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Biology, Chemistry, Sustainability

How does soap work?

I’m purposefully choosing to write about topics that aren’t specific to the biology and epidemiology of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2, ie., the virus that causes coronavirus-19) for a few reasons. First, I typically post once or twice a week but things with this virus are changing daily so I won’t be able to keep up with updates. Second, there are already some really well done pieces about the virus. Third, there are so many other things that are connected to or highlighted by the outbreak and our responses to it that are interesting to explore. So that’s what I’m going to do.

I realized something when I consciously started washing my hands for 20 seconds. Actually, I realized two things. One, I definitely was not washing for a full 20 seconds before. Yep, I’m embarrassed to admit that (especially to myself), but I also don’t think I’m alone based on what I see in public washrooms. A 2013 study by Borchgrevink, Cha, and Kim supports my observations, finding that men are worse than women but only 67% of people washed their hands with soap and water, 23% used water only, and 10% didn’t wash their hands at all. We fared even worse on the length of time with only 5% spending more than 15 seconds. So all I can say is eww. And I swear that I now am significantly better, even going back to repeat the action when I realize that I fell back into old habits.

The second thing I realized is that I had never really thought about how soap works against germs. I have talked about soap and grease but not germs. Therefore, I’m going to briefly address grease because it gives us a good primer for understanding how soap works on germs.

Soap is made up of molecules that have two different ends. One end likes water (hydrophilic), the other end does not (hydrophobic). So soap can be attracted to water molecules on one end and attracted to water hating molecules on the other). In this way, soap is a little like the person who navigates between two people who don’t get along with each other: all three of them can hang out together as long as the mediator (the soap) is present, but remove the soap and they just don’t mix (like oil and water). When we clean our dishes the soap hangs onto both the water and the grease/oil making a mixture of the two and letting the water carry the grease down the drain (hopefully not tons of grease because we should be storing that and putting it in the garbage instead).

Now onto germs. Germs, including both bacteria and viruses, will stick to the oils on our skin, where they stay until they get access to the juicier parts on our insides via a wound or when we touch our faces. This is why soap works: it mediates between the oil on our skin and the water, pulling them into a mixture with each other and pulling the germs along for the ride. In other words, the soap doesn’t kill the germs (unless it contains additives designed for that – which, by the way, don’t make soap more effective) it just helps get them off your skin. Although I dislike the implied waste in the following analogy its a good description of what is happening. I have seen people do a picnic or a birthday party where they do the following: put down a disposable tablecloth, this is the oil your skin produces naturally. Now use all disposable dishes, these are the germs. When the event is done, fold the tablecloth around all the dishes, this is the soap attaching to the oils. And throw the whole package in the garbage, or wash the soap, water, oils, and germs down the drain.

Now, you might be asking how effective this actually is, and it’s good, but only if you do it for the right amount of time. The thing is that the right amount of time actually varies based on a whole bunch of factors so organizations like the Centers for Disease Control and prevention (CDC) recommend 15 to 30 seconds at a minimum.

And just in case you need a reminder, here are some tips for proper hand washing:

  • Wet hands then turn water off
  • Lather, rubbing palms, backs of hands, fingertips, and between fingers for 15 to 20 seconds.
  • Rinse thoroughly
  • Dry
  • Repeat often

About Tai Munro

I am passionate about making science, sustainability, and sport accessible through engaging information and activities.


3 thoughts on “How does soap work?

  1. This is a question that’s dangled in the back of my mind, too– very informative read. I’m gonna pass it along tonight.


    Posted by TheChattyIntrovert | March 17, 2020, 10:35 pm


  1. Pingback: Adopted Q #124: How does soap work? – The Chatty Introvert - March 17, 2020

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