Melting, Freezing, Boiling and Evaporation

8/10/2019 Melting, Freezing, Boiling and Evaporation

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Melting and Freezing,

Boiling and Evaporation

Done by : Fan Yiheng


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Melting and Freezing


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Why they happen at the same

temperature?

My Hypothesis: It is because the two differ

only in the direction from which the change instate is approached.

For instance, if you have ice you warm it up to

OC so it can melt, if you have water you coolit down to OC so it freezes.


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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.


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Further Explanations

Substances freeze at exactly the same

temperature as they melt. As a result the

temperature at whichunder a specified

pressure

liquid and solid exist in *equilibriumis defined as the melting or freezing point.

*Equilibrium is the condition existing when a

chemical reaction and its reverse reactionproceed at equal rates.


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Further Explanations

At a fundamental level freezing and meltingrepresent changes in the energy levels of themolecules of the substance under consideration.

Freezing is a change from a high energy state to

one of lower energy, the molecules are movingless as their temperature falls. They becomemore ordered and fixed in shape.

When a substance melts the average energy level

of the constituent molecules increases. Themolecules are moving more rapidly and in a lessordered manner in a liquid than in a solid.

This consideration of the energy of the molecules

is known as the kinetic molecular theory.


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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 thetemperature required.


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Additional facts

The purity of the compound can influence thetemperature at which the solid-liquid change takes place.For example, adding sodium chloride (common salt) towater depresses the freezing point, which is why salt isput on roads to stop their icing over.

When a solid is melted by heating or a liquid frozen whilecooled, the temperature remains constant. Thus, if agraph of temperature is plotted against heat added ashoulder or plateau will be seen which represents the

freezing or melting point. With an impure substance, thisshoulder will not be so precise. A graph of this nature isknown as a heating curve. The conversion between solidand liquid occurs at a constant temperature.


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Boiling and Evaporation


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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 isbetween them.

To break the bond between two particles, one

particle has to be moving fast enough toovercome the pull of the other, until it gets so far

away that pull is diminished.


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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 thetemperature is uniform throughout.

Boiling


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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 vaporpressure 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.


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Evaporation

Not all particles in the liquid are moving at thesame speed

As a result, the faster particles are more likely to

overcome the forces they feel from theirneighbors.

The particles at the surface of the liquid are onlyheld in place by forces from the neighboring

particles beneath them, whereas particles in themiddle of the liquid have forces holding them onall sides. Thus, particles at the surface find iteasier to break away from the liquid.


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Consider two molecules in a liquid, as shown in thefigure. 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 thesurface feels forces of attraction or cohesive forces

from the molecules only on one side. The other side it

is exposed to air.


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The force of cohesion in the case of molecule 1 is less

than that compared to the cohesive forces experiencedby molecule 2.

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

We now know that energetic molecules escape fromthe surface of a liquid during evaporation. This lowersthe average kinetic energy of the molecules left behindin the liquid. This lowers the temperature of the liquid.Thus evaporation lowers the temperature of the liquid.


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Conclusion

Evaporation is a surface phenomenon.

Boiling is a bulk phenomenon.

My hypothesis is wrong.


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References

Enotes.com

http://www.educationalelectronicsusa.com/p

/heat-III.htm

http://www.learner.org/courses/essential/phy

sicalsci/session3/closer2.html


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Thank You!

Melting, Freezing, Boiling and Evaporation

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