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  • There are over one billion cars in the world today,

  • getting people where they need to go,

  • but cars aren't just a mode of transportation,

  • they're also a chemistry lesson waiting to be taught.

  • The process of starting your car begins in the engine cylinders,

  • where a spritz of gasoline from the fuel injector

  • and a gulp of air from the intake valve

  • mix together before being ignited by a spark,

  • forming gases that expand and push the piston.

  • But combustion is an exothermic reaction, meaning it releases heat.

  • Lots of it.

  • And while much of this heat escapes through the tail pipe,

  • the heat that remains in the engine block needs to be absorbed, transported, and dissipated

  • to protect the metal components from deforming or even melting.

  • This is where the cooling system comes in.

  • A liquid gets circulated throughout the engine,

  • but what kind of liquid can absorb all that heat?

  • Water may seem like an obvious first choice.

  • After all, its specific heat,

  • the amount of energy required to raise the temperature

  • of a given amount by one degree Celsius,

  • is higher than that of any other common substance.

  • And we have a lot of heat energy to absorb.

  • But using water can get us into deep trouble.

  • For one thing, its freezing point is zero degrees Celsius.

  • Since water expands as it freezes,

  • a cold winter night could mean a cracked radiator and a damaged engine block,

  • a chilling prospect.

  • And considering how hot car engines can get,

  • the relatively low boiling point of 100 degrees Celsius

  • can lead to a situation that would get anyone steamed.

  • So, instead of water, we use a solution,

  • a homogeneous mixture consisting of a solute and a solvent.

  • Some of the solution's properties will differ depending on the proportion of solute present.

  • These are called colligative properties, and as luck would have it,

  • they include freezing point depression and boiling point elevation.

  • So, solutions have both a lower freezing point and a higher boiling point than pure solvent,

  • and the more solute is present, the bigger the difference.

  • So, why do these properties change?

  • First of all, we need to understand that temperature is a measure

  • of the particle's average kinetic energy.

  • The colder the liquid, the less of this energy there is,

  • and the slower the molecules move.

  • When a liquid freezes, the molecules slow down,

  • enough for their attractive forces to act on each other,

  • arranging themselves into a crystal structure.

  • But the presence of solute particles gets in the way of these attractions,

  • requiring a solution to be cooled down further before the arrangement can occur.

  • As for the boiling point, when a liquid boils,

  • it produces bubbles filled with its vapor,

  • but for a bubble to form, the vapor pressure must become as strong

  • as the atmosphere constantly pushing down on the surface of the liquid.

  • As the liquid is heated, the vapor pressure increases,

  • and when it becomes equal to the atmospheric pressure,

  • the bubbles form and boiling occurs.

  • A solution's vapor pressure is lower than that of pure solvent,

  • so it must be heated to an even higher temperature

  • before it can match the strength of the atmosphere.

  • As an added bonus, the pressure in the radiator

  • is kept above atmospheric pressure,

  • raising the boiling point by another 25 degrees Celsius.

  • The solution commonly used for a car's cooling system

  • is a 50/50 mixture of ethylene glycol and water,

  • which freezes at -37 degrees Celsius and boils at 106 degrees Celsius.

  • At the highest recommended proportion of 70 to 30,

  • the freezing point is even lower at -55 degrees Celsius,

  • and the boiling point rises to 113 degrees Celsius.

  • As you can see, the more ethylene glycol you add,

  • the more protection you get, so why not go even higher?

  • Well, it turns out you can have too much of a good thing

  • because at higher proportions,

  • the freezing point actually starts to go back up.

  • The properties of the solution head towards the properties of ethylene glycol,

  • which freezes at -12.9 degrees Celsius,

  • a higher temperature than we attained with the solution.

  • The solution flows through the engine, absorbing heat along the way.

  • When it reaches the radiator, it's cooled by a fan,

  • as well as air rushing through the front of the car

  • before returning to the hot engine compartment.

  • So, an effective and safe engine coolant

  • must have a high specific heat, a low freezing point, and a high boiling point.

  • But instead of searching all over the world for the perfect liquid to solve our problem,

  • we can create our own solution.

There are over one billion cars in the world today,

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B1 US TED-Ed boiling point celsius boiling liquid solution

【TED-Ed】Under the hood: The chemistry of cars - Cynthia Chubbuck

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    Chia-Lin Hsin posted on 2014/09/15
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