|Why salt will make water freeze slower than the
In liquid water, all of the water molecules are
constantly moving very quickly. The motions that
are most important for liquid water are rotations
(the water molecules spin) and translations (the
water molecules move from place to place).
To freeze into ice, the water molecules need to
lose these motions to become "stuck" into place in
a specific pattern, called the ice crystal
structure. As liquid water is made colder and
colder, the motions of liquid water molecules
become slower and slower. When water is at its
freezing point, the liquid water molecules that
"crash" into the surface of ice are moving slow
become "stuck" in place. This is how ice forms at
As salt is added to water, the salt water
solution is now composed of both water molecules
and dissolved salt ions. As a result, these salt
ions "replace" some of the water molecules.
For example, one gallon of pure water contains
many more water molecules than one gallon of ocean
water, because that gallon of ocean water now also
contains many salt ions.
Salt water freezes more slowly than pure water
because many of the water molecules that would be
"crashing" into the surface of the ice in pure
water are replaced by these salt ions. The salt
ions still "crash" into the surface of the ice
like the water molecules do, but the salt ions do
not become "stuck," so they end up slowing the
growth of ice.
As water is made more and more salty, less and
less water molecules are available to "crash" into
the surface of ice, and the freezing point becomes
lower and lower. At the same time, the ice that is
formed is still pure water. This is the reason
that the glaciers that form at the surface of
arctic oceans melt into fresh, pure water!
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To understand the process of freezing, it’s
important to realize that there are two processes
at play: water turning into ice and ice turning
into water. Even when water is freezing into ice,
there are still “ice molecules” that are turning
into liquid water. Salt messes with the balance
between the freezing and the melting process.
One way to think of this is that when a liquid
water molecule hits the ice, it becomes part of
the ice. But if there’s salt on the ice, the
salt will sort of “block” the liquid water
molecules from becoming part of the ice. This
slows down the freezing process because less water
per time can be incorporated into the ice.
Meanwhile, the melting rate of the ice is
unchanged by the salt. Since the ice/water
mixture now has a higher melting rate than its
freezing rate, the ice will melt. This also causes
the freezing temperature of the water to go down
so that it can be liquid at lower temperatures
than pure water. If salt is added to ice, it will
often melt the ice by slowing the freezing rate.
The easiest way to think of this is that the salt
interferes with the ice forming a crystal. The
salt likes (or at least is very energetically
stable) being mixed with the water. To freeze
water with salt, it's actually kindof a two-step
process: getting the salt out of the way, and then
forming the structure of the crystal. So it
takes extra "coldness" to force the salt out of
the way to form crystals. This image should help
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Salt lowers the freezing temperature of water.
Water freezes less readily because it requires
even colder temperatures in order to freeze.
You are asking two questions in one! So let's
break up your question in two parts:
1. Why salt addition in pure water makes the
salty solution freeze at an even lower temperature?
2. Does it mean that the salty solution will
freeze slower than the pure water?
Part I: we can add a lot of salt or a
little salt. If you add a lot of salt, then it
starts to matter what kind of salt you added. So
for now, let's see what happens when you add a bit
- First of all, it turns out it doesn't matter
what kind of salt you add! For example, you can
add sodium chloride (table salt) or potassium
chloride (also found in the supermarket) or sodium
iodide. Because they each have one positive and
one negative charge they will behave identically.
- Secondly, we will assume that your solution is
"ideal". This means that the interaction between
the salt and the water is the same as between the
water and the water.
So, then what difference does adding salt
By adding salt, we actually decrease the
fraction of water. This is what matters. Now,
in the same space, instead of having say 100 water
molecules we only have 90, so if we want to make
ice fewer water molecules are available to escape
to the solid ice phase. We need to cool more to
make them escape to the solid phase.
This simple but amazing property is what keeps
sea water still liquid when the temperatures fall
below 0 C, keeps fish alive in arctic waters,
keeps the streets of Boston without ice in the
winter so cars and people drive and walk without
Part II : Part I had to do with a static
view of freezing. As the scientists call it, "an
equilibrium" view. But you asked how fast. That
depends on how fast we cool the system. Again, for
dilute salty solutions that are ideal, since water
and salt interactions are assumed to be the same,
it should be exactly as fast in the case of pure
water and salty water. But, in the case of salty
water, more cooling will need to get done to get
to lower temperatures.
For example, pure water freezes at 0 C. Upon
addition of 10g of sodium chloride in 100g of
water, the freezing point will decrease from 0 C
to -5.9 C. So, if I place it in my freezer at -3C,
the salty solution will NEVER freeze no matter how
long I wait.
When a liquid (like water) cools past its
freezing temperature, its molecules no longer have
enough energy to move freely against each other.
Instead, the molecules become locked in place, as
a solid. The temperature where a particular
material freezes (or melts, depending on the
direction) depends on the forces between the
molecules and the size of the molecules – the
stronger the forces are and the bigger the
molecules, the more energy is needed to escape the
solid state and become a liquid.
Water is peculiar because it has a rigid solid
structure that is less dense than its liquid state
(which is why ice floats in liquid water). This is
due to the partial negative charge on water and
partial positive charge on hydrogen. This polarity
allows for hydrogen bonding, where hydrogen
associates strongly with the oxygen on other
molecules in a network. When you freeze water,
this network becomes rigid and locked in place. If
you add salt (NaCl) to the water, the ions disrupt
the formation of this network because they are too
big to be included in it. This lowers the freezing
temperature of the water – you have to remove more
thermal energy from the solution before you can
lock these ions in solid water.
The Celsius temperature scale was defined to
match the phase transitions of pure water, with
100°C at water’s boiling point and 0°C at its
freezing point. Fahrenheit was also designed with
water in mind, but using salt water instead – 0°F
is when seawater begins to freeze. The salt
content in the water lowers its freezing
temperature by 32°F (17.8°C)!
This is a general phenomenon too – dissolving a
small amount of any solid in any liquid will
decrease its freezing point and increase its
boiling point, regardless of what the solid and
liquid are. These effects are called the
colligative properties of solutions.
The properties of solutions are different than
those of pure liquids, and these effects are
called colligative properties.
Interestingly, these properties don't depend on
the type of salt, but rather the number of salt
molecules that are dissolved. When you add salt
ions to water, it stabilizes the liquid water. In
other words, mixing ions takes work (you have to
move them around the whole container), but it
happens without you doing work because the end
solution is more stable, so the stability gives
you the energy to make it happen spontaneously,
that is to say, without you doing any work.
(If the solution wasn't more stable, it
wouldn't dissolve, in the same way that you can't
go up a hill spontaneously -- you have to do work
-- but you can go down a hill spontaneously).
Because the solution is more stable than the
pure liquid, it is harder to turn it into a had or
into a solid. For this reason, the boiling point
increases and the melting point (freezing point)
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