1.4.8Physical Chemistry
Properties of Covalent Substances — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 24 of 102
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.
Section 1 of 4Simple Molecular Substances
Low melting points
- Low melting points Only weak intermolecular forces between molecules are overcome on melting,
- so little energy is needed and many are gases or liquids at room temperature.
Poor conductors
- Poor conductors Molecules are uncharged and have no delocalised electrons or free ions, so they cannot carry electric current.
Solubility
- Solubility Most are insoluble in water but dissolve in non-polar solvents, because similar intermolecular forces can form between solute and solvent.
Examples
- Examples Iodine, water, carbon dioxide and methane are simple molecular substances whose covalent bonds remain intact during changes of state.
Section 2 of 4Giant Covalent Substances
Giant Covalent Substances
Very high melting point
- Very high melting point Every atom is joined to its neighbours by strong covalent bonds,
- and a great amount of energy is needed to break so many bonds.
Diamond
- Diamond All four outer electrons of each carbon are used in bonding, so diamond is a very hard electrical insulator.
Graphite
- Graphite Layers of carbon atoms are joined by weak forces so they slide easily, and delocalised electrons make graphite a good conductor.
Silicon dioxide
- Silicon dioxide Each silicon is bonded to four oxygen atoms in a tetrahedral network, making SiO₂ hard with a very high melting point.
| Property | Simple molecular | Giant covalent |
|---|---|---|
| Melting point | Low | Very high |
| Conductivity | Poor | Poor (except graphite) |
| Hardness | Soft, brittle solids | Very hard (except graphite) |
Section 3 of 4Answering Structure Questions
1
Identify whether the substance is simple molecular or giant covalent from the description given.
2
State what is being overcome during melting: intermolecular forces for simple molecules, covalent bonds for giant lattices.
3
Link the strength of what is being overcome to the energy required and so to the melting point.
4
For conduction, say whether charged particles are present and free to move, naming electrons or ions explicitly.
5
Mention that graphite is the exception among giant covalent substances because of its delocalised electrons.
Section 4 of 4Exam Checklist & Tips
1.4.8 – Key Points for the Exam
- Simple molecular Made of discrete molecules held together in the solid by weak intermolecular forces.
- Giant covalent Atoms joined into a continuous lattice by covalent bonds.
- Allotrope A different structural form of the same element, such as diamond and graphite.
- Insulator A substance that does not conduct electricity.
Common Mistakes
- Stating that covalent bonds break when iodine sublimes; only the forces between molecules are overcome.
- Saying all covalent substances have low melting points, which ignores diamond and silicon dioxide.
- Claiming graphite conducts because of ions; it conducts because of delocalised electrons.
- Forgetting that a molecule with weak forces can be a gas, liquid or solid depending on its size.
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