Educational Resources Lets Talk Science Challenge participants

Soapy hands

A person washes hands with soap (jacqueline macou, Pixabay)

STEM in Context

Stupendous Suds

Soumya Satheesan

Summary

Every time you wash your hands with soap, there are complex chemical reactions taking place.

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!
 

What is Soap?  

The basic ingredients of any soap are a fat (which contains a fatty acid) and an alkali (chemically a base) which is usually lye or potassium hydroxide. When a base and acid meet, a neutralization reaction takes place. This reaction forms the salt (soap) plus water. So all soaps are actually salts! 

The neutralisation process that forms soap is called saponification. To make soap, the base has to have one hydroxide ion (like lye or potassium hydroxide). The acid needs to be a triglyceride (three fatty acids, attached to a single molecule of glycerol). Typically, the triglyceride is in the form of animal fat, or vegetable fat such as olive oil. Two reactions occur. First, the glycerol turns into glycerin, which is a humectant. Then the base and acid form the soap. 

Soap manufacturers add various ingredients to make the soap smell, feel, look, and function better. For example, they add perfumes and fragrances to the soap mixture to make the soap smell fresh. They also add abrasives, such as talc or pumice (volcanic ash), to give it a nice texture. Then they add dyes to make the soap look nice.

Some manufacturers may also add enzymes to help the soap remove biological stains such as grass stains or blood from your clothes.

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.

High (A), moderate (B), low (C) and very low (D) surface tension
High (A), moderate (B), low (C) and very low (D) surface tension (Source: MesserWoland [CC BY-SA 3.0] via Wikimedia Commons).

 

Illustration - Text Version

A: High surface tension means the surface is not wet. B: Moderate surface tension means that the surface is somewhat wet C: Low surface tension means that the surface is fairly wet. D: Very low surface tension means that the surface is very wet.

 

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. 

Soap molecules in water
Each soap molecule has a hydrophilic end and a hydrophobic end. When mixed with water and oil, small droplets form that allow the oil to be removed from the water (© 2019 Let’s Talk Science).

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.

Starting Points

Connecting and Relating
  • What soaps or soap products do you use regularly in your daily life? 
  • Have you ever taken a bath and noticed a ring of soap scum around the bathtub when you were finished? Do you wonder why the soap always collects there? 
  • Do prefer a certain type of soap? What do you like about it? 
     
Connecting and Relating
  • What soaps or soap products do you use regularly in your daily life? 
  • Have you ever taken a bath and noticed a ring of soap scum around the bathtub when you were finished? Do you wonder why the soap always collects there? 
  • Do prefer a certain type of soap? What do you like about it? 
     
Relating Science and Technology to Society and the Environment
  • Soaps come in many different forms. What are some of the different forms of soap? Why do you think there are so many types?
  • Are you aware of any environmental issues regarding soaps? What ingredients in soaps are potentially harmful for the environment? 
  • Many companies now sell eco-friendly soaps. What are “environmentally friendly” soaps? 
     
Relating Science and Technology to Society and the Environment
  • Soaps come in many different forms. What are some of the different forms of soap? Why do you think there are so many types?
  • Are you aware of any environmental issues regarding soaps? What ingredients in soaps are potentially harmful for the environment? 
  • Many companies now sell eco-friendly soaps. What are “environmentally friendly” soaps? 
     
Exploring Concepts
  • What is soap? List the main ingredients in any soap.
  • What is saponification? Write out the chemical equation for saponification.
  • Distinguish between the terms hydrophilic and hydrophobic.
  • What is an emulsion? Provide some examples of emulsions.
     
Exploring Concepts
  • What is soap? List the main ingredients in any soap.
  • What is saponification? Write out the chemical equation for saponification.
  • Distinguish between the terms hydrophilic and hydrophobic.
  • What is an emulsion? Provide some examples of emulsions.
     
Nature of Science/Nature of Technology
  • People have been making soap since 2 800 BCE. How has the science of chemistry changed the process of soap-making? 
Nature of Science/Nature of Technology
  • People have been making soap since 2 800 BCE. How has the science of chemistry changed the process of soap-making? 
Media Literacy
  • Laundry soap manufacturing is a very competitive business. Why do you think this is the case? How do laundry soap companies distinguish their soap products from the competition?
  • Do you ever watch soap operas on television? Can you name any current soap operas? How did these television programs come to be called soap operas?
     
Media Literacy
  • Laundry soap manufacturing is a very competitive business. Why do you think this is the case? How do laundry soap companies distinguish their soap products from the competition?
  • Do you ever watch soap operas on television? Can you name any current soap operas? How did these television programs come to be called soap operas?
     
Teaching Suggestions
  • This article can be used in Chemistry for teaching and learning related to organic chemistry, decomposition reactions and molecular bonding. Concepts introduced include surfactants, emulsifiers, saponification, enzymes and surface tension.
  • After reading the article, teachers could have students do a Concept Definition Web learning strategy to consolidate their understanding of a key concept introduced in the article (e.g., saponification, surface tension, emulsion). Download ready-to-use Concept Definition Web reproducibles in [Google doc] or[PDF] formats.
Teaching Suggestions
  • This article can be used in Chemistry for teaching and learning related to organic chemistry, decomposition reactions and molecular bonding. Concepts introduced include surfactants, emulsifiers, saponification, enzymes and surface tension.
  • After reading the article, teachers could have students do a Concept Definition Web learning strategy to consolidate their understanding of a key concept introduced in the article (e.g., saponification, surface tension, emulsion). Download ready-to-use Concept Definition Web reproducibles in [Google doc] or[PDF] formats.

Learn more

Dear Science: How does soap make things clean? (2017)

Article by Sarah Kaplan for the Washington Post on why soap works to make things clean.

Soap and Detergent

Article from Encyclopaedia Britannica on the history of soap, types of soap, and the uses for soap.
 

References

Amrita Online Lab. (2019). Saponification: The process of making soap Ministry of Electronics and Information Technology.

Helmenstine, A. M. (2019). What is soap made of and how does it clean? ThoughtCo.