IB Biology Water Explained (A1.1 SL): Polarity, Hydrogen Bonds & Properties Made Simple

Feb 19
2 min read

If you’re starting IB Biology, water might look like the easiest topic.

Two hydrogens. One oxygen. Done… right?

Not quite.

Water is actually the topic that determines whether students understand biology or just memorise it. Because every major unit later depends on understanding why water behaves the way it does. Whether it be enzymes, membranes, respiration, or transport for example.

So in this guide, we’re going to break down IB Biology Topic A1.1 Water clearly and logically.

By the end, you should understand:

why water molecules stick together
why ice floats
why sweating cools you
how plants pull water up against gravity
and why life literally cannot exist without it

👉 Download the IB Biology A1.1 Water Study Guide & Quiz here

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1. Water as the Medium for Life

Life began in water, and every cell today is still mostly water (around 70–80%).

Water is not just in organisms, it is the environment where chemistry happens.

Inside your body:

cytoplasm reactions occur in water
blood transports dissolved substances
enzymes only function in aqueous solutions

Biology is essentially chemistry happening in water.

2. Why Water is Polar (The Most Important Idea)

Water molecules share electrons unequally because oxygen attracts electrons more strongly than hydrogen.

So the molecule has:

a slightly negative oxygen (δ–)
slightly positive hydrogens (δ+)

This makes water a polar molecule.

And this single idea explains almost everything else in the topic.

3. Hydrogen Bonds: Weak but Powerful

Because opposite charges attract, water molecules stick to each other using hydrogen bonds.

These bonds:

are weak individually
but strong collectively

This creates the unique properties of water.

4. Cohesion, Adhesion and Surface Tension

Cohesion

Water sticks to itself → allows continuous water columns in plants.

Adhesion

Water sticks to other surfaces → helps water climb xylem walls.

Together they create capillary action, letting plants move water upward against gravity.

Surface tension happens because surface molecules are pulled inward — creating a “skin” that insects can walk on.

5. Water as a Solvent

Because water is polar, it surrounds charged and polar particles and dissolves them.

This allows:

diffusion of ions
transport in blood plasma
metabolic reactions

Hydrophilic substances dissolve.

Hydrophobic substances (like lipids) do not.

This is the reason cell membranes form.

6. Temperature Properties of Water

Water resists temperature change because hydrogen bonds require energy to break.

This gives water:

high specific heat capacity
cooling by evaporation (sweat)
stable aquatic environments

Without this, organisms would constantly overheat or freeze.

Why Students Struggle With This Topic

Most students memorise:

“cohesion, adhesion, capillary action…”

But IB exam questions test cause and effect.

You must link:

Structure → Bonding → Property → Biological importance

That’s the real skill.

Practice It (Where My Resource Helps)

To help students check understanding, I created a short structured study guide and summary quiz covering the full A1.1 syllabus.

It includes:

simplified notes
definitions
biological links
quick self-test questions