AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
These compounds have high melting points and high boiling points because…
Full specification wording
Properties of ionic compounds: These compounds have high melting points and high boiling points because of the large amounts of energy needed to break the many strong bonds.
Ionic compounds have high melting and boiling points because melting or boiling requires breaking the strong electrostatic forces of attraction between ions throughout the giant lattice.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 2 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Ionic compounds have high melting and boiling points because melting or boiling requires breaking the strong electrostatic forces of attraction between ions throughout the giant lattice. These forces are strong and act in all directions, so a large amount of energy is needed to overcome them. For example, sodium chloride melts at about 801 °C and boils at about 1413 °C. The energy is needed to separate the ions from their fixed positions, not to break individual covalent bonds. The strength of the forces increases with the charge on the ions, so compounds such as magnesium oxide have even higher melting points than sodium chloride.
What earns the marks
- States that ionic compounds have high melting and boiling points.
- Explains that a large amount of energy is needed to overcome the strong electrostatic forces of attraction between ions.
- Refers to the many strong forces throughout the giant lattice, not just a few bonds.
- Links the energy required to the strength and number of electrostatic forces in the lattice.
- Uses a named example, such as sodium chloride or magnesium oxide, to illustrate the high melting point.
Common mistakes and how to put them right
Saying that covalent bonds are broken when an ionic compound melts.
Put it right: Electrostatic forces between ions are overcome.
Stating that only a few bonds need to be broken.
Put it right: The whole lattice is held by many strong forces in all directions.
Confusing melting with dissolving.
Put it right: Melting involves breaking the lattice, while dissolving separates ions by interaction with water molecules.
Exam tips
- Use the term 'electrostatic forces of attraction' rather than 'bonds' to avoid confusion with covalent bonding.
- Quantify the energy needed as 'a large amount' and link it to the strength of the forces.
- Compare with a simple molecular substance to highlight the difference in melting point.
Try the lesson
The start of the Properties of ionic compounds: These compounds have high melting points and high boiling points because of the large amounts of energy needed to break the many strong bonds. lesson, made from checked teaching notes for this specification point.
Have a go first
Which property is typical of an ionic compound such as sodium chloride?
Listen
Ionic compounds have high melting and boiling points because melting or boiling requires breaking the strong electrostatic forces of attraction between ions throughout the giant lattice. These forces are strong and act in all directions, so a large amount of energy is needed to overcome them. For example, sodium chloride melts at about 801 °C and boils at about 1413 °C. The energy is needed to separate the ions from their fixed positions, not to break individual covalent bonds. The strength of the forces increases with the charge on the ions, so compounds such as magnesium oxide have even higher melting points than sodium chloride. A strong answer earns marks for these points. States that ionic compounds have high melting and boiling points. Explains that a large amount of energy is needed to overcome the strong electrostatic forces of attraction between ions. Refers to the many strong forces throughout the giant lattice, not just a few bonds. Links the energy required to the strength and number of electrostatic forces in the lattice. A common slip: Saying that covalent bonds are broken when an ionic compound melts. Instead, electrostatic forces between ions are overcome. Exam tip: Use the term 'electrostatic forces of attraction' rather than 'bonds' to avoid confusion with covalent bonding.
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Key words
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Energy changes and bonding
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Structure and properties
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Strength of electrostatic forces
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Melting and boiling as physical changes
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See every definition
- Energy changes and bonding
- A large amount of energy is needed to break the many strong bonds, which is why these compounds have high melting points and high boiling points.
- Structure and properties
- The strong electrostatic forces of attraction between ions act in all directions throughout the giant lattice.
- Strength of electrostatic forces
- The strength of the forces increases with the charge on the ions, so compounds such as magnesium oxide have even higher melting points than sodium chloride.
- Melting and boiling as physical changes
- Melting or boiling requires breaking the strong electrostatic forces of attraction between ions throughout the giant lattice, not breaking individual covalent bonds.
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