One of the best protections we have against SARS-CoV-2, the coronavirus that causes the severe respiratory disease Covid-19, is to wash our hands, well and often.

That's because when it comes to virus-busting, soap is an oldie but a goodie. And it turns out that soap is particularly effective against coronaviruses.

"We've got to find relatively benign molecules that are deadly to viruses - and of everything that is out there soap is a damn good one," says chemist Professor Allan Blackman, from the Auckland University of Technology.


Know your enemy

"A virus is a very intriguing package of nucleic acid that is wrapped up in a protein exterior and then, in some cases, surrounded by a lipid – or a fat - envelope."

That's according to University of Otago virus expert Professor Kurt Krause, who says the lipid envelope is important for maintaining the integrity of viruses such as those in the coronavirus group.

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In the case of a coronavirus, however, that fatty envelope is both a strength – and a weakness.

The reason why comes down to chemistry.

The chemistry of fat and water
Grease and water, as we all know, don't mix. That's because water is a polar molecule, while grease or oil is a nonpolar molecule.

"What we mean by polar is that there is some separation of charge," says Blackman. "One end of the molecule is slightly positively charged and the other is negatively charged."

So a polar molecule such as water has both a positive and a negative end, just as a battery does, while a nonpolar molecule has its charge evenly spread.


"And there's a rule in chemistry," says Blackman, "that 'like dissolves like'. Polar molecules like other polar molecules … so polar molecules will dissolve quite happily in water."

You can happily stir sugar into your tea, for example, because both water and sugar are polar molecules.

Try and wash anything oily or greasy off your hands with just water, however, and it won't work. The nonpolar oil is insoluble in water and much prefers sticking to your slightly greasy skin.

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Add soap, however, and the oil washes away.

That's down to the unique molecular structure of soap, and it works just as well against a virus protected by a fatty envelope as it does against any greasy object.