LESSON 3: FREEZING AND MELTING
When you have completed this lesson, you should be able to:
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Operationally define freezing temperature in terms of observable and measurable
characteristics.
Construct and interpret heating and cooling graphs from experimental data.
Determine the freezing temperature of a liquid from a temperature versus energy
(or temperature versus time) graph
State the normal freezing temperature of water.
Predict the ranking of normal freezing temperatures of a series of compounds, and
explain reasons for the ranking.
Describe and give examples of the practical value of knowing the freezing
temperatures of substances (e.g., propane, alcohol versus mercury thermometers,
water/ethylene glycol mixture or antifreeze).
Phase Changes
A solid will change to a liquid when heat is added or absorbed. This change is called
melting or fusion. Energy is needed to move the particles apart and overcome the forces
of attraction in the solid.
Why do you feel colder when getting out of the lake or the shower? What is happening
on your skin that causes this feeling?
Energy is needed to change a liquid to a gas. Since the particles of a gas are
much farther apart than in a liquid, the intermolecular forces must be overcome
to allow the particles to move more freely. Overcoming forces of attraction
requires the input of energy. This change is known as vaporization. There are
two types of vaporization, evaporation and boiling. Evaporation is the conversion
of a liquid to gas on the surface of the liquid. Boiling occurs when vaporization
takes place throughout the liquid. We will define these further later in the unit.
The water on your skin is evaporating, taking your body heat with it in doing so!
When a gas changes to a liquid the particles move closer together. As a gas
cools, the kinetic energy of the particles decreases. The particles slow down and
their kinetic energy can no longer overcome the forces of attraction from the
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other particles. The particles are drawn closer together by the intermolecular
forces. The energy lost is released to the surroundings. The conversion of a gas
to a liquid is called condensation or liquefaction.
Freezing or solidification is the conversion of liquid to solid. Like condensation, as
the particles slow down and move closer together energy is released.
Sublimation occurs when a solid changes directly to a gas, without passing
through the liquid state. Dry ice is solid carbon dioxide that goes directly to the
gas phase. This is why it is called dry ice, it does not get "wet" with liquid. This is
quite convenient for storing frozen foods, as the food containers do not get wet
from the melting ice, like they would with water ice. Much of the snow in the
winter disappears due to sublimation. Some winters, more snow disappears due
to sublimation than melting. Before clothes dryers were popular, people would
hang clothes outside to dry after washing. In the winter the clothes would freeze,
then eventually dry by sublimation. Most freeze-dried foods are first frozen at a
very low temperature and the water sublimes after being attached to a vacuum.
Deposition is the opposite of sublimation. It is the conversion to a solid from a
gas. The formation of frost in the winter, or in your freezer, is a good example of
deposition. In the winter, it is too cold for the formation of a liquid yet ice is
formed on trees and your car windshield from water vapour in the air.
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Defining Freezing and Melting
Freezing , solidification or crystallization is known as the change in state from solid to
liquid. The freezing point of a substance is the temperature at which a substance freezes.
As a substance freezes (energy is lost), the particles begin to slow down and the
forces of attraction between the particles, or intermolecular forces begin to
increase and take hold. As the intermolecular forces increase, the particles
arrange themselves into an organized repeating pattern called a crystal.
Melting or fusion is the change in state from solid to liquid. The melting point of a
substance is the temperature at which a substance melts or fuses.
As a solid is heated (energy is absorbed), the forces of attraction between the
particles of the crystal are slowly overcome, allowing some particles to move
more freely. As more particles are free from the crystal, the substance melts.
Lab Activity: Heating Curves
Freezing/Melting Points and Intermolecular Forces
According to the Kinetic Molecular Theory, intermolecular forces must be overcome in
order for a substance to melt.
Here are some normal melting / freezing points of several substances:
Melting Points of Similar Carbon Compounds
Compound
Mass (amu)
Melting Point (°C)
Propane, C3H8
44
-190
Butane, C4H10
58
-138
Octane, C8H18
114
-56
What conclusions can we make from this data?
It makes sense then, the greater the forces of attraction between the particles, the
more energy needed to overcome these forces. If more energy is required, the
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temperature at which the substance melts or freezes must be higher. So, the greater the
intermolecular forces, the higher the melting or freezing point.
When comparing similar covalent (molecular) compounds, as mass increases,
melting point also increases. This indicates that octane requires more energy to
change states, and thus has larger intermolecular forces than propane and
butane. Most covalently bonded substances are gases or liquids at room
temperature, indicating that their melting points are generally low.
When comparing molecular compounds with ionic compounds, ionic compounds
typically have higher melting. Recall that ionic compounds, generally solids at
room temperature, are made of positive (often metals) and negatively charged
(often non-metals) ions held together by electrostatic forces. Ions in ionic
compounds are held in a crystal lattice, where each ion is held in place by
several others of opposite charge
When ionic compounds melt, the ions separate or dissociate and move freely as
liquid ions. These electrostatic forces are extremely strong, hence the melting
point of ionic compounds is very large. For example, the melting point of sodium
chloride is 801°C
Melting and freezing point are characteristic physical properties. A characteristic
physical property is a property that can be used to distinguish one substance
from another. Since many substances have different freezing points, we can use
freezing point to separate substances. For example, water can be separated
from a mixture of water and ethanol by slowly decreasing the temperature. At
standard pressure, water will (freeze) crystallize at zero degrees Celsius, while
ethanol will not freeze until about –117°C. Therefore, after the water has
solidified the ethanol can be poured or decanted from the mixture.
The Special Case for Water
The following table compares the melting points of other substances to water's
melting point.
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Melting Points of Various Compounds
Compound
Mass (amu)
Melting Point (°C)
Methane, CH4
16
-182
Carbon Monoxide, CO
28
-190
Ammonia, NH3
17
-78
Water, H2O
18
0.0
Methane, carbon monoxide and ammonia have similar masses to water, but
have much lower melting points than water.
A special force of attraction called hydrogen-bonding holds water molecules
together. These are not really bonds, but a special type of intermolecular force.
These forces are responsible for the higher than expected melting point of water
and, to a lesser extent, ammonia.
This hydrogen bonding is due to the non-symmetrical shape of the water
molecule and the uneven sharing of electrons between hydrogen and oxygen
atoms. The oxygen atom "hogs" the hydrogens' electrons,
resulting in a partial negative charge on the oxygen side of
the atom and a partial positive charge on the hydrogens'
side of the atom. We will discuss the structure of water
further in a later module.
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As a result of these intermolecular forces, as energy is released during freezing,
the molecules arrange themselves into six-sided crystals which occupy a space
greater than that occupied by the liquid. The
larger water crystal results in the solid form of
water being less dense than the liquid form of
water, while most solids are more dense than
their respective liquids. The crystal structure
of water is evident in each snowflake which
always has six points.
Applications
Since freezing is an exothermic process, man has used the heat released by freezing water
for many years.
Florida is a source of much of the fresh produce, especially oranges, that we use
during the winter months. Florida will often be hit with a cold period when frost
could damage these sensitive crops. In order to prevent frost damage, farmers
spray their crops with water continuously during the coldest part of the night. As
the water freezes, heat is released to the plants. The heat is sufficient to prevent
the plants from freezing. If you watch the news, you may see pictures of orange
trees with long icicles hanging from them. It seems bad, but that's just what the
farmers wanted.
Why don't farmers just spray the crops with water once and let the water freeze?
Well, as you know, melting is endothermic (energy absorbing). The ice will
absorb heat from the plants, causing them to freeze. The farmers spray water
continuously so water freezes continuously. Eventually the sun comes out in the
morning and melts the ice.
Farmers in Manitoba will do the same to protect delicate crops, like tomatoes and
lettuce, in the event of late frosts in the spring or an early frost in the fall.
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Placing a few ice cubes into a warm drink cools the drink because as the ice
melts, it absorbs heat energy from the drink. The drink cools as it loses heat and
the ice cubes melt. The only problem is your drink gets "watered down", which
isn't a problem if all you're drinking is water.
Lesson Summary
In this lesson, you have learned:
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A heating curve shows the graph of a substances temperature as it's heated, as a
function of time.
At the freezing/melting point, both solid and liquid phases are present.
The melting point is a substance is the temperature at which its heating curve
plateaus.
As a substance is heated, the temperature of a pure substance remains unchanged
at its melting point because energy is used to overcome intermolecular forces,
rather than increasing temperature.
As a substance is cooled, the temperature of the substance remains unchanged at
its freezing point because energy is released as the molecules become arranged
into a crystal The released energy maintains the temperature at the freezing point.
The normal melting point is the temperature at which a substance melts or fuses at
standard pressure.
The freezing point can be found at the plateau of a cooling curve.
The freezing point and melting point of a substance are the same temperature.
The greater the intermolecular forces, the higher the melting or freezing point.
Hydrogen bonding is a special intermolecular force of attraction between
hydrogens and oxygens in different water molecules.
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