O

(a) Electron cloud model

(b) Space-filling model figure 2-8

The oxygen region of the water molecule is weakly negative, and the hydrogen regions are weakly positive. Notice the different ways to represent water, H2O. You are familiar with the electron cloud model (a). The spacefilling model (b) shows the three-dimensional structure of a molecule.

The positive region of a water molecule attracts the negative region of an ionic compound, such as the Cl~ portion of NaCl. Similarly, the negative region of the water molecule attracts the positive region of the compound—the Na+ portion of NaCl. As a result, NaCl breaks apart, or dissolves, in water.

Solubility of Water

The polar nature of water allows it to dissolve polar substances, such as sugars, ionic compounds, and some proteins. Water does not dissolve nonpolar substances, such as oil because a weaker attraction exists between polar and nonpolar molecules than between two polar molecules. Figure 2-9 shows how water dissolves the ionic compound sodium chloride, NaCl. In your body, ions, such as sodium and chloride, are essential to bodily functions, such as muscle contraction and transmission of impulses in the nervous system. In fact, dissolved, or dissociated ions, are present in all of the aqueous solutions found in living things and are important in maintaining normal body functions.

figure 2-10

The dotted lines in this figure represent hydrogen bonds. A hydrogen bond is a force of attraction between a hydrogen atom in one molecule and a negatively charged region or atom in a second molecule.

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Hydrogen H Jv bond

Hydrogen H Jv bond

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