Polar solvents have molecules with a slight electrical charge due to their shape, allowing them to dissolve polar materials like salt and sugar. Non-polar materials do not dissolve in polar solvents, but surfactants can be used to create stable emulsions of polar and non-polar materials. Dielectric constant and polarity index are used to classify polar solvents.
A polar solvent is a liquid with molecules that have a slight electrical charge due to their shape. For example, water is a molecule with one oxygen atom and two hydrogen atoms. The two hydrogen atoms are not on opposite sides of the oxygen, but rather at an angle. This creates a slight imbalance of electrical charge in the water molecule, also known as polarity.
When a solid molecule is placed in a polar solvent, it can dissolve if it has its own polarity. This occurs because the molecules of the solid are attracted to the weak electrical charges of the solvent. Examples of polar materials include salt and sugar, both of which dissolve easily in water, Earth’s most common polar solvent.
A polar solvent does not normally dissolve non-polar materials or vice versa. Salt and sugar don’t dissolve in most organic solvents, because there’s no electric charge to attract the molecules. The term “similar prefers similar” is often used to indicate the preference of polar materials for polar solvents and, likewise, for non-polar materials. There are some rare exceptions to this rule, because non-polar solvents can have slight electrical charges that can mimic polar ones.
When a solid is placed in a solvent and dissolves, the solid molecules are dispersed or evenly distributed in the solvent. This dissolved mixture will remain stable as long as enough solvent remains to surround the solid molecules. Precipitation, or separation of the solid from the mixture, can occur if more solid is present than can be dissolved. These solutions are called saturated, and temperature changes can cause solids to precipitate from the dissolved mixture.
A polar solvent is often classified by stating its dielectric constant or polarity index. The dielectric constant is a measure of the electrical properties of a solvent in a sample relative to an empty capacitor, which holds the material while electric current passes through it. Polar index is a relative measure of a solvent’s ability to dissolve various standard polar materials. In both tests, the measured constant or index is entered into a table of common solvents, which can be used to identify solvents for chemical processes.
Another type of solvent, a surfactant, can be used to make mixtures of polar and non-polar materials. Surfactants are polar and non-polar molecules at each end. These materials will create molecular bonds of the polar end with similar molecules and likewise with the non-polar end.
An example of this effect is hand cream. Water and oily moisturizers don’t normally mix and, when agitated, eventually separate. The addition of a surfactant causes the two insoluble materials to form a stable emulsion. Oil and water do not dissolve, because they are not soluble in each other, being polar and non-polar. A surfactant bonds the two materials and they remain stable emulsions for long periods.
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