Transcript MELTING AND BOILING

MELTING AND BOILING
SEC 3 PHYSICS
On the Molecular Level
• We have learnt previously that when
molecules absorb heat energy, they will
start to move faster.
• When the molecules have reached a
certain speed, the energy that they
absorb will now be used to break the
attractive forces between them.
On a Molecular Level (cont’d)
• When the energy absorbed is used to
break the attractive forces between
them, there is no increase in speed.
• Which means energy absorbed can only
be used for either one of the purpose
(break attractive forces OR increase in
speed). Only one change is taking place
at any time.
So what do we see?
• When the energy is used to increase the
speed of the molecule, we see a rise in
temperature of the substance.
• When the energy is used to break attractive
forces between molecules, we see the
substance changing in state.
This energy which brings about a change in
state is known as LATENT HEAT
Changes in States
Gas
CONDENSATION
Loss in heat
Boiling
Liquid
Gain in heat
FREEZING
melting
Solid
MELTING AND SOLIDIFICATION
• When a solid changes to a liquid on heating,
we call it melting
In this instance, heat is ABSORBED by the
substance
• When a liquid changes to a solid on cooling,
we call it solidification (freezing)
In this instance, heat is LOST by the
substance
MELTING AND SOLIDIFICATION
• In both cases, the heat that is absorbed
or lost is known as the latent heat of
fusion.
• When this latent heat is absorbed or
lost, there is no change in temperature.
• This means when a substance is
undergoing a change in state, the
temperature will remain constant.
The Temperature-time Graph
temperature
This part of the graph shows a
constant temperature which
means this is when the
substance is changing state
time
Effect of Impurities on Freezing Point
• When impurities are added to water, it
would lower the freezing point of water.
This means that water now freezes at a
lower temperature.
Effect of Impurities on Freezing Point
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 lower until the temperature reaches –10oC.
When A becomes solid ice at 0oC, B is still liquid.
Effect of Impurities on Freezing Point
The reverse situation would be true. (When melting.)
A (no salt added)
B (salt added)
Ice cube A would melt at 0oC.
But B would melt(turn into liquid) at a temperature
even lower.
Effect of Pressure on Melting Point
• Melting point of ice is lowered by an increase
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.
Effect of Pressure on Melting Point
• Example of pressing two ice cubes together.
• When pressed together, the pressure they experience
is higher, so the melting point is lowered causing the
ice to melt.
• When hand releases, the pressure they experience is
lowered, so the water will now freeze at higher
temperature.
• Another example is walking on ice.
• And ice skating.
boiling AND condensation
• When a liquid changes to a gas on heating,
we call it boiling
In this instance, heat is ABSORBED by the
substance
• When a gas changes to a liquid on cooling, we
call it condensation.
In this instance, heat is LOST by the
substance
Boiling AND condensation
• In both cases, the heat that is absorbed
or lost is known as the latent heat of
vaporisation.
• When this latent heat is absorbed or
lost, there is no change in temperature.
• This means when a substance is
undergoing a change in state, the
temperature will remain constant.
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.
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.
Specific Latent Heat
• Recall that the energy absorbed or
released causing a change in state is
known as latent heat.
• Example: latent heat of fusion
latent heat of vaporisation
Specific Latent Heat
• This is the heat absorbed or released
when 1 kg of the substance
undergoes a change in state.
• There are 2 types of specific latent
heat:
Specific latent heat of fusion
Specific latent heat of vaporisation
Specific Latent Heat of Fusion
• This is the heat required by 1 kg of a
substance to change from solid to liquid
or vice versa without a change in
temperature
• Formula: Lf = mlf
• Unit: Jkg-1
Specific Latent Heat of Vaporisation
• This is the heat required by 1 kg of a
substance to change from liquid to gas
or vice versa without a change in
temperature
• Formula: Lv = mlv
• Unit: Jkg-1
Example:
4400 J of heat was supplied to 2kg of water at
100oC. All of the water turned into steam. What
is the specific latent heat of vaporisation of
water?
Solution:
Lv = m l v
4400 = 2 (lv)
lv= 4400 / 2
lv=2200 Jkg-1
Evaporation
• Liquid changing into gas.
• Seems similar to boiling but there are
differences.
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
Factors Affecting rate of Evaporation
• Temperature
Temperature of liquid and surroundings.