MELTING AND BOILING

1
MELTING AND BOILING

• SEC 3 PHYSICS

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Melting

• Let us recall
• Melting is a change of state from solid to
  liquid.
• Melting occurs at a fixed temperature.
• This temperature is known as the melting point of
  the substance.
• Eg. Melting point of water is 0oC or 273 K

3
Melting

• Let us remove an ice cube from the freezer and
  place it under the hot sun which is about 35oC.
• Assuming that original temperature of ice cube is
  about -20oC. What will you observe after
  sometime?
• It is obvious that after sometime, the ice cube
  will start to melt and eventually solid ice cube
  will turn into water totally.

4
Melting

• If you were to measure the temperature of the ice
  cube as it slowly turns from a solid state to a
  liquid state, which would be the situation that
  you would observe? Graph A or B?

-20oC
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Melting

• The result that one would observe is that of
  Graph B!
• Interpreting the Graph
• P-Q Temperature of ice cube increases from -20 ?
  0oC. Ice cube remains a solid.
• Q-R Ice cube starts to melt. Solid changing to
  liquid. During this stage, substance is both in
  solid-liquid state. No change in temperature.
• R-S Solid ice cube has changed totally to liquid
  water. Temperature of water increases from 0 ?
  35oC.

Graph B is known as the Heating Curve
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Melting

• This observation is TRUE for almost ALL pure
  solid substance during melting.
• During melting, temperature of the substance
  remains constant (fixed) at the melting point for
  some time until all solid has changed to liquid,
  before its temperature increases again.
• WHY?

7
Melting

• To explain this phenomena, we have to look at it
  from the kinetic theory of matter.
• P-Q
• Ice is at a low temperature compared to hot air.
  So thermal energy from surrounding will move into
  ice.
• This thermal energy absorbed by ice, is changed
  to KE. The result we see is particles of ice
  vibrating faster.
• When particles vibrate faster, it results in a
  change (increase) in temperature.
• Thermal energy will be continually absorbed as
  long as there is a temperature difference btw ice
  and surrounding.

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Melting

• Q-R
• The amount absorbed will reach a level such that
  the KE of particles are enough to break the
  particles from the inter-molecular forces of
  attraction holding them together.
• During this stage, whatever thermal energy
  absorbed will not change into KE, but is used to
  break the forces of attraction.
• This is why the temperature during melting is
  constant.

9
Melting

• R-S
• Once all solid ice has turned into liquid state,
  energy is no longer needed to break the
  attractive forces.
• Since liquid water is at a lower temperature
  compared to surrounding, it will still absorb
  thermal energy from hotter surrounding.
• This thermal energy is changed into KE and as a
  result, particles move faster, causing an
  increase in temperature.
• When the temperature of liquid water has reached
  that of the surrounding, no more thermal energy
  transfer will happen. Its temperature will be
  equal to the surrounding.

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Melting molecular level
MELTING
LIQUID
SOLID

• KE constant
• Thermal energy used
• to break attractive
• forces btw particles
• Temperature constant

• KE increases
• Particles vibrate faster
• Temperature increases

• KE increases
• Particles vibrate faster
• Temperature increases

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Freezing

• The reverse to melting is freezing or
  solidification
• Freezing is the process of change from liquid to
  solid state.
• Freezing occurs at a fixed temperature.
• This temperature is known as the freezing point.
• A pure substance freezes at a temperature equal
  to its melting point.
• This means that for the same pure substance, its
  freezing point is the same as its melting point.

12
Freezing

• A typical cooling curve of a substance can be
  represented by the freezing of napthalene (moth
  balls)
• Again it can be observed that during the freezing
  stage, K?L, temperature remains constant at 78oC.
• This means that 78oC is the freezing point of
  napthalene.

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Freezing

• Again to understand this phenomena, we have to
  approach the problem from the kinetic theory of
  matter.

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Freezing

• J?K
• Napthalene is at a higher temp. compared to
  surrounding
• Thermal energy is being transferred out of
  napthalene
• Losing thermal energy means that KE of particles
  drops
• Particles are slowing down, resulting in
  temperature drop.

15
Freezing

• K?L
• KE of particles are at such a level that the
  particles are no longer able to ignore
  inter-molecular force of attraction
• As particles are being drawn into a fixed
  structured pattern, the average KE of all
  particles will remain the same until ALL
  napthalene changes to solid state
• Hence there is no change in temperature although
  thermal energy is still being released to the
  surroundings. This energy released comes from
  bond formation.

16
Freezing

• L?M
• Once all napthalene has changed into solid state,
  the continual release of thermal energy due to
  temperature difference btw napthalene and
  surrounding, causes KE of particles to drop
  further.
• The result of KE drop is that temperature of
  napthalene will also drop.
• Temperature will drop until temperature of both
  napthalene and surrounding are the same.

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Effect of Impurities on Freezing Point

• When impurities are added to a substance, it
  would lower its freezing point.
• Impurities are other foreign substances.
• Example
• I have 2 cups of water, A and B.
• I add salt to B. When I try to freeze the water
  in both cups, A would freeze at 0oC but B would
  freeze maybe at 10oC.
• This means B would stay as a liquid even at 0oC
  or -2oC or lower until the temperature reaches
  10oC.
• When A becomes solid ice at 0oC, B is still
  liquid.

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Effect of Impurities on Freezing Point
(optional)

• Reason for this is that presence of impurities
  causes disruption to the formation of
  inter-molecular forces of attraction.
• The particles of original substance would need to
  be slower (this means the average KE has to be
  lower) before they can be drawn into a fixed
  structured pattern by the forces.

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Effect of Impurities on Freezing Point

• Applications
• In cold countries, impurities are added to water
  in radiators of vehicles to prevent water from
  freezing
• Salt are spread on roads during winter, so that
  water from melted snow will not form into ice
  easily. Ice is very slippery and is dangerous to
  traffic.

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Effect of Pressure on Melting Point

• Melting point of ice is lowered by an increase in
  pressure and is increased by a drop in pressure.
• At higher pressure, an ice cube would melt at
  temperatures lower than 0oC.
• At lower pressure (on the mountain), ice would
  only melt at temperatures higher than 0oC. Which
  is why there can be snow on the mountain top even
  when surrounding temperatures are above 0oC.

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Effect of Pressure on Melting Point

• Try this at home. Press two ice cubes just newly
  removed from freezer together for sometime.
• Release the ice-cubes.
• Both ice-cubes are stuck together.
• WHY?

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Effect of Pressure on Melting Point

• (optional)
• When a solid substance is subjected to high
  pressure, this pressure helps particles to
  overcome the inter-molecular forces holding them
  together.
• As a result, it is not necessary for KE of
  particles to reach a high level before they are
  able to break free.
• When pressure is lower, substance is more stable.
  Hence these particles would need more KE before
  they can break free from the forces of
  attraction. Result is that its melting point
  would be higher.

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Boiling

• Boiling is a change of state from liquid to gas.
• Boiling occurs at a fixed temperature. During
  boiling, temperature remains constant.
• Heat is taken in / absorbed during this process.
• This temperature is known as the boiling point of
  the substance.
• Liquid will remain at boiling point until all the
  liquid has changed to gas.
• (phenomena is pretty similar to melting)

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Boiling

• A typical boiling curve for this instance, for
  water.
• Q?R where the temperature remains constant is
  the boiling stage.

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Boiling

• P?Q
• Water absorbs thermal energy, causing its
  particles to increase in speed (KE increases)
• The result of KE increase is the rise in
  temperature

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Boiling

• Q?R
• When KE of particles reach a certain level, it is
  strong enough to overcome the inter-molecular
  forces holding them together
• At this stage, all thermal energy absorbed goes
  towards breaking the attractive forces
• Particles are all now free to move freely and
  randomly
• Substance has changed from liquid to gas state
  completely

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Boiling

• R?S
• Once in gas state, particles will speed up as it
  absorbs more thermal energy (from heating)
• This results in temperature increase of substance

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Condensation

• The reverse of boiling is condensation.
• It is the process of change from gas to liquid.
• It occurs at a fixed temperature which is the
  boiling point of the substance.
• Heat is given off during this process.

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Condensation

• A typical cooling curve for condensation
• A?B
• Thermal energy is being transferred out of water
• Losing thermal energy means that KE of particles
  drops
• Particles are slowing down, resulting in
  temperature drop

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Condensation

• B?C
• KE of particles are at such a level that the
  particles are no longer able to ignore
  inter-molecular force of attraction
• As particles are being drawn into a structured
  pattern, the average KE of all particles will
  remain the same until ALL water vapour changes to
  liquid state
• Hence there is no change in temperature although
  thermal energy is still being released to the
  surroundings. This energy released comes from
  bond formation.

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Condensation

• C?D
• Once all water vapour has changed into liquid
  state, the continual release of thermal energy
  due to temperature difference, causes KE of
  particles to drop further.
• The result of KE drop is that temperature of
  liquid water will also drop.
• Temperature will drop until temperature of both
  liquid water and surrounding are the same.

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Melting and Boiling Curve of a Substance

• A typical melting and boiling curve of a
  substance is as shown. Note the following

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Heating and Cooling Curve

• From a heating or cooling curve, we can determine
  the melting (or freezing) and boiling (or
  condensation) points of a substance.
• All we need to do is to look for horizontal
  straight line.

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Effect of Impurities on Boiling Point

• Adding impurities will raise the boiling point of
  an object.
• That means to say the liquid will now boil at a
  higher temperature.

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Effect of Pressure On Boiling Point

• Lower pressure would lower the boiling point of
  water.
• Water will boil very quickly on the mountain top
  but the temperature reached is lower than 100oC.
• Increasing pressure would raise the boiling point
  of water.
• Water will boil at a higher temperature above
  100oC.

Can we boil water at room temperature, 30oC?
36
Evaporation

• Evaporation is the change of state from liquid to
  gas at any temperature.
• Note that evaporation is not the same as boiling.

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Differences
Boiling Evaporation
Occurs at fixed temperature Occurs at any temperature
Quick process Slow process
Takes places within liquid Takes places only on surface
Bubbles are formed No bubbles are seen
Temperature remains constant Temperature may change
Heat supply by energy source Heat supplied by surroundings
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Kinetic Theory of Evaporation

• Molecules at surface of liquid have higher energy
  than the average kinetic energy of the other
  molecules when these molecules absorb energy from
  the surrounding air and objects.
• These molecules gain energy and move faster and
  are thus able to overcome forces of attraction
  between the molecules and hence leaving behind
  less energetic ones.
• When energy is lost for evaporation to take
  place, the remaining molecules have lower kinetic
  energy. Since temperature is directly
  proportional to average kinetic energy, the
  temperature of the remaining liquid decreases.
  (slightly)
• As a result, the surface where evaporation has
  taken place feels cooler.
• Conclusion Evaporation causes cooling.

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Some everyday effects and uses of evaporation.

• Cooling effect on skin upon application of
  cologne
• Evaporation of perspiration cools the body.
• Wet clothes or puddles of water dries up in the
  open air.
• Sponging someone who is having fever will cause
  the temperature to drop as the evaporation of the
  water from the persons skin will cause cooling.

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Factors affecting evaporation

• Temperature
• A liquid with a higher temperature will evaporate
  faster than one at a lower temperature. So
  heating will increase rate of evaporation
• Humidity of surrounding air
• Humidity refers to presence of water vapour in
  the air. The rate of evaporation decreases with
  increasing humidity. Rate of evaporation
  increases with decreasing humidity.
• Surface area of liquid
• Rate of evaporation increases with increasing
  surface area. The bigger the surface area, the
  faster the evaporation.

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Factors affecting evaporation

• Presence of wind
• Rate of evaporation increases in the presence of
  wind.
• Pressure
• Rate of evaporation increases with decreasing
  atmospheric pressure. Wet objects dry faster at
  higher altitudes.
• Nature of liquid
• A liquid with a low boiling point has a higher
  rate of evaporation than a liquid with high
  boiling point.