The water molecule has many special properties that make life possible on this planet. It has a high heat index, high surface tension, and is often called the universal solvent. There are plenty of things that will dissolve in water. Most salts and sugars do. However, as anyone who has had to wash greasy dishes knows, oil and water do not mix. There's a reason for that. Chemistry can explain why oil won't dissolve in water.
Water is a Polar Molecule
Water is a polar molecule and this is a key reason for water and oil not mixing. Polarity means one end is positively charged while the other end is negatively charged. Dr. Kent Simmons explains the water molecule on his webpage “Polar Bonds of Water”. The water molecule is made of two hydrogen atoms and one oxygen, but the atoms are not arranged in a neat line. Rather, the two hydrogen atoms cling to one side of the oxygen, looking similar to a Mickey Mouse head. The electrons in the molecule spend more time on the oxygen side of the molecule, giving this side a negative charge and the hydrogen side a positive charge.
The Importance of Water Polarity in Dissolving Oil
Water is an excellent solvent because of its polarity. However, as the University of Southern Maine explains on its “Solvents” page, in general only other polar molecules will dissolve in water. They call this the “Like dissolves like” adage. Polar molecules will dissolve in polar solvents and non-polar molecules will dissolve in non-polar solvents. Estrella Mountain Community College offers a picture of a polar salt molecule dissolving in water on their page “Chemistry II: Water and Organic Molecules”. The salt, NaCl (sodium chloride), is attracted to the water molecules so that the positive Na+ ion (sodium) is pulled by the negative side of a water molecule while the negative Cl- ion (chlorine) is pulled by the positive side of a water molecule.
Oil Does Not Mix With Water Because of Polarity
Oil and water don’t mix because oil is made up of non-polar molecules while water molecules are polar. Estrella Mountain Community College explains how water molecules are attracted to other water molecules and thus exclude the oil molecules which are not electrically charged. This exclusion causes the oil molecules, or lipids, to clump together. Ultimately, the fact that water and oil don’t mix is due to the polarity of the molecules. This affects people’s daily life and is the reason people use soaps and shampoos when cleaning.
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