How Does Soap Work Chemically?
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Soap Basics
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Even when they're in the bathtub or shower, most people don't take the time to stop and think about how the soap they're using actually works. Many people have never given a second thought, as long as it lathers properly and they get clean. Soap itself is a complex chemical combination that works in a number of different ways. Whether it's scented or unscented, soap all works in the same basic way.
How Does it Clean?
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Soap is a chemical compound consisting primarily of fatty acids. Soap acts as an emulsifying agent, causing otherwise incompatible materials to join and be washed away. Dirt and oil on a person's skin is typically unable to be washed away by water alone. Because the oils aren't readily able to combine with plain water, the water washes away and leaves the dirt and oils behind. When soap is applied to the skin, however, the potassium or sodium fatty acids within the soap combine and latch onto the dirt and oil molecules that are stuck to the skin. The soap has an organic component which is negatively charged and hydrophilic. That means that when the soap binds with the dirt molecules and lifts them off the skin, the water used to rinse the soap also carries away the dirt and oils as well. The micelles within the soap are hydrophobic, meaning that they aren't readily dispersed in water. That's why when soap is dissolved into bathwater, it doesn't sink to the bottom or float to the top. The water itself actually holds the soap, oil and dirt mixture in suspension until it's drained away. The soap molecules are slightly acidic and work best in water with a neutral PH level. Typically, tap water contains chlorine, which helps ensure that the soap acts as desired.
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What Leaves Soap Scum and Tub Rings?
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Many of the negatively charged particles within the soap aren't readily soluble in some acidic water. That's why, after a bath or shower, you often see a ring around your bathtub. These soap molecules still have dirt and oils attached to them and cling readily to the bath or shower wall. The more acidic your water, the more likely you are to see a noticeable ring form. You can have your water tested either by putting some in a lidded jar and taking it to your local pool supply store or by purchasing a home testing kit. They're inexpensive and can help you determine which soaps will work best with your water's chemistry.
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