AP Biology

Polarity of the water molecule

A water molecule is made of one oxygen atom joined to two hydrogen atoms by polar covalent bonds. Oxygen attracts the shared electrons more strongly than hydrogen does, so the oxygen end of the molecule carries a slight negative charge ($\delta^-$) and each hydrogen carries a slight positive charge ($\delta^+$). The molecule is also bent rather than straight, so these charges do not cancel and the molecule as a whole is polar.

Opposite partial charges on neighboring molecules attract one another. The attraction between the $\delta^+$ hydrogen of one water molecule and the $\delta^-$ oxygen of another is a hydrogen bond. A single hydrogen bond is weak, much weaker than a covalent bond, but every water molecule can form up to four of them, and in liquid water they break and re-form constantly. The large number of them gives water most of the properties that matter for living things.

Hydrogen bonds are not special to water. They also form between and within biological molecules wherever a hydrogen attached to oxygen or nitrogen sits near another oxygen or nitrogen. They hold the two strands of DNA together and help fold proteins into shape.

Water as a solvent

Polar molecules and ions dissolve in water because water molecules cluster around them, with $\delta^-$ oxygens facing positive charges and $\delta^+$ hydrogens facing negative charges. Substances that dissolve this way are hydrophilic. Nonpolar substances, such as the hydrocarbon tails of fats, cannot form these attractions and are pushed together by the surrounding water, which is why they are hydrophobic. This separation of hydrophilic and hydrophobic regions is what drives the formation of cell membranes.

Cohesion, adhesion and surface tension

Three properties follow directly from hydrogen bonding between water molecules and between water and other surfaces.

• Cohesion is the attraction of water molecules to one another. It holds a column of water together, which matters in plants because water is pulled upward through xylem as a continuous column when it evaporates from leaves.
• Adhesion is the attraction of water molecules to other polar or charged surfaces, such as the walls of xylem vessels. It stops the column from pulling away from the walls and helps water climb narrow tubes.
• Surface tension is the resistance of the water surface to being broken. Molecules at the surface have no water above them to bond to, so they bond more strongly to their neighbors beside and below, and the surface behaves like a thin elastic layer. It lets small animals such as water striders stand on a pond, and it makes water form droplets on a clean surface.

Adding a substance that disrupts hydrogen bonding between surface molecules, such as a detergent, lowers surface tension, and a simple way to test this is to count how many drops of each solution fit on a coin before spilling.

Temperature and heat

High specific heat capacity

The specific heat capacity of a substance is the amount of heat needed to raise the temperature of 1 gram of it by 1 °C. Water's value is about $4.18\ \mathrm{J\,g^{-1}\,^\circ\mathrm{C}^{-1}}$, which is high compared with most liquids. Heating water has to break hydrogen bonds before the molecules can move faster, so a large amount of heat is absorbed for a small rise in temperature, and the same heat is released again as the water cools.

For example, warming 250 g of water by 4 °C takes

This is why large bodies of water change temperature slowly, and why the water that makes up most of an organism's body helps hold its temperature steady. An animal that produces heat quickly, for example during exercise, does not overheat as fast as it would if its body were made of a substance with a lower specific heat.

High heat of vaporization

The heat of vaporization is the heat needed to turn a liquid into a gas. Water's is high because hydrogen bonds must be broken for a molecule to escape the liquid. The fastest-moving molecules leave first, taking their energy with them, so the water that remains is cooler. This is evaporative cooling. Sweating in humans and panting in dogs lower body temperature this way, and a plant cools its leaves by transpiration.

Ice

Below 0 °C, water molecules lock into a regular lattice in which each one is held at a fixed distance by four hydrogen bonds. The lattice is more open than the liquid, so ice is less dense than liquid water and floats. Ponds freeze from the top down, and the layer of ice insulates the water beneath, so aquatic life can survive a cold winter.

Properties and their biological effects