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2.2.10Additional lessons

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

5 min read 5 parts Lesson 34 of 172

📚 IB Chemistry HL 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 5Simple Molecular Substances

The properties of a covalent substance depend on whether it forms small molecules or a giant network.

Low melting and boiling points
  • Low melting and boiling points Only weak intermolecular forces are broken, so little energy is required.
Volatility
  • Volatility Many are liquids or gases at room temperature because of these weak forces.
Poor conductors
  • Poor conductors They contain no mobile charged particles, so they do not conduct electricity.
Trend
  • Trend Melting and boiling points rise with larger molecules and with stronger forces such as hydrogen bonds.

Part 2 of 5Giant Covalent Substances

Giant Covalent Substances
High melting point
  • High melting point Many strong covalent bonds must be broken, so melting points are very high.
Hardness
  • Hardness Diamond and silicon dioxide are hard because the network resists distortion.
Insolubility
  • Insolubility They do not dissolve in water or organic solvents.
Conductivity
  • Conductivity Most are insulators, but graphite and graphene conduct through delocalised electrons.

Part 3 of 5Solubility Patterns

1
A polar solute dissolves in a polar solvent because new attractions to solvent molecules replace those lost.
2
Sugar and ethanol dissolve in water because they form hydrogen bonds with water molecules.
3
Non-polar substances such as iodine and hexane dissolve in non-polar solvents through London forces.
4
Non-polar substances do not dissolve well in water, because they cannot replace the hydrogen bonds between water molecules.
5
Some covalent compounds, such as hydrogen chloride, react with water to form ions and then conduct.

Part 4 of 5Comparing Structures

Comparing Structures
Ionic
  • Ionic Lattice of ions, high melting point, conducts when molten or dissolved.
Simple molecular
  • Simple molecular Low melting point, no conduction, solubility depends on polarity.
Giant covalent
  • Giant covalent Very high melting point, usually non-conducting, insoluble.
PropertySimple molecularGiant covalent
Melting pointLowVery high
ConductivityNoneNone, except graphite
Solubility in waterDepends on polarityInsoluble

Part 5 of 5Exam Checklist & Tips

2.2.10 – Key Points for the Exam

  • Volatility The ease with which a liquid evaporates.
  • Simple molecular A substance made of discrete molecules held by intermolecular forces.
  • Polar solvent A solvent whose molecules have a permanent dipole, such as water.
  • Like dissolves like Polar solutes dissolve in polar solvents and non-polar in non-polar.

Common Mistakes

  • Saying that covalent bonds are broken when a simple molecular substance melts.
  • Stating all covalent substances have low melting points.
  • Claiming that molecular substances conduct because they contain electrons.
  • Forgetting that graphite is a conducting covalent substance.

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