Water

1. Overview

Water is the most abundant molecule in living things, making up roughly 70-80% of most cells, and life depends on its properties. Almost every one of those properties comes from a single feature: water is a polar molecule, so neighbouring water molecules attract one another through hydrogen bonds. These many weak bonds, acting together, make water behave very differently from other small molecules. This note links hydrogen bonding to three biologically important properties — its action as a solvent, its high specific heat capacity and its high latent heat of vaporisation — and shows why each property matters to organisms.

Key Definitions

• Polar molecule: a molecule with an uneven distribution of charge, giving it a slightly negative region and a slightly positive region while remaining overall neutral.
• Hydrogen bond: a weak attraction between the slightly positive hydrogen of one polar molecule and a slightly negative atom (such as oxygen) of another.
• Solvent: a liquid in which other substances (solutes) dissolve to form a solution.
• Specific heat capacity: the amount of energy needed to raise the temperature of one kilogram of a substance by one degree Celsius.
• Latent heat of vaporisation: the energy needed to change a liquid into a gas (to evaporate it) without a change in temperature.
• Water potential: a measure of the tendency of water molecules to move from one region to another; water moves from a higher to a lower water potential.

Content

Why water is polar and how hydrogen bonds form

A water molecule is made of one oxygen atom covalently bonded to two hydrogen atoms. Oxygen attracts the shared bonding electrons more strongly than hydrogen does, so the electrons sit closer to the oxygen. This gives the oxygen a small negative charge (written $\delta^-$) and each hydrogen a small positive charge ($\delta^+$). A molecule with separated regions of charge like this is described as polar.

Because opposite charges attract, the $\delta^+$ hydrogen of one water molecule is attracted to the $\delta^-$ oxygen of a neighbouring molecule. This attraction is a hydrogen bond. A single hydrogen bond is weak and easily broken, but:

• In liquid water, each molecule forms hydrogen bonds with several neighbours.
• There are vast numbers of these bonds at any moment.
• Acting together, they make water molecules "stick" to one another.

This is why water is a liquid at the temperatures found in living organisms, even though it is a very small molecule.

Solvent action

Water is an excellent solvent for polar molecules and ions (charged substances). When an ionic substance such as sodium chloride is added to water, the partly charged regions of water are attracted to the ions:

• The $\delta^-$ oxygen of water is attracted to positive ions.
• The $\delta^+$ hydrogens of water are attracted to negative ions.

So every ion, whether positive or negative, becomes surrounded by a "shell" of water molecules pointing the correct way round. This shell pulls the ions apart from one another, and the substance dissolves. Crucially, the water shell also keeps the separated ions apart and stable in solution, so oppositely charged ions cannot re-attract and re-form the solid.

This matters because:

• Most chemical reactions in cells (metabolic reactions) take place in solution; substances must be dissolved to react.
• Dissolved substances can be transported around organisms — for example glucose, mineral ions, amino acids and urea travel dissolved in blood plasma, and sugars travel dissolved in the phloem of plants.

Note that water does not dissolve non-polar substances such as lipids; these are described as hydrophobic ("water-fearing"), which is itself biologically useful (for example in cell membranes).

High specific heat capacity

Because so many hydrogen bonds hold water molecules together, a large amount of heat energy must be absorbed before the molecules can move faster — and it is the speed of molecular movement that we measure as temperature. Much of the added energy first goes into weakening hydrogen bonds rather than raising the temperature. This gives water a high specific heat capacity: its temperature changes only slowly when heat is added or removed.

Biologically this means:

• The water inside cells and in body fluids resists rapid temperature change, so temperature stays fairly stable inside organisms even when the surroundings change. This helps keep enzymes working near their optimum and avoids damaging fluctuations.
• Large bodies of water (oceans, lakes) warm and cool slowly, providing a thermally stable habitat for aquatic organisms.

High latent heat of vaporisation

To evaporate water (turn liquid into vapour), enough energy must be supplied to break the hydrogen bonds holding the molecules together so individual molecules can escape into the air. Because there are so many hydrogen bonds, a large amount of energy is needed — water has a high latent heat of vaporisation.

This is useful for cooling:

• When sweat evaporates from the skin, the energy needed for vaporisation is taken from the body, so the body loses heat and cools down efficiently. Only a small mass of water needs to evaporate to remove a large amount of heat.
• In plants, transpiration (evaporation of water from leaves) has a similar cooling effect.

Supporting precision: water movement and water potential

This last point is not a fourth property to learn — it is precision you need when you use water's solvent and transport roles to describe osmosis.

Because water is the medium for transport and reactions, the direction it moves matters. Water moves by osmosis from a region of higher water potential to a region of lower water potential. Adding solute to water lowers its water potential. It is the difference in water potential — not the "amount" or "concentration" of water — that drives this movement, and using the correct term is essential when describing osmosis.

Worked example

Exam-style question: A student exercises on a hot day and begins to sweat. Sweating helps the body to cool down. Using your knowledge of the properties of water, explain how the evaporation of sweat cools the body. [3]

Model answer:

• Sweat is mostly water, and water has a high latent heat of vaporisation because many hydrogen bonds between the molecules must be broken before it can evaporate.
• The energy needed to break these bonds and evaporate the water is taken (absorbed) from the skin/body.
• Because the body loses this heat energy, its temperature falls, and only a small mass of water needs to evaporate to remove a large amount of heat.

Key Equations

This topic is qualitative — you explain water's properties in terms of hydrogen bonding rather than performing calculations, so no equations are required.

Common Mistakes to Avoid

• Saying "concentration of water" when describing osmosis. Always use the term water potential, and describe water moving from a higher water potential to a lower water potential.
• Treating a single hydrogen bond as strong. One hydrogen bond is weak; it is the large number of them acting together that gives water its properties. Always mention that many hydrogen bonds are involved.
• Confusing hydrogen bonds with covalent bonds. The O-H bonds within a molecule are covalent; the attractions between molecules are hydrogen bonds. Do not mix them up.
• Vague answers about heat capacity. Do not just say "water heats up slowly". Link it to energy being needed to break/weaken hydrogen bonds, so water resists temperature change and keeps internal conditions stable.
• Saying water dissolves "everything". Water dissolves polar substances and ions, not non-polar substances such as lipids. Be specific.
• Forgetting where the energy comes from in cooling. When sweat evaporates, the energy is taken from the body/skin, which is why cooling occurs — say this explicitly, not just "water evaporates".
• Oven-drying living tissue before weighing it in an osmosis investigation. Heating drives off the water held inside the tissue, ruining the very mass change you are trying to measure. Instead, gently blot the surface dry with a paper towel to remove only the outside liquid before each weighing.

Exam Tips

• When explaining any property, start from hydrogen bonding and then build to the property and finally its biological role — this clear chain of reasoning earns the marks.
• Use the symbols $\delta^+$ and $\delta^-$ (or the words "slightly positive/negative") to show you understand why water is polar.
• For "explain the importance to organisms" questions, give a named example (sweating, transpiration, transport in blood/phloem, stable aquatic habitat) rather than a general statement.
• Match the number of distinct points to the marks available: a 3-mark question usually needs three separate, linked ideas.
• Keep your terminology precise: polar, hydrogen bond, solvent/solute/solution, water potential — these specific words earn the marks.