Soap allows to clean dirt thanks to its molecules that have a part hydrophilic attracted by water and a part hydrophobic attracted by dirt and grease, thus allowing to dislodge them and carry them away during rinsing.
Soap is made up of a long chain primarily composed of carbon and hydrogen atoms (called a hydrocarbon chain) that loves grease, and an end called a polar head with charged atoms (often sodium or potassium) that loves water. This gives soap its unique characteristics: one part that clings to grease (hydrophobic), and the other part that easily bonds with water (hydrophilic). In short, it acts as an intermediary between two substances that usually do not mix. This dual chemical property is soap's main advantage for dislodging greasy dirt.
Soap is special because it has a double personality. On one side, it has a hydrophilic head (which loves water) and on the other, a hydrophobic tail (which avoids water but loves fats). When you rub soap on a surface covered in grease or dirt, the hydrophobic part clings directly to these oily and sticky areas. The soap molecules therefore "capture" the fats by enveloping them with their hydrophobic tail, while their hydrophilic head is attracted to the water around. As a result, soap acts as a sort of intermediary that helps to detach fats and dirt that are normally impossible to remove with just water.
Soap has a part that loves water (hydrophilic) and another part that prefers fats (hydrophobic). When in contact with water and dirt, several soap molecules spontaneously organize into small bubbles called micelles. They group their hydrophobic tails inside, attracting and trapping greasy dirt. On the outside, the hydrophilic heads remain in contact with the water, allowing the dirt-laden micelle to easily wash away with the rinse water. Thanks to this phenomenon, grime can no longer cling to the cleaned surface and easily disappears during rinsing.
Water has an invisible "skin" on its surface that keeps the molecules together: this is surface tension. Soap acts as a disruptor, significantly reducing this tension. As a result, water penetrates more easily into fabrics or dirt, instead of forming droplets that slide off. With reduced surface tension, it glides better, wets surfaces more effectively, and easily dislodges small particles stuck or embedded, carrying them away during rinsing. The lower the surface tension, the more water behaves like a true cleaner capable of effectively dislodging stubborn dirt.
Soap produces little lather in very hard water because the calcium and magnesium ions present react with its molecules to form an insoluble precipitate. This explains why soap seems less effective in certain areas.
Some bacteria are resistant to alcohol-based antibacterial gels, but the mechanical action of soap combined with water remains effective in eliminating most germs and viruses present on the skin.
Historically, soap was considered a valuable product and was sometimes even taxed or regulated by authorities. In the Middle Ages, only the wealthy classes could afford to use it regularly.
Solid soap generally has a lower ecological footprint than liquid soaps. In fact, it often requires less packaging and water for its production, as well as less transportation energy.
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