The big idea: Every living thing is mostly water, and almost everything a cell does happens in water.
That is why water is called the medium for life: the substance life happens in.
Think of a packet of dried beans. In the jar they can sit for years and nothing happens. Soak them overnight, and the next day they swell and start to grow.
The beans were alive all along. What was missing was water: without it, the chemistry of life cannot run.
Where water matters
- The first cells formed in water, in the early oceans, billions of years ago.
- Inside every cell today, the reactions of life happen in the watery cytoplasm.
- Moving things around: blood in animals and sap in plants are mostly water.
Two facts to know: Where life started: the first cells arose in water.
Where life happens now: most of the processes of life still take place in water, inside and between cells.
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Picture a tug-of-war between a strong adult and a small child, both pulling on one rope. They both hold it, but the rope ends up closer to the adult.
A water molecule has the same kind of contest. One oxygen atom is joined to two hydrogen atoms, and each join is a pair of electrons the two atoms share (a covalent bond).
Equal sharing
- Both atoms pull on the shared electrons equally hard
- The electrons sit halfway between them
- Neither end of the bond is charged (non-polar)
Unequal sharing: water
- Oxygen pulls on the shared electrons harder than hydrogen
- The electrons sit closer to the oxygen
- Each O–H bond has a slightly charged end (polar)
Why oxygen wins: Oxygen pulls shared electrons much more strongly than hydrogen does (oxygen is more electronegative).
So the sharing inside water is unequal. A bond like this is a polar covalent bond.
- Covalent bond
- Two atoms held together by sharing a pair of electrons.
- Polar covalent bond
- A covalent bond where one atom pulls the shared electrons closer, so the sharing is unequal.
- Electronegativity
- How strongly an atom pulls shared electrons towards itself. Oxygen's is much higher than hydrogen's.
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Because the shared electrons sit closer to the oxygen, the oxygen end of the molecule is slightly negative. Each hydrogen, left a little short, is slightly positive.
These slight charges are written with the Greek letter delta: δ− and δ+. A molecule with a slightly negative end and a slightly positive end is a polar molecule.
One oxygen (δ−) joined to two hydrogens (δ+). The two bonds sit at an angle.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Reading the symbols: δ− on the oxygen = slightly negative. δ+ on each hydrogen = slightly positive.
δ means partial: much smaller than the full charge on an ion. Water is still a neutral molecule overall.
The molecule is not a straight line. The two hydrogens sit at an angle of about 104.5°, like the two arms of a boomerang, so both δ+ ends are on one side and the δ− end is on the other. That is what gives water two distinct ends.
The trap: full charges: Never write a bare + or − on water. A full charge means an ion, and water is not an ion.
Always write δ+ and δ−.
How this is tested: A multiple-choice question shows a water molecule and asks which symbol belongs on the oxygen, or what kind of bonding holds the atoms together.
A short written part asks you to draw one water molecule.
Blood plasma is about 90% water. Draw the structure of one water molecule.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
The drawing to aim for: one O, two H, single bonds at an angle, partial charges added.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
The trap: a straight line: H–O–H drawn in a straight line loses the second mark. The bonds must sit at an angle.
A circle labelled H₂O is not a structure either: show each atom and each bond.