A Universal Solvent

A Universal Solvent - A Solvent is any substance in which another substance (called a solute) can dissolve. Water's polar nature makes it an excellent solvent. As we discussed earlier, because there is an unequal distribution of electrons between the atoms of the molecule, the atom of oxygen behaves as if it had a negative charge (or as if it were electronegative). The hydrogen regions behave as if they had positive charges (or as if they were electropositive). When a polar molecule is placed with other polar substances, there is an electrical interaction and the molecules are dispersed spread out. In other words, they dissolve.

A Universal Solvent

		Polar molecules that form weak hydrogen bonds with water molecules and thus dissolve are said to be hydrophilic (water-loving). Some molecules are not polar, however, so they do not dissolve in water. Instead they are usually insoluble in water, so they are called hydrophobic, or water-fearing. For example, if you mix oil and water, the oil (a liquid fat) is nonpolar, so it remains in the form of droplets in the water (Figure 2.14). The oil and water separate. That is why you need to remix your salad dressing every so often. As we will see when we discuss cell membranes, the hydrophobic and hydrophilic properties of molecules are very important in the movement of molecules through membranes.

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