Frozen car windows

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When the child of morning, rosy-fingered Dawn, appeared… the windscreen of our car was frozen. This is a problem that did not occur in Homer‘s time but it does in ours. It happens when dew droplets freeze together in the early hours of the morning, thus delaying your morning commute. Luckily though, while physics may have gotten you into this mess, it can also get you out of it. Here are three branches of science to help you get that ice off and get going.

The gut-wrenching feeling when discovering your windshield is frozen is like being the conductive filament in a plasma pinch.

1. Use thermodynamics

The first thing that you could do is to get your engine running and let your heating system start warming up the windscreen. The heat transfer will melt the ice, albeit very slowly as the windscreen itself absorbs the heat first. A shortcut is to throw some hot water on the iced window. However, besides glass being able to crack under sudden thermal expansion, hot water also freezes more quickly than cold water. So, your cracked windscreen may freeze up again afterwards. However, the glass should be safe if the layer of ice is thick enough to absorb most of the heat.

2. Use Newtonian mechanics

Another approach is to get yourself a little commercial ice scraper and physically force the ice off the window. What I recommend though is to scale up this idea by using any large rigid piece of plastic to scrape clean the window with. The larger the better! The plastic won’t scratch the glass, just like that of the commercial scraper doesn’t. When you’re done, throw that piece of plastic in the boot and off you go. As they say, go big or go home.

3. Use chemistry (and thermodynamics)

Finally, you could fill a spray bottle with an ethanol-water mixture (although any other type of alcohol should also work). If you spray this mixture onto the windscreen, it helps to melt the ice. While waiting for the ice to melt this way, you could consider some of the underlying physical principles as follows.

The melting point of ethanol is -114 degrees Celsius. This means that it must be that cold before ethanol freezes. When spraying room-temperature (20 degrees Celsius) ethanol onto ice, this can melt some ice via heat transfer from the warm ethanol into the cold ice. Any molten ice (water) mixes with the ethanol and thereby obtains a lower freezing point. This means that this new mixture cannot refreeze. This even works when the ice and ethanol are in thermal equilibrium at zero degrees Celsius (the melting point of ice). At that temperature, ice and water co-exist so that A) ice can melt and B) water can freeze. The above mixing effect cuts out the re-freezing while keeping the melting. Pretty cool eh?