Hydrogen bonds from the polar covalent bonds in water
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Question
Define a hydrogen bond (in water).
Answer
A **weak attraction** between a **δ+ hydrogen** of one water molecule and a **δ− oxygen** of another molecule.
Question
Define a polar molecule.
Answer
A molecule with an **uneven spread of charge** — a slightly negative end and a slightly positive end.
Question
Does a hydrogen bond act within or between water molecules?
Answer
**Between** separate water molecules (a covalent bond acts **within** one molecule).
Question
Define a covalent bond.
Answer
A bond in which two atoms **share a pair of electrons**.
Question
What is electronegativity?
Answer
How strongly an atom **pulls shared electrons** toward itself.
Question
Which two parts attract to form a hydrogen bond?
Answer
A **δ+ hydrogen** of one molecule and a **δ− oxygen** of a neighbouring molecule.
Question
In water, which atom is δ− and why?
Answer
**Oxygen** — it is more electronegative, so it pulls the shared electrons closer.
Question
Why can water molecules form hydrogen bonds at all?
Answer
Because each molecule is **polar** — it has a δ− oxygen and δ+ hydrogens.
Question
Is a hydrogen bond weak or strong compared with a covalent bond?
Answer
**Weak** — much weaker than a covalent bond.
Question
In water, which atoms are δ+?
Answer
The **two hydrogen** atoms.
Question
If each hydrogen bond is weak, why do they matter?
Answer
Because there are a **huge number** of them, so a **lot of energy** is needed to separate the water molecules.
Question
What does δ (delta) mean on an atom?
Answer
A **partial** (slight) charge — much smaller than a full ionic charge.
Question
What is the approximate H–O–H bond angle?
Answer
About **104.5°** — the molecule is **bent**.
Question
Why is a whole water molecule polar (not just its bonds)?
Answer
Its **bent shape** means the partial charges **do not cancel** — they act on the same side.
Question
How is a hydrogen bond drawn in a diagram?
Answer
As a **dashed line** from a δ+ hydrogen of one molecule to the δ− oxygen of another, labelled **'hydrogen bond'**.
Question
What type of bond joins O and H within a water molecule?
Answer
A **polar covalent** bond (electrons shared, but unequally).
Question
Why is water liquid at room temperature when methane is a gas?
Answer
Water forms **many hydrogen bonds** that hold its molecules together; methane is **non-polar** and forms none.
Question
Why does water's polarity matter biologically?
Answer
It lets water molecules attract each other and other charged particles — the basis of water's life-supporting properties.
Question
Name two of water's properties that come from hydrogen bonding.
Answer
High **boiling point**, **cohesion** (and a high **heat capacity**) — any two.
Question
Can one water molecule hydrogen-bond to more than one neighbour?
Answer
Yes — each molecule can hydrogen-bond to **several** neighbours at once.
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