How Does Soap Clean Dishes?

Water and Oil

If all foods would just dissolve in water, there would be no need for soap to clean dishes. Unfortunately, many foods contain fats, oils and grease, and no amount of rinsing will clean those sorts of things off dishes.

Chemistry

The problem lies in the chemistry of water. Water is made up of molecules that consist of two atoms of hydrogen and one atom of oxygen. The atoms of hydrogen are both located on the same side of the oxygen atom, and have a small positive electrical charge. The oxygen atom has a slight negative charge. This kind of molecule is said to be "polar," meaning that it has a positive pole and a negative pole. Polar substances like water are very good at dissolving other polar substances, like salt. They are not very good at dissolving non-polar substances, like fats.

One Solution

One solution would be to use another solvent, such as gasoline or turpentine. That would clean the grease and oil from the dishes, but most people would object to having the taste of turpentine in their pasta.

A Better Solution

A better solution to the problem is soap. Soap is made up of long chain molecules, which are lipophilic (love fats) on one end, and hydrophilic (love water) on the other. The lipophilic end behaves like a non-polar substance, and the hydrophilic end behaves like a polar one. One end lifts the grease and oils from the dishes, water sticks to the other, and the whole thing rinses cleanly down the drain.

In its simplest form, soap is made from fat and an alkali. Our ancestors obtained the fat from pigs, and the alkali from water that was strained through wood ashes. Wood ashes contain potassium oxide, which becomes potassium hydroxide ("lye") when mixed with water (thus "lye soap"). When the alkali and fat are heated and stirred together, the alkali neutralizes the fatty acids in a process called saponification.

Still Another Choice

Soaps are not always the final solution. "Hard" water contains calcium and magnesium ions (charged particles), which can replace the potassium ions in the soap, creating new chemical compounds that water cannot dissolve. For this reason, chemists have developed another class of compounds called detergents, which clean dishes better in hard water.

References

• Naturally Pure Alternatives