid gain enough energy to overcome the bonding forces holding them firmly in place. Typically, during melting, the particles start to move about, staying close to their neighbouring particles, then move more freely. For pure substances, the temperature at which this change occurs is quite precise and is called the melting point of the substance.
As shown in Figure 5, the refrigerator contains 1 an electrically-powered compressor that does work on Freon gas, and 2 a series of coils that allow heat to be released outside on the back of the refrigerator or absorbed from inside the refrigerator as Freon passes through these coils.
This is a schematic diagram of the major functional components of a refrigerator. The major features include a compressor containing Freon CCl 2 F 2 gas, an external heat-exchange coil on the outside back of the refrigerator in which the Freon passes and condenses, an expansion valve, and a heat-exchange coil inside the insulated compartment of the refrigerator blue in which the Freon is vaporized, absorbing heat from inside the refrigerator and thus lowering its temperature.
Figure 6 below traces the phase transitions of Freon and their associated heat-exchange events that occur during the refrigeration cycle. The steps of the refrigeration cycle are described below the figure. The numbers in the figure correspond to the numbered steps below. This diagram shows the major steps in the refrigeration cycle. For a description of each step indicated by the green numbers , see the numbered steps below.
In this figure, blue dots represent Freon gas, and solid blue areas represent liquid Freon. Small arrows indicate the direction of heat flow into or out of the refrigerator coils. Please click on the pink button below to view a QuickTime movie showing an animation of the refrigeration cycle shown in the figure above and described below. Click the blue button below to download QuickTime 4. Outside of the refrigerator, the electrically-run compressor does work on the Freon gas, increasing the pressure of the gas.
As the pressure of the gas increases, so does its temperature as predicted by the ideal-gas law. Next, this high-pressure, high-temperature gas enters the coil on the outside of the refrigerator. Heat q flows from the high-temperature gas to the lower-temperature air of the room surrounding the coil.
This heat loss causes the high-pressure gas to condense to liquid, as motion of the Freon molecules decreases and intermolecular attractions are formed. Hence, the work done on the gas by the compressor causing an exothermic phase transition in the gas is converted to heat given off in the air in the room behind the refrigerator.
If you have ever felt the coils on the back of the refrigerator, you have experienced the heat given off during the condensation of Freon. Next, the liquid Freon in the external coil passes through an expansion valve into a coil inside the insulated compartment of the refrigerator.
Now, the liquid is at a low pressure as a result of the expansion and is lower in temperature cooler than the surrounding air i. Since heat is transferred from areas of greater temperature to areas of lower temperature, heat is absorbed from inside the refrigerator by the liquid Freon, causing the temperature inside the refrigerator to be reduced.
The absorbed heat begins to break the intermolecular attractions of the liquid Freon, allowing the endothermic vaporization process to occur. When all of the Freon changes to gas, the cycle can start over. The cycle described above does not run continuously, but rather is controlled by a thermostat. When the temperature inside the refrigerator rises above the set temperature, the thermostat starts the compressor.
Once the refrigerator has been cooled below the set temperature, the compressor is turned off. This control mechanism allows the refrigerator to conserve electricity by only running as much as is necessary to keep the refrigerator at the desired temperature. Refrigerators are essentially heat engines working in reverse. Whereas a heat engine converts heat to work, reverse heat engines convert work to heat.
In the refrigerator, the heat that is generated is transferred to the outside of the refrigerator. To cool the refrigerator, a "working substance", or "coolant", such as Freon is required. The refrigerator works by a cycle of compressing and expanding the Freon, combined with phase transitions between the gaseous and liquid phases of Freon. Work is done on the Freon by a compressor, and the Freon then releases heat to the air outside of the refrigerator as it undergoes the exothermic condensation from a gas to a liquid.
To regenerate the gaseous Freon for compression, the Freon passes through an internal coil, where it undergoes the endothermic vaporization from the liquid phase to the gaseous phase. This endothermic process causes the Freon to absorb heat from the air inside the refrigerator, cooling the refrigerator. Louis, MO Figure 1 In a heat engine, an input of heat causes an increase in the temperature of the working substance, allowing the working substance to perform work.
Figure 2 In a reverse heat engine, a work input is converted to a heat output. Figure 3 This schematic diagram shows the differences in physical properties and particle arrangement between a substance in the solid, liquid, and gas phases. A chemical that exists in its liquid state can be changed back into a solid state in a process known as fusion , which is more commonly-known as "freezing.
During condensation , a substance is changed from the gaseous to the liquid state of matter. Vaporization , which is more often referred to as "boiling," is the complementary process in which a chemical is converted from the liquid state of matter to a gaseous physical form. Many people erroneously label this transformation as "evaporation.
While the particles in a liquid must remain in direct contact with one another, gaseous atoms or molecules exist independently. Therefore, in order to change a substance from the liquid to the gaseous state of matter, heat must be applied to overcome the attractive forces between the liquid's constituent particles. In contrast, evaporation is a passive transformation, meaning that heat is not required to either initiate or sustain its corresponding processes.
Because of these subtle, yet significant, differences, "evaporation" is not synonymous with "vaporization," and, therefore, these terms should not be used interchangeably.
Finally, sublimation is the process in which a solid is change to a gas. Its complement, deposition , is defined as a conversion from the gaseous state of matter to the solid state of matter. While these complementary transformations are the least common among the six phase changes, both can be exemplified by analyzing the transformations of carbon dioxide and water.
Solid carbon dioxide, which is commonly-known as "dry ice," does not melt at room temperature, but instead generates a white, "smoky" vapor by subliming directly into its gaseous state.
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