Explain hydrogen bonding and water as a solvent
Water’s polarity allows hydrogen bonds between molecules and helps it dissolve many ionic and polar substances.
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain syllabus concepts; self-checked practice does not measure practical competence or qualify as teacher-reviewed assessment.
Before you begin
- Recognise that biological molecules have structures that relate to their roles in cells and organisms.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Where does a hydrogen bond form in liquid water?
What you will learn
- Explain how hydrogen bonds form between water molecules
- Relate water polarity and hydrogen bonding to solvent action
Water molecules are polar
Within a water molecule, oxygen attracts shared electrons more strongly than hydrogen, giving oxygen a partial negative charge and the hydrogen atoms partial positive charges. The molecule has an uneven charge distribution and is polar.
A hydrogen bond forms between the partially positive hydrogen of one water molecule and the partially negative oxygen of another. It is an attraction between molecules, not the covalent O–H bond within one molecule.
Polarity supports solvent action
Water molecules can surround ions and many polar molecules. Their partial charges orient around charged particles, helping separate and keep them dispersed in solution. This solvent action supports transport in blood and cytoplasm and brings dissolved reactants together in cells.
Water does not dissolve all substances equally. Non-polar molecules such as many lipids do not mix readily with water. Use “many ionic and polar substances” rather than saying water dissolves every molecule.
Pause and explain
Why is water useful as a solvent for ions?
Worked example
A dissolved salt separates into ions in water. Explain how water molecules assist, distinguishing hydrogen bonding from the covalent bonds within water.
Show the worked solution
- Water has partial charges.
- Water molecules orient around ions.
- Hydrogen bonds are between molecules.
Answer Water is polar: its oxygen is partially negative and its hydrogens partially positive. Water molecules orient around charged ions and help keep them dispersed in solution. Hydrogen bonds are intermolecular attractions; the O–H bonds within each water molecule are covalent.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe how a hydrogen bond forms between two water molecules.
Give me a hint
Name the partial charges on the interacting atoms.
Compare my reasoning
- Oxygen has a partial negative charge and hydrogen a partial positive charge.
- A partial positive hydrogen is attracted to another molecule’s partial negative oxygen.
A hydrogen bond forms when the partially positive hydrogen atom of one water molecule is attracted to the partially negative oxygen atom of a neighbouring water molecule.
Look for these in your work
- Partial charges stated
- Intermolecular attraction described
Explain how water’s polarity supports solvent action for an ionic substance.
Give me a hint
Describe how water surrounds charged particles.
Compare my reasoning
- Water has partial positive and negative regions.
- These interact with ions and help keep them dispersed.
Water’s partial charges interact with oppositely charged ions. Water molecules surround the ions and help separate and keep them dispersed in solution, supporting transport and reactions.
Look for these in your work
- Polarity linked to ions
- Role of solution stated
A student says water dissolves oil because water has hydrogen bonds. Correct the claim.
Give me a hint
Distinguish polar from non-polar substances.
Compare my reasoning
- Water is polar; oil molecules are largely non-polar.
- Non-polar lipids do not mix readily with water.
Hydrogen bonding and water’s polarity support dissolving many ionic and polar substances, but non-polar oils do not mix readily with water. Water does not dissolve every substance.
Look for these in your work
- Polarity distinction
- Limitation stated
Common mistakes
- Calling an intermolecular hydrogen bond a covalent O–H bond
- Saying water dissolves every substance
- Ignoring the polar partial charges
What can you explain now?
Where does a hydrogen bond form in liquid water?
Compare with the explanation
Between neighbouring water molecules
It is an intermolecular attraction.
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Make it stick
How do water’s partial charges and hydrogen bonds support solvent action?
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