Dry cleaning chemistry. And some revision along the way….

Well, I just picked up my dry cleaning in time for the races…

It gave me some inspiration for this blog post – there is plenty of chemistry involved.

Dry cleaning isn’t ‘dry’ as such, a liquid is still used in the cleaning process.  The word ‘dry’ is used to indicate that water isn’t used.

So, using what we know already:

Polar substances are soluble in other polar substances.  That is, polar substances will dissolve in water.  We normally wash in water, so any polar substances should be removed.  Sound reasonable so far?

In the dry cleaning process, a chemical called perchloroethylene is often used.

(Can you give the systematic name for this chemical?)

The next part of your revision… Can you describe the polarity of this molecule?

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Well, you might recall the two conditions necessary for a molecule to be polar.

1. Polar bonds must be present

2. The poles created by the polar bonds must be arranged in such a way that they don’t cancel each other out.  That is, the molecule must not have multiple axes of symmetry.

So, back to perchloroethylene. There are polar bonds present, as there is a difference in electronegativity between the chlorine atom and the carbon atom. The shape of the molecule is planar, and there are multiple axes of symmetry.  This in effect cancels out the sight positive charges and slight negative charges (dipoles), resulting in a non-polar molecule. (Or, the net vector sum of the dipoles is zero, as Reece might prefer to say).

In summary, perchloroethylene is non-polar.

So what does this mean in regards to the dry cleaning (that I’m sure you think I’ve forgotten about…)?

Just as polar substances will dissolve in other polar substances, non-polar substances will also dissolve in other non-polar substances.

That is, non-polar substances will dissolve in perchloroethylene.

Which is really handy, as a number of stains in clothing come from grease and other substances that are organic (and therefore largely non-polar).

To watch more about how dry cleaning works, check out this video (that I couldn’t embed for you).

We will also learn more about other cleaning agents and how they work later in the year.

But if you’re interested in some more related reading, check out the following sites:

How Stuff Works – The Chemistry of Cleaning Clothes

RSC – Dry Cleaning and Green Chemistry

PS.  The systematic name of perchloroethylene? Tetrachloroethene. (Don’t be tricked, it’s not going to be 1,1,2,2-tetrachloroethene… There is only one possible way to get tetrachloroethene, so we won’t need the numbers in this case).

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