She Said–Mommy, how does soap work?

A while back, my kids asked me how soap cleaned, and I realized I didn’t know, so I made up the answer, “the dirt sticks to the bubbles, and then washes away.”  Feeling a little guilty, I decided to look up the correct answer, and found that I wasn’t that wrong.

After wading through much chemistry, I think I found the answer.  Basically, soap has two “hands”  one hand likes to hold hands with water (and hates oils) and is called hydrophilic (Latin for water loving)  The other “hand” likes to hold hands with oils (and hates water) , and is called hydrophobic (Latin for water hating).  Here is a diagram of a soap molecule.  The end with the purple dot loves water.  The other end hates water.

Dirt sticks to our hands one of two ways:  it enters the crevices of our hands, or sticks by moisture to our hand.  In both of these cases, a bit of water and a little scrubbing would get your hands clean.  However, if oil is present water alone won’t work.  Oil will repel water. You could stick your hand into a sink filled with alcohol, kerosene, or gasoline, and that would dissolve the oil.  Your hand would be rid of the oil, but gasoline, alcohol, and kerosene aren’t gentle or even good for our skin (never mind the smell.)  Dry-cleaners use chemicals like these to get our clothes clean.  Soap doesn’t dissolve the oils, it just sticks to the oil.   For that reason, soap is the bond/bridge/emulsifier that attaches oil and water.  The oil attaches to the soap, and then the soap attaches to the water and is rinsed off.  You could also say that soap surrounds the oils (germs attach to oils) and allows the water to wash the oils away.  Here is a diagram of all the oil loving hands surrounding an oil.  The water loving ends end up all facing outwards, then they grab water’s hand and wash away.

Technically speaking, soap isn’t a disinfectant.  Soap doesn’t kill germs.  Germs stick to oils and soap washes the oils away.  A disinfectant (like that gel hand sanitizer) kills germs, but doesn’t wash them away.

Before moving on, let me add one more thing.  Soap also causes water to flatten out (instead of being in a ball like a raindrop).  Therefore, soap actually helps water get into the creases and crevices of our hands to get the oils (with germs attached to them) out.

So now you can answer the question, “Mommy, why do I have to wash my hands with soap?”  Stay tuned for a post about soap making from my friend Lynda on the day after tomorrow.

your chemistry lesson is over,

class is dismissed,


P.S.  emulsifier = to immerse one liquid into another immiscible liquid.  immiscible = incapable of being mixed; cannot undergo mixing or blending; example: oil and water.

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1 Response to She Said–Mommy, how does soap work?

  1. Bel McCoy says:

    Just what I really wanted to know. 🙂


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