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Soap

Watch this to unpack the science of soap for Higher Chemistry (and why handwashing actually works)

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If there is one thing I took from the coronavirus global pandemic it was a new view on handwashing.

Our lives changed completely for months, with lockdowns, face masks, one-way systems…but it was the frequent handwashing that made me realise how little I did it before Covid. How disgusting.

It now seems that our new more frequent hand-washing may be even reducing the number of colds and the spread of influenza we see.

But why is that? What are we doing when we wash our hands that could stop the spread, kill the virus.

It’s that five thousand years old chemical that can get rid of grease, stains, muck or even nasty microorganisms.

Let’s clean up this part of Higher chemistry.

This is thinkfour.

Soaps have been an essential part of societies all over the globe and across the ages to keep utensils, clothing, surfaces basically anything clean.

Soaps are basically salts produced from the alkaline hydrolysis of edible fats and oils. This reaction means the addition of a hydroxide ion to the esters present in fats and oils. The OH- will come from an alkali such as Sodium Hydroxide (NaOH). Natural oils found in the surrounding nature were historically used. Savon de Marseille (Marseille Soap) in the south of France is produced using the abundant regional production of olive oil.

The products of this reaction will be one Glycerol and three fatty acid molecules. These last ones will be charged negatively and neutralised by the alkali present in solution, Sodium in our example, to form water soluble ionic salts that we call soap.

Their odd shape with an “head” and a long “tail” gives to soaps their amazing properties:
- They will be able to dissolve in water at their charged ionic “head” making them hydrophilic
- And they will also be able to dissolve non-polar substances thanks to their non-polar “tail” making they hydrophobic as well.

Hydrophilic at one end, hydrophobic at the other, this duality will allow soaps to dissolve oils, grease, fats with their tails while their heads stay in water. A little bit of shaking or brushing or tumbling and little spheres of grease surrounded by soap molecules will be able to detach. Because of the negatively charged head on the outside of this tiny sphere, they will repel each other and stay suspended in the water.

Bam! Washed!!

Ah! That would be too easy, water is rarely just water. There are always metal ions like Calcium, Magnesium, etc dissolved in tap water.

Check for yourself. Have a look at the back of any water bottle and you’ll see a list of ions present. When there is a high concentration of metal ions, we say the water is “hard”. In this situation, soap and these dissolved metals will be able to form an insoluble precipitate called scum. Have you ever wondered why it was easier or harder to wash your hair in different parts of the country (Water is especially hard in the south east of England).

But do not despair Londonians, there are also soap-less detergents. Like regular soap, they have a non-polar hydrophobic tails and ionic hydrophilic heads. They work the same way to remove grease and oils, but they are not fatty acids, and they have the advantage of not forming scum in hard water.

And of course, both types of soap will help solve our virus problem. The membrane of a virus is called a lipid membrane. Lipid means fat. So, soap will annihilate the virus’ membrane like it was a simple stain to eliminate.

So now you know everything about soap.

There is a good reason we have been using it for so long and why it is far preferable to antibacterial wipes and gels.

Knowing about soap will also hopefully help to give you a clean bill of health when it comes to Higher Chemistry.

This was Think Four. Thanks for watching.

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