AQA · GCSE · Combined Science

    Course version: 8464 · 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 such as sodium chloride have high melting points and high boiling points.

    Checked against the AQA 8464 specification (opens in a new tab) on .

    • Point 2 of 3

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Ionic compounds such as sodium chloride have high melting points and high boiling points. This is because a giant ionic lattice contains many strong electrostatic forces of attraction between oppositely charged ions, and these forces act throughout the structure. Melting or boiling requires enough energy to overcome these forces so that the ions can move apart. Because there are many strong attractions in a giant lattice, a large amount of energy is needed, so the melting point and boiling point are high. For example, sodium chloride melts at about 801 °C. The energy is used to break the many electrostatic attractions, not to break the ions themselves. This links the structure of ionic compounds directly to their physical properties.

    What earns the marks

    • High melting and boiling points are caused by strong electrostatic forces between oppositely charged ions.
    • The lattice is giant, so there are many strong attractions to overcome.
    • A large amount of energy is needed to break these many strong attractions.
    • Melting and boiling involve overcoming the forces between ions, not breaking the ions themselves.
    • A named example, such as sodium chloride melting at about 801 °C, can support the explanation.
    • The explanation should link structure to property rather than simply stating that ionic compounds have high melting points.

    Common mistakes and how to put them right

    • Saying that covalent bonds between ions are broken.

      Put it right: The electrostatic forces of attraction between ions are overcome.

    • Stating that only a few bonds need to be broken.

      Put it right: A giant lattice contains many strong attractions, so a large amount of energy is needed.

    • Confusing melting with dissolving.

      Put it right: Melting is the change from solid to liquid when the lattice is broken down by heating, not by a solvent.

    Exam tips

    • Always mention both the strength and the number of electrostatic forces when explaining high melting points.
    • Use the phrase 'large amounts of energy' rather than vague wording such as 'lots of power'.
    • Link the property to the giant lattice structure in the same sentence to show cause and effect.

    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

    Sodium chloride is an ionic compound. Which statement explains why sodium chloride has a high melting point?

    Listen

    Ionic compounds such as sodium chloride have high melting points and high boiling points. This is because a giant ionic lattice contains many strong electrostatic forces of attraction between oppositely charged ions, and these forces act throughout the structure. Melting or boiling requires enough energy to overcome these forces so that the ions can move apart. Because there are many strong attractions in a giant lattice, a large amount of energy is needed, so the melting point and boiling point are high. For example, sodium chloride melts at about 801 °C. The energy is used to break the many electrostatic attractions, not to break the ions themselves. This links the structure of ionic compounds directly to their physical properties. A strong answer earns marks for these points. High melting and boiling points are caused by strong electrostatic forces between oppositely charged ions. The lattice is giant, so there are many strong attractions to overcome. A large amount of energy is needed to break these many strong attractions. Melting and boiling involve overcoming the forces between ions, not breaking the ions themselves. A common slip: Saying that covalent bonds between ions are broken. Instead, the electrostatic forces of attraction between ions are overcome. Exam tip: Always mention both the strength and the number of electrostatic forces when explaining high melting points.

    See it

    A flowchart running from top to bottom. The first box reads 'Giant ionic lattice' and points to a second box reading 'Many strong electrostatic forces of attraction between oppositely charged ions'. This points to 'Forces act throughout the structure', which points to 'Melting or boiling requires energy to overcome these forces'. That box points to three boxes: 'Large amount of energy needed', which leads to 'High melting point and high boiling point'; 'Ions move apart'; and 'Energy overcomes forces between ions, not the ions themselves'.
    It shows how the giant ionic lattice with many strong electrostatic forces leads to a large amount of energy being needed, and so to high melting and boiling points.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A flowchart running from top to bottom. The first box reads 'Giant ionic lattice' and points to a second box reading 'Many strong electrostatic forces of attraction between oppositely charged ions'. This points to 'Forces act throughout the structure', which points to 'Melting or boiling requires energy to overcome these forces'. That box points to three boxes: 'Large amount of energy needed', which leads to 'High melting point and high boiling point'; 'Ions move apart'; and 'Energy overcomes forces between ions, not the ions themselves'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

    There is more to this lesson

    The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.

    Carry on with the full lesson

    A week of Premium from your first lesson · Then the free plan continues · No card, no charge

    Test your understanding of 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.

    Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.

    Take topic quiz

    A week of Premium from your first lesson · Then the free plan continues · No card, no charge

    Start free

    Cookies on MasteryMind

    Essential cookies keep you signed in.

    Analytics and advertising cookies stay off unless you accept.

    Read our Privacy Policy.