Why Do Some Things Dissolve in Water While Others Do Not?

On the average, adults will maintain about 60 percent of their body weight as water. Since water is the predominant substance in the body, it is important to understand how other substances interact with it. What
we are really talking about is a substance’s ability or inability to dissolve into water.
 

If a substance dissolves easily into water it is said to be water soluble. On the other hand, if a substance does not dissolve into water it is said to be water insoluble. As a general rule, water-insoluble substances will dissolve in lipid substances, such as oil (fat). Therefore, we can call these substances either water insoluble, lipid soluble, or fat soluble.
 

Examples of water insolubility are often obvious. Some of us have been frustrated by the inability of traditional salad dressings, such as vinegar (water-based) and oil, to stay together and not separate into two layers. Meanwhile, others have witnessed oil tanker spills whereby the oil does not dissolve into the body of water but rather forms a layer on top of the water, posing a threat to the aquatic life. As with many water-insoluble substances, the oil from the tanker or in the salad dressing is less dense than water, allowing it to float on top of the water or water-based fluid.

Some elements and molecules easily dissolve in water while others (for example, lipids) do not.
 

The key to understanding water solubility requires a closer look at the bonds between hydrogen and oxygen atoms in a water molecule. As Figure 1.8 shows, two hydrogen atoms share electrons with one oxygen atom. Hydrogen atoms are the smallest atom (element) and contain only one proton (positive charge); meanwhile the larger oxygen atom has eight protons. As a result, oxygen tends to pull the shared electrons (negative charge) in the bond closer to it because it has a greater positive charge in its nucleus. This leads to a partial negative charge associated with oxygen atoms and a partial positive charge associated with hydrogen atoms. It is an electron tug-of-war, with hydrogen atoms having a weaker pulling force. It is important to see that even though the electrons in the bond spend more time closer to oxygen, they still some spend time closer to hydrogen. So, the charge associated with hydrogen and oxygen is not a full charge, but partial charges. This is like having extra money 25 percent of the time and owing money the remaining 75 percent of the time or vice versa. Partial charge will be displayed with the Greek lowercase letter delta in superscript (δ+ or δ−).
 

The partial charges associated with hydrogen and oxygen in a water molecule allows it to be somewhat electrical. And, partially charged water molecule atoms can then interact with other water molecules because of opposite charge attraction as displayed in Figure 1.8. This is the glue that holds water together. This glue helps us understand how you can fill a glass up with water and briefly exceed the rim of the glass before the water begins to spill over. The water molecules at the top of the

glass are attracted to the other water molecules beneath them and they “hold on” electrically, which keeps the too-full glass from overflowing, to a point.
 

Since atoms in a water molecule bear partial charges it only makes sense that they can interact with other substances that have a charge. This includes sodium (Na+), potassium (K+), and chloride (Cl−). When these
atoms (and other charged chemicals) are dissolved in water, the resulting fluid becomes even more electrical and can carry an electric current. This is why scientists often refer to charged atoms and some molecules as electrolytes, which means “electricity loving.” Sodium and chloride are the main electrolytes in sports drinks. These beverages are often called fluid and electrolyte replacements, because they are water based and contain electrolytes such as sodium, chloride, potassium, calcium, and magnesium.
 

Certain elements (atoms), such as sodium, can have a charge and are called electrolytes.
 

On the other hand, lipids, such as fats and cholesterol, do not have a significant charge and as a result they are water insoluble. In general, the partial charges of water atoms do not find lipid molecules electrically attractive. Therefore, the two substances do not mix. Or, from another perspective, the partial charges of water molecules are more attracted to water and other charged substances and as a result lipid substances get pushed aside.
 

Since lipid molecules fail to dissolve into water, they tend to clump together. As mentioned previously, because lipids are generally less dense than water, they tend to sit on top of water. This explains why some salad dressings separate with the oil on top. It also explains why oil spills lay on top of water and can be cleaned up by using a corralling device called a boom