The three states of matter: explain the different temperatures at which changes of state occur in terms of energy transfers and types of bonding

The idea

Melting and boiling temperatures differ between substances because the energy needed to overcome the forces between particles differs. In a simple molecular substance, such as iodine, the molecules are held together by weak intermolecular forces, so only a small energy transfer is needed to separate them; melting and boiling points are low. In a giant ionic lattice, such as sodium chloride, strong electrostatic forces between oppositely charged ions must be overcome, so the melting point is high. In a giant covalent structure, such as diamond, many strong covalent bonds must be broken, giving a very high melting point. Metals have strong metallic bonding and generally melt at high temperatures. Energy transferred during heating increases the kinetic energy of particles until the forces can no longer hold them in place.

What earns the marks

• Link the temperature of a change of state to the energy transfer needed to overcome the forces between particles.
• Identify the type of bonding or structure present, for example simple molecular, giant ionic, giant covalent or metallic.
• Explain that weak intermolecular forces in simple molecular substances require less energy to overcome, giving lower melting and boiling points.
• Explain that strong electrostatic forces in giant ionic lattices or strong covalent bonds in giant covalent structures require more energy, giving higher melting and boiling points.
• Use the idea that heating transfers energy to particles, increasing their kinetic energy until the forces holding them in the solid or liquid arrangement are overcome.
• Compare two named substances and relate the difference in their melting or boiling points to the difference in bonding and forces.

Common mistakes and how to put them right

• Saying that covalent bonds break when a simple molecular substance melts.

  Put it right by stating that intermolecular forces between molecules are overcome, not the covalent bonds within molecules.

• Confusing the strength of bonding with the amount of bonding.

  Put it right by referring to the strength of the forces or bonds that must be overcome, not simply how many there are.

• Describing ionic bonding as sharing electrons.

  Put it right by stating that ionic bonding involves electrostatic attraction between oppositely charged ions such as Na⁺ and Cl⁻.

Exam tips

• Name the structure and bonding type first, then link it to the energy needed to overcome the forces.
• Use comparative language such as weaker or stronger forces to make the explanation explicit.
• If asked to compare, give a sentence for each substance and a sentence linking the difference to bonding.