1.1 Structure of Water and Hydrogen Bonding

A water molecule (H₂O) is one oxygen atom bonded to two hydrogen atoms by covalent bonds, which are bonds where atoms share electrons. Oxygen is much more electronegative than hydrogen, meaning it pulls shared electrons toward itself more strongly. The electrons spend more time near the oxygen, so the oxygen end gets a slight negative charge (written δ−) and each hydrogen gets a slight positive charge (δ+). A bond with this uneven sharing is a polar covalent bond.

The molecule is also bent, not straight, with an angle of about 104.5° between the two hydrogens. Because of that bend, the partial charges don't cancel out. The whole molecule has a negative side (oxygen) and a positive side (the hydrogens). That is what it means for water to be a polar molecule.

Opposite charges attract. The δ+ hydrogen of one water molecule is attracted to the δ− oxygen of a neighboring molecule. That attraction is a hydrogen bond. Each water molecule can hydrogen-bond with up to four neighbors, so liquid water is a constantly shifting web of these bonds.

Keep the two kinds of bonds straight. Covalent bonds hold the atoms inside one water molecule together and are strong. Hydrogen bonds form between molecules (or between different parts of one large molecule) and are much weaker. Individually they break and re-form constantly, but because there are so many of them, together they have a big effect.

Hydrogen bonds aren't only about water. They also hold the two strands of DNA together (1.6) and help proteins fold into their shapes (1.7).

• Cohesion: water molecules stick to each other. In plants, water is pulled up through xylem tubes as a continuous column, because when water evaporates from leaves, each molecule tugs on the one behind it.
• Adhesion: water molecules stick to other polar or charged surfaces, such as the walls of xylem cells or a glass tube. Adhesion helps water climb narrow tubes against gravity (capillary action).
• Surface tension: at the surface, water molecules hydrogen-bond to the molecules beside and below them, forming a 'skin' that resists breaking. This is why a water strider can stand on a pond.
• High specific heat: specific heat is the amount of energy needed to raise the temperature of 1 gram of a substance by 1 °C. Water's is high because added energy must first break hydrogen bonds before the molecules can move faster. So water warms and cools slowly.
• High heat of vaporization: a lot of energy is needed to turn liquid water into vapor, because every escaping molecule must break its hydrogen bonds. When water evaporates, it carries that energy away, which cools the surface it left (evaporative cooling).

Organisms are mostly water, so water's high specific heat keeps their body temperature from swinging wildly when the environment heats up or cools down. Large bodies of water do the same thing for coastlines and for the organisms living in lakes and oceans.

Evaporative cooling is how you cool off when you sweat, and how plants cool their leaves when water evaporates from them (transpiration). Both are ways to keep body temperature in a safe range, which is part of homeostasis (keeping internal conditions stable).

Polarity also makes water an excellent solvent for ions and other polar molecules, since water's partial charges surround and separate them. Molecules that dissolve or mix with water are called hydrophilic ('water-loving'). Nonpolar molecules like fats don't mix and are called hydrophobic ('water-fearing'). This split is the key to how membranes form in Unit 2.