Strong attractions, high melting points

The idea

Ionic compounds generally have high melting and boiling points because of strong inter-ionic attractions, whereas covalent compounds usually have low melting and boiling points.

Melting salt and camphor

Common salt is ionic. Camphor is covalent. A teacher heats each one and notes the temperature at which it melts.

Which do you think needs the higher temperature to melt?

• Salt, the ionic compound
• Camphor, the covalent compound
• They melt at the same temperature

Ionic or covalent?

Sort each statement into the type of compound it describes.

Strong attractions

In an ionic compound the attractions between the ions are strong, so a lot of energy is needed to separate them. That is why ionic compounds generally have high melting and boiling points. Covalent compounds usually have low ones.

Ionic compounds generally have high melting and boiling points because of strong inter-ionic attractions, whereas covalent compounds usually have low melting and boiling points.

Notes

Ionic compounds generally have high melting and boiling points (strong inter-ionic attractions). Covalent compounds usually have low ones.

Check yourself

Why do ionic compounds generally have high melting and boiling points?

What are the melting and boiling points of covalent compounds usually like?

Sodium chloride is ionic and camphor is covalent. Which is expected to have the lower melting point?

What are the boiling points of ionic compounds generally like?

• Because their molecules are very large. Ionic compounds do not form molecules. Their high melting and boiling points are due to strong inter-ionic attractions.
• Because of strong inter-ionic attractions — correct. Yes!
• Because they dissolve in water. Dissolving in water is a different property. The high melting and boiling points are due to strong inter-ionic attractions.
• High. Ionic compounds generally have high melting and boiling points. Covalent compounds usually have low ones.
• Exactly the same as ionic compounds. Ionic compounds generally have high points and covalent compounds usually have low ones.
• Low — correct. Yes!
• Camphor — correct. Yes!
• Sodium chloride. Ionic compounds like sodium chloride generally have high melting points.
• Neither, both have the same. Ionic compounds generally have high melting points and covalent compounds usually have low ones.
• Low. Ionic compounds generally have high boiling points because of strong inter-ionic attractions.
• High — correct. Yes!
• Lower than their melting points. The boiling point of a substance is not lower than its melting point; for ionic compounds both are generally high.