Construct an explanation of how the properties of water (polarity, hydrogen bonding, cohesion, and its role as a solvent) support life (Tennessee Academic Standards for Science, Biology I, BIO1.LS1).

What this topic is asking

The Tennessee LS1 standards build biochemistry on a foundation of water, asking you to explain how water's properties support life. For the Biology I EOC that means connecting one fact, that water is a polar molecule that forms hydrogen bonds, to the properties that follow: cohesion, adhesion, a high specific heat, and being a good solvent. Items often give you an observation (water rising in a tube, sweat cooling skin, salt dissolving) and ask which property explains it and why.

Why water is polar

This single fact, polarity, is the root of almost everything else about water. Because opposite charges attract, the slightly positive hydrogen of one water molecule is drawn to the slightly negative oxygen of another, forming a weak attraction called a hydrogen bond. Each water molecule can hydrogen-bond to several neighbors, and although each bond is weak, the huge number of them gives water its remarkable behavior.

Cohesion, adhesion, and surface tension

On the EOC, an observation of water beading up, a strider standing on a pond, or water climbing a thin tube should point you to cohesion and adhesion, explained by hydrogen bonding.

High specific heat: a temperature buffer

Because hydrogen bonds must be broken before water molecules can move faster, water can absorb a lot of heat with only a small rise in temperature. This high specific heat means water resists temperature change. For living things this is vital: it keeps the temperature of cells, of bodies (which are mostly water), and of whole habitats like oceans and lakes relatively stable. The related property of evaporative cooling, where the most energetic molecules escape as vapor and carry heat away, is how sweating cools the body.

The universal solvent

Water's polarity makes it an excellent solvent: it surrounds and separates other polar molecules and ions, pulling them into solution. So much dissolves in water that it is called the universal solvent. This matters because the chemical reactions of life happen in water solution, and substances are transported dissolved in water, in blood, in plant sap, and across the cytoplasm. Nonpolar substances such as oils do not dissolve in water, which is exactly why the nonpolar tails of phospholipids form a stable membrane.

Ice floats

Most substances are denser as solids, but water is unusual: when it freezes, the hydrogen bonds lock the molecules into an open lattice that is less dense than liquid water. So ice floats. This protects aquatic life: a layer of ice on top of a pond insulates the liquid water below, so fish and other organisms can survive the winter.

Try this

Q1. Explain why water is able to dissolve salt and sugar but not oil. [2]

• Cue. Water is polar, so it attracts and surrounds other polar or charged (ionic) substances like salt and sugar, pulling them into solution; oil is nonpolar, so water does not interact with it and it does not dissolve.

Q2. State why it is important for aquatic life that ice floats. [2]

• Cue. Water expands and becomes less dense when it freezes, so ice forms a floating layer that insulates the liquid water below, letting organisms survive under it through winter.