The three states of matter: The amount of energy needed to change state from solid to liquid and from liquid to gas depends on the strength of the forces between the particles of the substance. The nature of the particles involved depends on the type of bonding and the structure of the substance. The stronger the forces between the particles the higher the melting point and boiling point of the substance.

The idea

Changing state requires energy to overcome the forces between particles, not to break the particles themselves. Melting needs enough energy to weaken the regular solid arrangement so particles can slide past one another; boiling needs more energy to separate particles completely into a gas. The energy required therefore depends on how strong those forces are. The forces depend on the type of bonding and the structure: a giant ionic lattice such as sodium chloride has strong electrostatic forces between oppositely charged ions, giving a high melting point, while simple molecular substances such as iodine have weak forces between molecules, giving low melting and boiling points. In general, the stronger the forces between particles, the more energy is needed to change state, so the higher the melting point and boiling point.

What earns the marks

• States that energy is needed to overcome the forces between particles when a substance melts or boils, not to break the particles themselves.
• Links the strength of the forces between particles to the amount of energy required for a change of state.
• Explains that the nature of the particles and the forces between them depend on the type of bonding and the structure of the substance.
• Uses the relationship that stronger forces between particles give higher melting and boiling points, and applies it to a named example.
• Distinguishes between energy needed to overcome forces between particles and energy needed to break chemical bonds within particles.

Common mistakes and how to put them right

• Saying that melting breaks the particles apart.

  Put it right by stating that the particles stay intact and the forces between them are overcome.

• Saying that all substances with strong bonding have high melting points without considering structure.

  Put it right by linking the forces to the arrangement, for example simple molecular substances have weak forces between molecules.

• Confusing the energy needed to melt with the energy needed to boil.

  Put it right by stating that boiling requires more energy because particles must be completely separated.

Exam tips

• When comparing melting points, name the substance, state the type of bonding and structure, then link the strength of the forces to the energy needed and the melting point.
• Use comparative language such as stronger forces, more energy and higher melting point to make the relationship explicit.
• If a question gives data, quote the values and use them to support the comparison rather than describing the trend in general terms only.