Hard to melt

The idea

Ionic compounds have high melting and boiling points because a considerable amount of energy is required to break the strong inter-ionic attraction.

Heating a salt

Sodium chloride, NaCl, is heated more and more.

Does it melt at a low or a high temperature?

• A high temperature
• A low temperature, like ice
• It melts as soon as it is warm

Table 3.4 as bars

Press Next step. Temperatures are in kelvin (K).

A lot of energy

The ions in an ionic compound are held by strong inter-ionic attraction. To melt or boil the compound, this attraction has to be broken, and that takes a considerable amount of energy. So the melting and boiling points are high.

Ionic compounds have high melting and boiling points because a considerable amount of energy is required to break the strong inter-ionic attraction.

• Compound | Melting point (K) | Boiling point (K)
• NaCl | 1074 | 1686
• LiCl | 887 | 1600
• CaCl₂ | 1045 | 1900
• CaO | 2850 | 3120
• MgCl₂ | 981 | 1685

Notes

Ionic compounds have high melting and boiling points: much energy is needed to break the strong inter-ionic attraction.

Check yourself

Why do ionic compounds have high melting and boiling points?

Which compound in the table has the highest melting point?

Which compound in the table has the lowest melting point?

How many kelvin higher is the boiling point of NaCl (1686 K) than its melting point (1074 K)?

Answer: 612 K

1686 − 1074 = 612 K.

• A lot of energy is needed to break the strong attraction between ions — correct. Yes!
• They are made of small molecules. Ionic compounds are aggregates of ions, not molecules.
• They are soft. They are somewhat hard solids; the reason is the strong inter-ionic attraction.
• LiCl. LiCl has the lowest melting point in the table, 887 K.
• CaO — correct. Yes!
• NaCl. NaCl melts at 1074 K, below CaO's 2850 K.
• MgCl₂. MgCl₂ melts at 981 K, which is above LiCl's 887 K.
• CaCl₂. CaCl₂ melts at 1045 K, which is above LiCl's 887 K.
• LiCl — correct. Yes!