IB Biology SLTopic 1 — Water & Its PropertiesPaper 1 & 2Core idea~10 min read
Hydrogen Bonds
Water looks like the most ordinary substance on Earth, and it behaves like nothing else. Almost every strange thing it does comes from one small fact about its shape — and that one fact is why life happens in water at all.
📘 What you need to know
A water molecule is two hydrogens covalently bonded to one oxygen, and the molecule is bent, not straight.
Oxygen pulls the shared electrons towards itself, so oxygen becomes slightly negative (δ–) and the hydrogens slightly positive (δ+).
A molecule with a positive end and a negative end is polar. Water is polar.
A hydrogen bond is the attraction between the δ+ hydrogen of one water molecule and the δ– oxygen of another.
One hydrogen bond is weak and keeps breaking and reforming. Billions of them together are strong.
Hydrogen bonds are not only in water — they hold DNA strands together, shape proteins, and give cellulose its strength.
Almost every property of water in the next page traces back to hydrogen bonding.
Why water is bent, and why that matters
Start with the covalent bonds. Oxygen shares a pair of electrons with each hydrogen, so all three atoms are properly bonded together. So far, nothing unusual.
The twist is that oxygen does not share fairly. Oxygen is much better at attracting electrons than hydrogen is, so the shared pairs sit closer to the oxygen. That leaves the oxygen end with a small negative charge and each hydrogen end with a small positive charge.
Now add the shape. The two hydrogens sit on the same side, at about 105° to each other, so the molecule is bent like a boomerang. That means the negative and positive parts end up at opposite ends of the molecule instead of cancelling out.
If water were straight instead of bent, the two positive ends would sit on opposite sides and cancel out. It would not be polar, and life as we know it would not work.
Watch that δ symbol. It means partial charge — a fraction of an electron’s worth. It is not the same as the full + or − charge on an ion like Na+. Examiners notice when students blur the two.
What a hydrogen bond actually is
Once you have polar molecules, the rest follows on its own. Opposite charges attract. So the δ+ hydrogen on one water molecule is pulled towards the δ– oxygen on a neighbouring molecule.
That attraction is a hydrogen bond. It is not a covalent bond — no electrons are shared, and it is roughly twenty times weaker. It is just an attraction between molecules that happen to have opposite ends.
Hydrogen bond
δ+ hydrogen of one molecule ↔ δ– oxygen of another molecule
A single hydrogen bond lasts only a fraction of a second before it breaks and a new one forms elsewhere. In a glass of water this is happening constantly, everywhere, all at once.
Weak on their own, strong in numbers
This is the idea students most often get half-right, so it is worth being precise.
An individual hydrogen bond is genuinely weak. It breaks from ordinary thermal movement, which is exactly why water is a runny liquid rather than a solid block — the molecules can still slide past each other.
But there are enormous numbers of them. Each molecule is attracted to several neighbours, and those neighbours to more neighbours, so the whole body of water is held together as a network. Weak × billions = strong, and that combination is what gives water its useful properties.
The best way to picture it: one strand of sticky tape peels off easily. A hundred strands side by side will hold up a chair. Nothing about each strand changed — there are just a lot of them working together.
Feature
Covalent bond
Hydrogen bond
Where it is
Inside a molecule, between O and H
Between two separate molecules
What holds it
A shared pair of electrons
Attraction between δ+ and δ–
Strength
Strong
Weak, roughly twenty times weaker
Does it break easily?
Needs a chemical reaction
Constantly breaking and reforming
Break them all and you get
Separate H and O atoms
Separate water molecules — steam
Boiling water does not break covalent bonds. It only breaks the hydrogen bonds holding the molecules to each other, which is why steam is still H2O. If boiling broke covalent bonds you would get hydrogen and oxygen gas, and kettles would be considerably more exciting.
Hydrogen bonds all over biology
Water is where you meet hydrogen bonds first, but they turn up in nearly every topic you will study this year. The same weak-but-numerous logic applies every time.
Where
What the hydrogen bonds do
Why weak bonds are the right choice
DNA double helix
Hold the two strands together between base pairs
Strands must separate for replication — strong bonds would make that impossible
Protein structure
Fold the chain into helices and sheets, then hold the 3D shape
Shape can change when needed, and heat can denature it
Cellulose in plant cell walls
Link neighbouring chains side by side
Huge numbers give real tensile strength to the wall
Enzyme and substrate
Help the substrate settle into the active site
The product must be able to leave again afterwards
mRNA and tRNA
Hold the anticodon on the codon during translation
tRNA has to let go and be reused
Spot the pattern. Every one of these needs a bond that holds firmly but can be undone without a chemical reaction. That is precisely what a hydrogen bond offers, and it is why biology uses them so heavily.
WORKED EXAMPLE
Explain why water is a polar molecule
Using the structure of a water molecule, explain how it becomes polar. (3 marks)
Point 1: name the bonding
Oxygen is joined to two hydrogens by covalent bonds, so electrons are shared.
Point 2: the sharing is unequalOxygen attracts the shared electrons more strongly than hydrogen does.
So oxygen becomes δ− and each hydrogen becomes δ+.
Point 3: the shape stops it cancelling
The molecule is bent at about 105°, so the negative and positive regions sit at opposite ends.
Unequal sharing + bent shape = a polar moleculethe shape mark is the one most students miss — unequal sharing alone is not enough
WORKED EXAMPLE
Why does DNA use hydrogen bonds rather than covalent ones?
The two strands of a DNA molecule are held together by hydrogen bonds between base pairs. Suggest why this is an advantage. (3 marks)
Point 1: what the strands have to do
During replication and transcription the two strands must be pulled apart.
Point 2: weak bonds allow thatHydrogen bonds can be broken by enzymes without breaking the strands themselves.
Covalent bonds would need far more energy and would damage the molecule.
Point 3: but the helix still has to be stable
There are millions of base pairs, so millions of hydrogen bonds.
Individually weak, collectively strong enough to hold the helix together.Weak enough to separate, numerous enough to stay stablethis “weak individually, strong together” argument earns marks across the whole course
💡 Exam tip
Always say partial or use δ when describing water’s charges. Never call the oxygen “negative” as though it were an ion.
In a diagram, draw hydrogen bonds as dashed lines and covalent bonds as solid lines. Marks are given for that distinction.
A hydrogen bond runs from a δ+ hydrogen to a δ– oxygen. Drawing one between two oxygens loses the mark.
If asked why water is polar, you need both unequal electron sharing and the bent shape.
“Weak individually, strong in large numbers” is a phrase worth learning word for word.
Remember hydrogen bonds outside water too — DNA, proteins and cellulose all come up in later topics.
⚠ Common mix-up
Calling a hydrogen bond a covalent bond. No electrons are shared in a hydrogen bond.
Thinking water is charged overall. It is neutral — the charge is just unevenly spread.
Saying boiling breaks the bonds in water. It breaks hydrogen bonds between molecules; the molecules survive.
Forgetting the bent shape. Unequal sharing on its own would not make water polar if it were straight.
Describing hydrogen bonds as strong. They are weak. Their strength comes from numbers.
Drawing hydrogen bonds inside a molecule. They form only between separate molecules.
Confusing δ+ with a full positive charge. The δ is doing important work.
Up next: Water’s Physical & Chemical Properties — cohesion, adhesion, water as a solvent, and the thermal properties that keep aquatic habitats liveable. Every one of them comes back to the hydrogen bonds on this page.
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