Phase Change


Introduction
pha1.gif
Image courtesy of hyper physics.

Phase Change refers to when matter changes states. Water is one of the few that can become a liquid, solid, and gas. For example, if you were to put an ice cube, a solid, outside on a hot day, it would turn to water, a liquid. If you were to boil that water, it would turn into steam, which is a gas. Then, during condensation, the gas is turned into a liquid. Freezing a liquid to a solid can be more rapid if the water is hot than when it is warm because of the rapid evaporation. Sublimation is where heat enters a solid to change the molecule bonding, turning the solid into a gas. All of these substances have the same composition, they just have a different form. Phase change depends on the temperature and the amount of pressure exerted on it, also usually involving a transfer of energy. Physical change and chemical change are the two different types of phase changes. Chemical change is when the composition of a substance changes. An example of chemical change is when a leaf changes color in the fall. A physical change is when only the form of the substance changes. A cloud is made through phase change by warm air rising and expanding. It then cools while expanding and water vapor molecules collide together and stick
. When there are larger and slower moving ions or parrticles, the water vapor condenses on the particles and forms a cloud.[1][2][11]

Vocabulary


  • Phases- plasma, solid, liquid, and gas. (However, only the last three will be addressed here)
  • Evaporation- change of phase from liquid to gas that takes place at the surface of a liquid.
  • Condensation- the changing of a gas to a liquid.
  • Saturated- the air when the water vapor limit is reached.
  • Relative Humidity- how much water vapor is in the air, compared with the limit for that temperature.
  • Equilibrium- a state of balance.
  • Boiling- change of liquid to a gas, the gas that forms beneath the surface causes bubbles and the bubbles are buoyed upward to the surface, where they escape into the surrounding air.
  • Freezing- when energy is extracted from water at a temperature of 0°C and at atmosphere pressure, ice is formed.
  • Regelation- The phenomenon of melting under pressure and freezing again when the pressure is reduced.[11]

Evaporation and Condensation


Evaporation and Condensation are practically the same process but go in the opposite direction. Condensation is the process of a gas turning into a liquid, and evaporation i
water%20cycle.gif
Image courtesy of cochrane
s the process of liquid turning into gas. Evaporation occurs below the boiling point, with the molecules of a liquid that have greater kinetic energy. These molecules have enough energy to escape the surface tension of the liquid and the molecular forces within the liquid. Escape of more energetic particles will make the average temperature of the leftover particles less. Wiping your forehead with a cool cloth is an example; it cools down your temperature. Condensation involves gaseous molecules cooling down to liquid stage. In this process, the mass of molecules have to give up a certain amount of heat that is equal to the amount of heat they have to gain to be vaporized or evaporated. Energy goes into the molecule and strength of bonds is only present in liquid, not in gas. The amount of heat given up is the same amount of work going into the molecular attraction when the molecules are in liquid form. Evaporation and condensation can be seen in the water drops on a glass on a hot day to the steam on the mirror after a hot shower. They are used a lot in heating and cooling technologies, including refrigeration. In some parts of the country, buildings are heated and cooled on the same steam system, which will run in condensing reverse in the summer from its winter evaporative steam heating procedure. There is also the process of equilibrium which is a state of balance, since evaporation and condensation have cancelling effects.This occurs when the molecules and energy leaving the liquid's surface by evaporation are counteracted by as many molecules and as much energy returning by condensation. [3][4][11]

Boiling and Freezing

----
To oversimplify only a little, temperature is a measure of how much energy there is in molecular motion. To begin with an explicit example, let's consider water. When water molecules are cold enough, they don't have much heat energy so they don't jostle around too much. Consequently, they can pack together in a very organised structure, called ice. At high enough temperatures, they have so much energy that they can escape the attraction of their neighbours. So they form steam, in which the molecules fly all over the place in a very disordered way. At medium temperatures, which means that the molecules have moderate amounts of energy (and if the pressure is high enough) they form liquid water. Here the molecules have enough energy to move around, but not enough to escape from their neighbours entirely. Molecules in liquid water are more ordered than in steam but less ordered than ice. (As an example of the order in a liquid, we can observe that the centre of each molecule is about one molecular diameter away from that of its nearest neighbours.) Whenever molecules evaporate from a liquid the boundary layer between the liquid and the vapor will move. When the liquid contains a non volatile solute, the moving boundary will transfer energy to the solute molecules. This energy is included in the evaporation process, therefore, it will affect the vapor pressure and boiling temperature of a solution. Similarly, during freezing, or melting, the boundary movement between the solution and pure solid will affect the freezing temperature. In both cases the moving boundary works against the osmotic pressure generated by the solute.[5][6]

Regulation

regelation.jpg
Image courtesy of tutorvista


When making a snow ball, regelation is used. Compressing the snow causes slight melting which binds the snowball. In the picture to the right is regelation of ice by round wires. The motion of wires pulled transversely through ice is in terms of pressure melting at the front of the wire and regelation behind it, the speed of the process being controlled by the rate of conduction of the heat of fusion through the wire and the ice. Measurements of the fractional volume of water in the trace show that above the transition heat flows to the moving wire from the surrounding ice. The low speed below the transition is due to the presence of accumulated solutes in the water layer around the wire, which concentrate toward the rear, lowering the freezing temperature there and hence the rate of heat flow toward the front. The transition occurs when the temperature at the rear reaches the triple point, which fixes the pressure there, so that with increasing driving stress the mean pressure around the wire increases and hence the mean temperature decreases, causing heat flow to the wire and formation of the trace, which carries away the dissolved solutes.[7][11]

Saturation, Relative Humidity, and Equilibrium


Saturation-The saturation level of the air is directly related to the air's temperature. As air temperature increases, more water can remain in a gas phase. As temperature decreases, water molecules slow down and there is a greater chance for them to condense on to surfaces. The graph below shows the relationship between air temperature and vapor pressure, a measure of the humidity, at saturation.
Relative Humidity-The amount of water vapor in the air at any given time is usually less than that required to saturate the air. The relative humidity is the percent of saturation humidity, generally calculated in relation to saturated vapor density. Relative humidity is the amount of moisture in the air compared to what the air can "hold" at that temperature. When the air can't "hold" all the moisture, then it condenses as dew. The equation to find relative humidity is:
rh1.gif

Equilibrium


Equilibrium is a state of balance, an example of this would be: A book sitting on a table top remains at rest because the downward force exerted by the earth's gravity acting on the book's mass is exactly balanced by the repulsive force between atoms that prevents two objects from simultaneously occupying the same space, acting in this case between the table surface and the book. If you pick up the book and raise it above the table top, the additional upward force exerted by your arm ends the state of equilibrium as the book moves upward. If you wish to hold the book at rest above the table, you adjust the upward force to exactly balance the weight of the book, this then becomes restoring equilibrium. [8][9][10][11]

References

1. id.mind
2.ric
3.chem.purdue
4.bookrags
5.animations.physics
6.ultra.tripod
7.rspa.royalsocietypublishing
8.uwsp
9.hyperphysics.phy-astr.gsu
10. chem1
11. Conceptual Science textbook