1 The Nature of Molecules Chapter 3

2 Water Chemistry Water consists of an oxygen atom bound to two hydrogen atoms by two single covalent bonds. – stable atom with no electrical charge most outstanding property is its ability to form weak chemical associations

3 Hydrogen Bonding In a water molecule, both the oxygen and hydrogen atoms attract the shared electrons in the covalent bond – Oxygen atom is more negative than the hydrogen atoms. distinct ends with partial charge (polar) hydrogen bonds

4 Water Chemistry Water clings to polar molecules. – Cohesion refers to attraction to other water molecules. responsible for surface tension: water strider leaves can pull water up the stem Seeds swell to germinate

5 – Adhesion refers to attraction to other substances. Water is adhesive to any other substance with which it can form hydrogen bonds. Capillary action on glass

6 Water Chemistry Water is an effective solvent as it can form hydrogen bonds. – Water clings to polar molecules causing them to be soluble in water. Hydrophilic - attracted to water( sugar/salt) – Water tends to exclude nonpolar molecules.

7 Water molecules Salt crystal Na + Cl – Na + Cl – Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

8 Hydrophobic - repelled by water- form their own layer Water and oil do not mix !

9 pH pH scale expresses hydrogen ion (H + ) concentration in a solution. – logarithmic scale ranging from 0-14 neutral = 7

10 pH Acids dissociate in water to increase the concentration of H +. – pH values lower than 7 Bases combine with H + ions when dissolved in water, thus decreasing H + concentration. – pH values above 7

11 pH Neutral More H ion Less H ion OH

12 How Acid and bases affect the homeostasis of our bodies CO 2 is acidic: high levels in blood will send message back to brain to breathe faster: remove CO2 and return to normal pH `