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2.2.10Additional lessons

Physical Properties of Covalent Substances — IB Chemistry SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 31 of 98

📚 IB Chemistry SL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 6Simple Molecular Substances

The covalent bonds inside molecules stay intact when a molecular substance melts or boils.

Low melting point
  • Low melting point Only weak intermolecular forces have to be overcome, so little energy is needed and melting points are low.
Volatile
  • Volatile Because the forces are weak, many molecular substances evaporate easily and are liquids or gases at room temperature.
Poor conductors
  • Poor conductors Molecules are neutral and have no free ions or electrons, so they do not conduct electricity.
Soft solids
  • Soft solids Molecules are held loosely, so molecular solids are usually soft and easy to break.

Part 2 of 6Giant Covalent Substances

Giant Covalent Substances
Very high melting point
  • Very high melting point A giant network of strong covalent bonds must be broken to melt the solid, which needs very large energy.
Hard and rigid
  • Hard and rigid Atoms are locked in position by directional bonds, so diamond and silicon dioxide resist deformation.
Insulators
  • Insulators Electrons are localised in bonds, so diamond, silicon dioxide and most giant covalent solids do not conduct.
Exceptions
  • Exceptions Graphite and graphene conduct because they have delocalised electrons, and silicon is a semiconductor.
PropertySimple molecularGiant covalent
Melting pointlowvery high
Particlesmoleculesatoms in network
Conductivitynonenone (except graphite)
Forces overcome on meltingintermolecularcovalent bonds

Part 3 of 6Solubility

1
A substance dissolves when attractions between solvent and solute are comparable to those it must break.
2
Polar molecules such as ethanol and sugars can form hydrogen bonds or dipole attractions with water, so they dissolve.
3
Non-polar molecules like hexane or iodine interact with water only weakly, so they dissolve very little.
4
Non-polar substances dissolve in non-polar solvents, where dispersion forces replace those lost in the pure liquid.
5
This idea is summarised as like dissolves like.

Part 4 of 6Conductivity and Other Cases

Pure molecular liquids
  • Pure molecular liquids Liquids such as water and ethanol have almost no free charges, so they conduct very poorly.
Reacting with water
  • Reacting with water Some polar molecules, such as HCl, react with water to form ions, giving a conducting solution.
Volatility trend
  • Volatility trend Boiling point rises as dispersion forces increase with molecular size, as in the alkane series.
Giant structures
  • Giant structures These substances are generally insoluble because solvent attractions cannot replace covalent bonds.

Part 5 of 6Worked Example

Question

  • A white solid melts at 120 °C, does not conduct when solid or molten, and dissolves in hexane.
  • Suggest the type of structure and explain.

  1. The low melting point shows that only weak forces are overcome, so the structure is simple molecular.
  2. No conductivity in the molten state means there are no ions or delocalised electrons.
  3. Solubility in non-polar hexane suggests the molecules are non-polar, held by London forces.

Answer A simple molecular, non-polar substance with weak intermolecular forces.

Part 6 of 6Exam Checklist & Tips

2.2.10 – Key Points for the Exam

  • Volatile Easily changing from liquid to gas, having a low boiling point.
  • Simple molecular Made of small discrete molecules held by weak intermolecular forces.
  • Like dissolves like Substances with similar intermolecular forces tend to mix.
  • Insulator A substance that does not conduct electricity.

Common Mistakes

  • Stating that covalent bonds are weak because molecular substances melt easily.
  • Saying graphite is an insulator.
  • Confusing simple molecular substances with giant covalent ones when predicting melting points.
  • Forgetting that molten molecular substances still do not conduct.

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