IB Biology HLWater & Its PropertiesPaper 1 & 2Core idea~10 min read
Hydrogen Bonds
Water is the most ordinary looking substance on Earth, and it behaves like nothing else. Nearly every strange thing water does comes from one small fact about its shape — and that one fact is the reason life happens in water at all.
📘 What you need to know
A water molecule is two hydrogen atoms covalently bonded to one oxygen atom, and the molecule is bent, not straight.
Oxygen pulls the shared electrons closer to itself, so oxygen becomes slightly negative (δ−) and each hydrogen becomes slightly positive (δ+).
This uneven spread of charge is called a dipole. 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 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 and collagen their tensile strength.
Life first evolved in water, and most life processes still happen in water because dissolved molecules can move and react there.
Water as the medium for life
Before the chemistry, it helps to know why the IB puts water first. The first cells are believed to have evolved in a watery environment, deep in the oceans near hydrothermal vents in the Earth’s crust. Water and solutes became trapped inside a membrane, chemical reactions started happening in that little bag, and cells were on their way.
The reason water works so well for this is simple. In its liquid state, dissolved molecules can move around freely, so they bump into each other and react. A solid would hold them still and a gas would spread them too far apart.
Water is the medium in which all metabolic reactions take place inside cells.
70–95% of the mass of a cell is water.
71% of the Earth’s surface is covered in water, so it is a major habitat too.
The link between water and life is so strong that astronomers searching for life on other planets and moons look for signs of water first. If a question asks why water matters, “it is the medium for metabolic reactions” is the sentence that earns the mark.
Why water is bent, and why that matters
Start with the covalent bonds. One oxygen atom shares a pair of electrons with each hydrogen atom, so all three atoms are properly bonded together. So far, nothing unusual.
The twist is that oxygen does not share fairly. Oxygen attracts electrons more strongly than hydrogen does, so the shared pairs sit closer to the oxygen. That leaves the oxygen end with a weak negative charge (δ−) and each hydrogen end with a weak positive charge (δ+). This separation of charge is called a dipole.
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 asymmetrical shape means the negative and positive parts end up at opposite ends 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 a partial charge — only a fraction of an electron’s worth. It is not the same as the full 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 water 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 simply an attraction between two 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 somewhere else. 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 all of 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.
🧠
Memory trick: “Inside is strong, between is weak.”
Inside a molecule → covalent bond → strong, electrons shared. Between molecules → hydrogen bond → weak, charges attracting.
If you can say which side of the molecule boundary the bond sits on, you will never mix the two up.
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
Dissolving solutes
Water forms hydrogen bonds with polar and charged particles, pulling them apart into solution
Solutes must be able to leave solution again and react
Cohesion and adhesion
Stick water molecules to each other, and to other polar surfaces such as cellulose
Lets water be pulled up the xylem of very tall trees in a continuous column
DNA double helix
Hold the two strands together between the base pairs
Strands must separate for replication — covalent bonds would make that impossible
Protein structure
Form the secondary structure (helices and sheets) and help hold the tertiary 3D shape
Shape can change when needed, and heat can denature it
Cellulose and collagen
Link neighbouring chains side by side
Huge numbers give these molecules real tensile strength
mRNA and tRNA
Hold the anticodon on the codon during protein synthesis
tRNA has to let go afterwards and be reused
Enzyme and substrate
Help the substrate settle into the active site
The product must be able to leave the active site again
Cell membranes
Surface effects between water and the polar phosphate heads
Keeps the bilayer arranged without locking it rigid
Same logic every time: firm enough to hold the structure, weak enough to be undone without a chemical reaction.
Spot the pattern. Every example above needs a bond that holds firmly but can be undone easily. That is exactly what a hydrogen bond offers, and it is why biology uses them so heavily.
Worked examples
WE 1
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+ — this is a dipole.
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
WE 2
Explain how hydrogen bonds form between water molecules
Describe how hydrogen bonds form between water molecules and explain why they are described as weak. (3 marks)
Point 1: start from polarity
Each water molecule has a δ− oxygen and two δ+ hydrogens.
Point 2: opposite charges attractThe δ+ hydrogen of one molecule is attracted to the δ− oxygen of an adjacent molecule.Point 3: why weak
No electrons are shared, so the attraction is easily broken by normal movement of the molecules.
They are constantly breaking and reforming.
Weak individually, but strong when large numbers are presentsay “adjacent molecules” — it shows you know the bond is between molecules, not inside one
WE 3
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 tips
Always say partial charge or use δ. 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, cellulose and collagen all come up in later topics.
Use the correct terms: dipole for the separated charge, polar for the molecule as a whole.
⚠ Common mistakes
Calling a hydrogen bond a covalent bond. No electrons are shared in a hydrogen bond.
Thinking water is charged overall. It is electrically 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 the molecule 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: Physical & Chemical Properties of Water — cohesion, adhesion, water as a solvent, specific heat capacity, thermal conductivity, buoyancy and viscosity. Every one of them traces back to the hydrogen bonds on this page.
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