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Soaps are complex combinations of molecules. On the surface of the soap there are many holes and spaces between the molecules that the water can enter. There the water molecules come in and probably penetrate many layers deep into the soap, and stay there. Thus the soap expands and becomes bigger. This happens most in pure water and then less in liquids which not like water. The reason for the difference is that inside the soap there are both "hydrophilic" (water liking) and "hydrophobic" (water hating) portions of the molecules. But importantly, the hydrophilic portions are on the outside of the molecules so water can get close to the molecules. So the more like water the liquid is, the more the water can get close to the molecules and stay with them (be absorbed into the soap). When the water has other particles dissolved in it, like iced tea, Sprite or salt water, the water molecules can't get as close to the hydrophilic portions. And of course the oil molecules are repelled by the hydrophilic portions of the soap. So the answer of why soap absorbs the water is that the water molecules penetrate the soap and stay with the hydrophilic portions of the soap molecules. This happens more when the liquid is more like water.

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โˆ™ 2009-03-28 16:47:00
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Q: Why soap absorb water?
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