How does soap actually work? The science behind soap, soft soap, and handwashing
Sara IdenforsShort answer
Soapa and soap are based on the same fundamental chemistry. Both contain fatty acid salts that act as surfactants—substances that bind to both water and grease. When hands or surfaces are washed, grease, dirt, and many microorganisms are loosened from the surface and rinsed away with running water.
That is why international public health agencies such as the CDC recommend washing hands with soap and water as the first choice in most everyday situations. When soap and water are not available, alcohol-based hand sanitizer is recommended as an alternative.
Soapa and soap—the same chemistry
Although the words are often used as though they referred to different types of products, they are based on the same chemical principle. (Sweden is one of the few countries with its own word for Soapa; many languages use the same word for all soap.)
Both soap and Soapa are formed through saponification, in which natural fats or oils react with a strong base to form fatty acid salts. These fatty acid salts are surfactants—the substances that make cleaning possible.
The main difference is which base is used during manufacturing:
| Bar soap | Soapa |
|---|---|
| Sodium hydroxide (NaOH) | Potassium hydroxide (KOH) |
| Sodium salts of fatty acids | Potassium salts of fatty acids |
| Usually solid | Usually liquid or soft |
Despite these differences, both work according to the same basic cleaning mechanism.
Why Soapa and soap clean
A surfactant molecule has two different ends.
- One part is attracted to water (hydrophilic).
- One part is attracted to grease (hydrophobic).
When soap or Soapa is worked into water, small structures called micelles form. The hydrophobic parts surround grease, oils, and other dirt, while the hydrophilic parts face outward toward the water.
The CDC explains that these micelles trap dirt, chemicals, and microorganisms. When the hands are then rinsed under running water, the contents are carried away. It is therefore the combination of surfactants, mechanical action, and rinsing that produces the cleaning effect.
Why soap and water are recommended
During the COVID-19 pandemic, many public health agencies emphasized the importance of handwashing.
The CDC currently states:
"Washing hands with soap and water is the best way to get rid of germs in most situations."
The CDC also explains why:
- soap and water remove dirt and microorganisms from the skin,
- hand sanitizer is a good alternative when soap and water are unavailable,
- but handwashing works better when hands are visibly dirty or greasy.
It is important to distinguish between removing microorganisms and killing them. Soap’s primary function is to help loosen and carry dirt and microorganisms away from the skin together with the water.
How does this relate to Soapa’s bar Soapa?
International research explains how soap and Soapa clean.
RISE, on the other hand, investigated a different question:
Can bacteria grow in the bar Soapa between uses?
To answer that question, a microbiological laboratory test was conducted in which the bacterium Escherichia coli was added to diluted bar Soapa.
After seven days, the bacterial count had decreased from approximately 1,700 bacteria per milliliter to less than 1 bacterium per milliliter. RISE concluded that the tested bar Soapa did not support bacterial growth under the conditions studied and assessed that the product’s high pH and water activity likely contributed to the result.
Two different scientific questions
It is important to distinguish between two different types of research.
Question 1—How do Soapa and soap clean?
There is extensive international research and guidance from organizations including the CDC describing how surfactants, mechanical action, and rinsing together remove dirt and microorganisms from the skin.
Question 2—Can bacteria grow in the product itself?
That question is answered by RISE’s laboratory test of VästerbottensSåpa’s bar Soapa, in which the bacterium tested, E. coli, did not grow under the conditions tested.
Conclusion
Soapa and soap are based on the same fundamental chemistry. Both contain surfactants that bind to both water and grease, allowing dirt and many microorganisms to be loosened and rinsed away with water. That is why international public health agencies recommend washing hands with soap and water as the first choice in most everyday situations. VästerbottensSåpa’s own RISE report complements this knowledge by showing that the tested bar Soapa, under the laboratory conditions studied, did not support the growth of the bacterium tested, E. coli.
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