The water molecule: bent and polar
A water molecule is H₂O: two hydrogen atoms joined to one oxygen atom by covalent bonds. The angle between the bonds is about 104.5°, so the molecule is bent.
Oxygen is more electronegative (it pulls shared electrons harder). The electrons spend more time near oxygen. So oxygen gets a small negative charge, written δ−, and each hydrogen a small positive charge, δ+.
A molecule with a plus end and a minus end is polar (a dipole). Because the molecule is bent, the charges do not cancel out.
Hydrogen bonds between water molecules
The δ+ hydrogen of one molecule is attracted to the δ− oxygen of a nearby molecule. This attraction is a hydrogen bond.
- One hydrogen bond is weak (about 1/20 of a covalent bond).
- But there are millions of them, always breaking and re-forming, so together they are strong.
- Each water molecule can make up to 4 hydrogen bonds.
Almost every special property of water comes from these bonds.
Properties of water and why they matter for life
1. A very good solvent
Ions and other polar molecules (salt, glucose, amino acids) dissolve in water because water molecules surround them. Dissolved substances can then react and be carried around. Non-polar substances such as oils and fats do not dissolve (they are hydrophobic).
2. High specific heat capacity
It takes 4.18 J to warm 1 g of water by 1 °C (4180 J kg⁻¹ °C⁻¹). Energy is first used to break hydrogen bonds. So water warms and cools slowly. Bodies, cells, lakes and oceans keep a steady temperature.
3. High latent heat of vaporisation
Turning water into vapour needs a lot of energy (about 2260 J per g). When sweat evaporates, it takes this heat from the skin, so we cool down while losing little water.
4. Cohesion, adhesion and surface tension
Cohesion = water sticks to water. Adhesion = water sticks to other surfaces. Together they pull long columns of water up the xylem of tall trees. Cohesion at the surface gives surface tension, so pond skaters can walk on water.
5. Ice is less dense than liquid water
Water is densest at about 4 °C. Ice floats and insulates the water below, so aquatic life survives winter.
6. A metabolite and a transport medium
Water is used in hydrolysis and in photosynthesis, and made in condensation reactions and respiration. Blood plasma, sap and urine are mostly water. Water pressure in cells (turgor) keeps plants upright.
Mineral salts (inorganic ions) in water
Water in living things carries dissolved mineral salts as ions. Each has a job:
- Na⁺, K⁺: nerve impulses, water balance.
- Ca²⁺: bones and teeth, muscle contraction, blood clotting.
- Fe²⁺: part of haemoglobin, carries oxygen.
- PO₄³⁻ (phosphate): in DNA, RNA and ATP.
- H⁺: sets pH; Cl⁻: stomach acid and salt balance.
About 60–70% of the human body is water, which is why losing water and salts (dehydration) is so dangerous. Oral rehydration solution (ORS) puts back both.
Try it: three kitchen experiments
- Coin drops: count how many drops of water fit on a coin before it spills. The dome is surface tension (cohesion).
- Floating ice: put an ice cube in a glass of water and one in oil. It floats in water and sinks in most cooking oils, showing ice is less dense than water but denser than oil.
- Salt vs oil: stir a spoon of salt and a spoon of oil into water. The salt disappears (dissolves); the oil forms drops (non-polar).
Key formulas and definitions
- Key terms: polar molecule, δ+ / δ−, hydrogen bond, solvent, solute, hydrophilic, hydrophobic
- Specific heat capacity of water c ≈ 4.18 J g⁻¹ °C⁻¹; heat Q = m × c × ΔT
- Latent heat of vaporisation of water ≈ 2260 J g⁻¹; heat Q = m × L
- Cohesion = water–water attraction; adhesion = water–surface attraction; liquid water is densest at 4 °C; ice is less dense and floats
Worked examples
1. How much heat is needed to warm 200 g of water from 20 °C to 70 °C?
Q = m × c × ΔT = 200 × 4.18 × 50 = 41 800 J ≈ 41.8 kJ. Water needs a lot of energy for a small rise in temperature.
2. A runner loses 300 g of sweat by evaporation. About how much heat leaves her body?
Q = m × L = 300 × 2260 = 678 000 J ≈ 678 kJ. This large loss of heat is why sweating cools us so well.
3. Explain why glucose dissolves in blood plasma but cholesterol needs to be carried in special particles.
Glucose has many polar –OH groups, so water molecules form hydrogen bonds with it and surround it: it dissolves. Cholesterol is mostly non-polar, so water cannot surround it. It is wrapped in lipoproteins to travel in blood.
Common mistakes
- Saying a hydrogen bond is the bond inside a water molecule. The O–H bonds inside are covalent; hydrogen bonds are between different molecules.
- Thinking water is polar because it has charged ions. It has no full charges, only small partial charges (δ+ and δ−).
- Mixing up specific heat capacity (heat to warm water) with latent heat of vaporisation (heat to turn water into vapour).
- Saying ice floats because it has air in it. It floats because hydrogen bonds hold the molecules further apart, so ice is less dense.