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There are over one billion cars
in the world today,
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getting people where they need to go,
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but cars aren't just
a mode of transportation,
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they're also a chemistry lesson
waiting to be taught.
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The process of starting your car
begins in the engine cylinders,
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where a spritz of gasoline
from the fuel injector
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and a gulp of air
from the intake valve
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mix together before
being ignited by a spark,
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forming gases that expand and push the piston.
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But combustion is an exothermic reaction,
meaning it releases heat.
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Lots of it.
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And while much of this heat escapes
through the tail pipe,
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the heat that remains in the engine block
needs to be absorbed, transported, and dissipated
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to protect the metal components
from deforming or even melting.
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This is where the cooling system comes in.
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A liquid gets circulated
throughout the engine,
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but what kind of liquid
can absorb all that heat?
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Water may seem like an obvious first choice.
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After all, its specific heat,
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the amount of energy required
to raise the temperature
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of a given amount
by one degree Celsius,
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is higher than that of
any other common substance.
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And we have a lot of heat energy to absorb.
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But using water can get us
into deep trouble.
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For one thing, its freezing point
is zero degrees Celsius.
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Since water expands
as it freezes,
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a cold winter night could mean
a cracked radiator and a damaged engine block,
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a chilling prospect.
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And considering how hot
car engines can get,
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the relatively low boiling point
of 100 degrees Celsius
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can lead to a situation
that would get anyone steamed.
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So, instead of water,
we use a solution,
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a homogeneous mixture consisting
of a solute and a solvent.
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Some of the solution's properties will differ
depending on the proportion of solute present.
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These are called colligative properties,
and as luck would have it,
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they include freezing point depression
and boiling point elevation.
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So, solutions have both a lower freezing point
and a higher boiling point than pure solvent,
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and the more solute is present,
the bigger the difference.
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So, why do these properties change?
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First of all, we need to understand that
temperature is a measure
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of the particle's average kinetic energy.
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The colder the liquid,
the less of this energy there is,
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and the slower the molecules move.
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When a liquid freezes,
the molecules slow down,
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enough for their attractive forces
to act on each other,
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arranging themselves into a crystal structure.
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But the presence of solute particles
gets in the way of these attractions,
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requiring a solution to be cooled down further
before the arrangement can occur.
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As for the boiling point,
when a liquid boils,
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it produces bubbles filled with its vapor,
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but for a bubble to form,
the vapor pressure must become as strong
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as the atmosphere constantly pushing down
on the surface of the liquid.
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As the liquid is heated,
the vapor pressure increases,
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and when it becomes equal
to the atmospheric pressure,
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the bubbles form and boiling occurs.
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A solution's vapor pressure is lower
than that of pure solvent,
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so it must be heated
to an even higher temperature
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before it can match
the strength of the atmosphere.
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As an added bonus,
the pressure in the radiator
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is kept above atmospheric pressure,
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raising the boiling point
by another 25 degrees Celsius.
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The solution commonly used
for a car's cooling system
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is a 50/50 mixture of
ethylene glycol and water,
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which freezes at -37 degrees Celsius
and boils at 106 degrees Celsius.
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At the highest recommended proportion
of 70 to 30,
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the freezing point is even lower
at -55 degrees Celsius,
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and the boiling point rises
to 113 degrees Celsius.
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As you can see,
the more ethylene glycol you add,
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the more protection you get,
so why not go even higher?
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Well, it turns out you can have
too much of a good thing
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because at higher proportions,
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the freezing point actually
starts to go back up.
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The properties of the solution head towards
the properties of ethylene glycol,
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which freezes at -12.9 degrees Celsius,
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a higher temperature than we
attained with the solution.
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The solution flows through the engine,
absorbing heat along the way.
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When it reaches the radiator,
it's cooled by a fan,
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as well as air rushing through
the front of the car
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before returning to the hot engine compartment.
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So, an effective and safe engine coolant
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must have a high specific heat,
a low freezing point, and a high boiling point.
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But instead of searching all over the world
for the perfect liquid to solve our problem,
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we can create our own solution.