SCNC1112 Chap.4 Water as a Special Molecule
Water as a Special Molecule
Three of the ten lectures in Module 1 are given to a single substance, and the reason is that water is an outlier on nearly every property that matters to life. The cleanest demonstration is a comparison with molecules of similar size. Methane, at 16 grams per mole, boils at about minus 161 degrees Celsius.
Hydrogen sulfide, at 34 grams per mole and therefore heavier than water, boils at about minus 60. Water, at 18, boils at 100. Molecular mass plainly does not explain the ordering. What explains it is the hydrogen bond. Each oxygen to hydrogen bond in water is strongly polar, and the molecule is bent rather than straight, so the two bond dipoles do not cancel.
Every water molecule therefore has a negative end and two positive ends and can attach to several neighbours at once. Separating water molecules costs far more energy than separating methane molecules, which interact only through weak fleeting attractions.
The same network explains the high heat capacity that lets water buffer temperature change, and the open structure of ice that makes it less dense than the liquid, so that lakes freeze from the top down and life survives beneath. Dissolving is the second theme, and it is where entropy first does real work in a biological setting.
Ionic solids dissolve because water molecules surround each ion with their oppositely charged ends, compensating for the bonds that were broken. Polar molecules dissolve through hydrogen bonding for the same reason. A non-polar molecule offers water nothing to bond to, so the water around it has to take up a more ordered arrangement, and an ordered arrangement is a low-entropy one.
When two oil droplets merge, the surface in contact with water shrinks and some of that constrained water is released. The entropy gained by the released water is what drives the droplets together. Oil is not repelled by water; the water is recovering its own freedom. The third theme turns the solvent into a reagent.
Every major class of biological polymer is assembled by a reaction that removes one water molecule for each link formed, and dismantled by adding one back. Nucleotides become nucleic acids, amino acids become polypeptides, monosaccharides become polysaccharides, and glycerol with fatty acids becomes a triglyceride.
Starch and cellulose are both built from glucose and differ only in how the units are joined, which is why humans digest one and not the other.
What this chapter covers
- 01
Boiling Points Against Molecular Mass
- 02
Hydrogen Bonding and Bent Geometry
- 03
Heat Capacity and the Density Anomaly
- 04
Solvation of Ions and Polar Molecules
- 05
The Hydrophobic Effect as Entropy
- 06
Dehydration Synthesis and Hydrolysis
Predicting solubility from what a surface offers water
- 2Classify sodium chloride. It is an ionic solid, so water molecules can surround each ion with their oppositely charged ends, and those new attractions pay for the ionic bonds broken. It dissolves.
- 2Classify ethanol. It carries an oxygen to hydrogen group that both donates and accepts hydrogen bonds, so water can keep bonding much as it did before. It mixes freely.
- 2Classify hexane and name the driver. Hexane offers only carbon and hydrogen with no significant charge separation, so surrounding water must order itself and lose entropy. Separation into two layers is driven by the entropy of the water, not by any repulsion between hexane and water.
Key terms
- Hydrogen Bond
- An attraction between a hydrogen atom already bonded to a strongly electron-attracting atom and a nearby atom carrying partial negative charge.
- Heat Capacity
- The energy needed to raise the temperature of a given mass by one degree, which is unusually high for water and buffers temperature change.
- Solvation
- The surrounding of a dissolved particle by solvent molecules arranged to interact favourably with it, which is what keeps it dispersed.
- Hydrophobic Effect
- The tendency of non-polar surfaces to associate in water, driven by the entropy gained when ordered water around them is released.
- Dehydration Synthesis
- A reaction that joins two smaller units into a larger molecule and releases one water molecule for each link formed.
- Hydrolysis
- The reverse reaction, which breaks a link between two units by adding one water molecule across it.
Water as a Special Molecule FAQ
Why does ice float when almost every other solid sinks in its own liquid?
Because the hydrogen bonds in ice hold the molecules in an open arrangement with more empty space than the liquid has, so a given mass takes up more volume. Water is unusual in reaching its greatest density a few degrees above freezing rather than on solidifying, and the consequence is that lakes freeze downwards from the surface.
Is the hydrophobic effect really about water rather than about oil?
Yes, and that is the point of the chapter. Nothing repels the oil. Water next to a surface it cannot bond to must arrange itself more rigidly, which costs entropy, so reducing the area of that surface returns freedom to the water. The driver is the entropy of the solvent, which is why the same effect folds proteins and forms membranes.
Do I need to memorise boiling points for the assessments?
Memorising the numbers matters far less than being able to use the comparison. The examinable move is recognising that a heavier molecule boiling far lower than a lighter one rules out mass as the explanation and points to an interaction between molecules, and then naming that interaction.
Exam move
Draw the boiling-point comparison from memory with three molecules on it and write one sentence saying what it rules out. Then practise the solubility prediction in both directions, and rehearse the entropy explanation for the hydrophobic effect until you can give it without using the words attract or repel.
