Melting and Freezing, Boiling and Evaporation

Done by : Fan Yiheng

Melting and Freezing

Why they happen at the same temperature?

• My Hypothesis: It is because the two differ only in the direction from which the change in state is approached.

• For instance, if you have ice you warm it up to O°C so it can melt, if you have water you cool it down to O°C so it freezes.

Explanations

• Freezing is the change that occurs when a liquid changes into a solid as the temperature decreases.

• Melting is the opposite change, from a solid to a liquid as the temperature increases.

• They are examples of phase changes in the opposite direction.

• My Hypothesis is right.

Further Explanations• Substances freeze at exactly the same

temperature as they melt. As a result the temperature at which—under a specified pressure—liquid and solid exist in *equilibrium is defined as the melting or freezing point.

• *Equilibrium is the condition existing when a chemical reaction and its reverse reaction proceed at equal rates.

Further Explanations• At a fundamental level freezing and melting

represent changes in the energy levels of the molecules of the substance under consideration.

• Freezing is a change from a high energy state to one of lower energy, the molecules are moving less as their temperature falls. They become more ordered and fixed in shape.

• When a substance melts the average energy level of the constituent molecules increases. The molecules are moving more rapidly and in a less ordered manner in a liquid than in a solid.

• This consideration of the energy of the molecules is known as the kinetic molecular theory.

Additional facts

• A change in pressure will change the temperature at which the change in the state of matter occurs.

• A decrease in pressure will decrease the temperature at which this occurs and an increase in pressure will increase the temperature required.

Additional facts

• A pure substance has a definite melting or freezing point.

• The addition of an impurity lowers this temperature as well as spreads it so that there is a less definite, more diffuse melting or freezing point.

• This means that we can use the freezing or melting point as an indicator of the purity of a substance.

Additional facts• The purity of the compound can influence the

temperature at which the solid-liquid change takes place. For example, adding sodium chloride (common salt) to water depresses the freezing point, which is why salt is put on roads to stop their icing over.

• When a solid is melted by heating or a liquid frozen while cooled, the temperature remains constant. Thus, if a graph of temperature is plotted against heat added a shoulder or plateau will be seen which represents the freezing or melting point. With an impure substance, this shoulder will not be so precise. A graph of this nature is known as a heating curve. The conversion between solid and liquid occurs at a constant temperature.

Boiling and Evaporation

General facts

Processes

Boiling - happens at a particular temperature, which is 100°C for water- happens throughout the whole liquid

Evaporation - happens at any warm temperature- happens only on the surface

• My Hypothesis: Evaporation only happens at the top layers so the liquid at the bottom basically remains cool and only the top layer is hot. For boiling, the liquid is heated throughout so the liquid will boil as the atoms are moving rapidly.

Why they happen at differenttemperatures?

Microscopic view

• In the cases of both boiling and evaporation, the force between two particles is always present.

• The greater the space between the particles becomes, however, the weaker the force is between them.

• To break the bond between two particles, one particle has to be moving fast enough to overcome the pull of the other, until it gets so far away that pull is diminished.

• Boiling occurs when the average motion of particles is fast enough to overcome the forces holding them close together. This happens evenly throughout a boiling liquid because the temperature is uniform throughout.

Boiling

Boiling

• When a liquid is heated, it eventually reaches a temperature at which the vapor pressure is large enough that bubbles form inside the body of the liquid.

• A liquid boils at a temperature at which its vapor pressure becomes equal to the atmospheric pressure.

• Once the liquid starts to boil, the temperature remains constant until all of the liquid has been converted to a gas.

• It is a state of phase transition.• A boiling liquid has steam and liquid in equilibrium.

Evaporation

• In evaporation, the liquid molecules escape the liquid surface without any external heat source.

• The process by which liquid directly changes to gaseous state is known as evaporation.

Evaporation

• Not all particles in the liquid are moving at the same speed

• As a result, the faster particles are more likely to overcome the forces they feel from their neighbors.

• The particles at the surface of the liquid are only held in place by forces from the neighboring particles beneath them, whereas particles in the middle of the liquid have forces holding them on all sides. Thus, particles at the surface find it easier to break away from the liquid.

• Consider two molecules in a liquid, as shown in the figure. One molecule (molecule 1) is at the surface and one molecule (molecule 2) is deep inside the liquid.

• The molecule 2 experiences attractive cohesive forces from molecules surrounding it. The molecule 1 on the surface feels forces of attraction or cohesive forces from the molecules only on one side. The other side it is exposed to air.

• The force of cohesion in the case of molecule 1 is less than that compared to the cohesive forces experienced by molecule 2.

• The probability of escape from the surface is larger for molecule 1, as its cohesive force holding it back to the liquid is less than that experienced by molecule 2. This is how evaporation takes place.

• We now know that energetic molecules escape from the surface of a liquid during evaporation. This lowers the average kinetic energy of the molecules left behind in the liquid. This lowers the temperature of the liquid. Thus evaporation lowers the temperature of the liquid.

Conclusion

• Evaporation is a surface phenomenon.• Boiling is a bulk phenomenon.• My hypothesis is wrong.

References

• Enotes.com• http://www.educationalelectronicsusa.com/

p/heat-III.htm• http://www.learner.org/courses/essential/

physicalsci/session3/closer2.html

Thank You!