Stupendous Suds

When I was a child, washing my hands took three long, extremely boring minutes. To make it more fun, I would rub my soapy hands together. I’d make an “O” with my thumb touching my index and middle fingers, creating a thin film of soap. I’d blow into that film I and a beautiful soap bubble would expand out. I’d try to make bigger and bigger bubbles every time.

I would play with soap several times a day. But I had no idea that I was playing with the product of a complex chemical reaction!

Did you know? 

Soap has been traced back as far as early Roman times. Back then, people made soap using a mix of water, ashes and animal fat.

Soaps may seem magical. The truth is, they’re so good at cleaning because they are surfactants and emulsifiers. As surfactants, soaps can wet surfaces easily because they reduce the surface tension of water. More on that in a minute. As emulsifiers, soaps can suspend oil or dirt so that it can be removed.

Let’s learn more about these processes in detail.

Did you know? 

You probably think of soap as something to clean with. But in the past, people used soap for all kinds of things - even as medicine!
 

Why does soap clean effectively?

In order to clean something, water needs to spread and dampen surfaces quickly. It can’t always easily do this on its own. That’s because it has a high surface tension. This means the molecules in the water are strongly attracted to each other, which prevents it from easily spreading. Lowering its surface tension helps water spread.

Substances that can lower surface tension are called surfactants (short for surface active agents). Soap is a type of surfactant called an emulsifier. Emulsifiers can cause two liquids to mix even if they normally wouldn’t. For example, think of oil and water. They don’t naturally mix. But since soap is an emulsifier, it can hold the oil in such a way that water can remove it. 

Did you know? 

You might think of an acid as dangerous and corrosive, and a base as something that cancels it out. But in the case of soap it’s the base (like lye) that needs to be handled with care.

One end of a soap molecule is hydrophilic - that is, it mixes with water. It holds onto compounds that can dissolve in water. The other end is hydrophobic - that is, it does not mix with water. It holds onto compounds that do not dissolve in water, such as oil. 

Both ends work together to hold all components in the mixture together, forming an emulsion that can easily be removed from a surface. This mixing is also what decreases the surface tension of water. The exact explanation is a bit complex, but a simple way to think of it is this: as the hydrophilic ends of the soap molecule get in between the water molecules, it interferes with the attractive forces between the water molecules.

So the next time you wash your hands, you’ll know that underneath those bubbles, some complex chemical reactions are taking place!

Did you know? 

Sodium-based soap is often called “hard soap,” and is often used as laundry soap. Potassium-based soap, a more water-soluble soap, is often called “soft soap.” It’s commonly used in shaving products.