Water is a molecule made up of hydrogen and oxygen atoms. Explain why water is a molecule and not a compound made of ions, and describe the type of bonding present in water and how it forms.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Water (H₂O) is essential for life. Chemists classify water as a covalent molecule rather than an ionic compound. Water has a low boiling point compared to many ionic compounds such as magnesium oxide.
Model answer (5 marks)
Water is made up of the non‑metal atoms hydrogen and oxygen, so it is a covalent molecule.
The H–O bonds are covalent: a pair of electrons is shared between the two non‑metal atoms.
Ionic compounds form when a metal atom loses electrons to a non‑metal, creating oppositely charged ions. This transfer of electrons does not occur in water, so it is not ionic.
Because the bonds are covalent, the molecules are held together only by weak intermolecular forces (dipole–dipole and hydrogen bonding), giving water a low boiling point compared with ionic compounds such as MgO.
The H–O bonds are covalent: a pair of electrons is shared between the two non‑metal atoms.
Ionic compounds form when a metal atom loses electrons to a non‑metal, creating oppositely charged ions. This transfer of electrons does not occur in water, so it is not ionic.
Because the bonds are covalent, the molecules are held together only by weak intermolecular forces (dipole–dipole and hydrogen bonding), giving water a low boiling point compared with ionic compounds such as MgO.
Examiner tips
- Mention non‑metal atoms first to justify covalent nature.
- Define covalent bond as shared electrons.
- Explain why no ion formation occurs.
- Link weak intermolecular forces to low boiling point.
Common mistakes
- Calling water ionic or stating it contains ions.
- Confusing covalent with polar covalent without noting shared electrons.
- Overstating the strength of hydrogen bonds as equivalent to ionic bonds.
Mark scheme (5 marks)
- Water is made up of non-metal atoms only (hydrogen and oxygen are both non-metals)
- The bond in water is a covalent bond
- A covalent bond is a shared pair of electrons between two non-metals
- Ionic compounds form when a metal loses electrons to a non-metal, forming oppositely charged ions — this does not happen in water
- Water has a low boiling point because intermolecular forces between water molecules are weak / weaker than the electrostatic forces in ionic compounds
Key terms in this question
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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