Topic 1.1 Structure of Water and Hydrogen Bonding: explain how the properties of water that result from its polarity and hydrogen bonding affect its biological function.

What this topic is asking

The College Board (Topic 1.1) wants you to explain how the polarity of the water molecule and the hydrogen bonding between water molecules produce the emergent properties (cohesion, adhesion, high specific heat, evaporative cooling, lower density of ice, and solvent ability) that make water essential to life. Free-response questions reward linking a property back to its molecular cause and then to a biological function.

Polarity and hydrogen bonding

Each water molecule can hydrogen bond to up to four neighbors. Individually a hydrogen bond is weak (about $1/20$ the strength of a covalent bond), but collectively the many hydrogen bonds in liquid water are responsible for nearly all of water's distinctive behavior.

The emergent properties

• Cohesion. Water molecules stick to one another through hydrogen bonding. Cohesion gives water surface tension and tensile strength, allowing a continuous column of water to be pulled up the xylem during transpiration.
• Adhesion. Water hydrogen bonds to other polar surfaces, such as the cellulose walls of xylem vessels, helping water resist gravity in narrow tubes (capillary action).
• High specific heat. A large amount of energy is needed to raise water's temperature because much of that energy first breaks hydrogen bonds. This buffers organisms and aquatic habitats against temperature swings.
• High heat of vaporization and evaporative cooling. Breaking the hydrogen bonds to vaporize water absorbs a great deal of heat, so evaporation (sweating, transpiration) cools the surface left behind.
• Lower density of ice. In ice, hydrogen bonds lock molecules into an open lattice, so solid water is less dense than liquid water and ice floats, insulating the water below.
• Solvent of life. The polar molecule surrounds and separates charged and polar solutes, forming hydration shells, which is why water is the medium for the cell's chemistry.

Linking structure to function

A recurring AP reasoning chain is: polarity gives hydrogen bonding, hydrogen bonding gives a property, and the property enables a biological function. For example, polarity to hydrogen bonding to cohesion to the transpiration stream; or polarity to hydrogen bonding to high specific heat to temperature homeostasis. Examiners reward answers that make every step in this chain explicit.

Try this

Q1. Identify the property of water responsible for the formation of a hydration shell around a sodium ion, and explain the role of polarity. [2 points]

• Cue. The solvent property; water's partial negative oxygen is attracted to the positive sodium ion, surrounding it (a hydration shell), because water is polar.

Q2. Predict what would happen to the cohesive properties of water if its molecules were nonpolar, and justify your prediction. [2 points]

• Cue. Cohesion would be greatly reduced because nonpolar molecules cannot form hydrogen bonds, so there would be no surface tension or tensile strength to support transport in xylem.