Changes of state, bonding and temperatures
Bonding, structure, and the properties of matter • Bonding and substance properties
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Key concepts
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Energy transfers during changes of state
Heating transfers energy into a substance, increasing kinetic energy of particles and, when required, supplying energy to overcome attractive forces. During melting and boiling, temperature remains constant while incoming energy breaks attractions; thermal energy is used as latent heat. Cooling removes energy, decreases particle kinetic energy, and allows attractions to form, producing solidification or condensation at constant temperature until the phase change completes.
Types of bonding and effect on melting and boiling temperatures
Ionic and metallic bonding produce strong electrostatic attractions between charged particles; these strong attractions require large energy inputs to separate particles and therefore produce high melting and boiling temperatures. Covalent network solids have atoms linked by strong covalent bonds throughout the structure; these bonds require very large energies to break and produce extremely high melting and boiling points. Simple molecular substances have molecules held together by weak intermolecular forces; these weak attractions require relatively small energy inputs and produce low melting and boiling temperatures.
Intermolecular forces versus covalent bonds in molecular substances
Intermolecular forces (London dispersion, dipole-dipole, hydrogen bonding) act between whole molecules and are much weaker than covalent bonds inside molecules. Molecular substances therefore retain strong covalent bonds within molecules while the weak intermolecular forces determine the bulk state and thermal behaviour. Weak intermolecular forces produce low melting and boiling points, poor electrical conductivity in all states, and volatility for many molecular liquids and gases.
Predicting states from temperature and data
Given melting and boiling point data and an ambient temperature, the state follows by comparing the ambient temperature with the given points: temperature below melting point produces a solid; temperature between melting and boiling point produces a liquid; temperature above boiling point produces a gas. Pressure dependence may shift melting and boiling points; standard predictions assume normal atmospheric pressure unless alternative pressure data are provided.
Latent heat and constant-temperature plateaus
Latent heat describes the energy required per unit mass to change phase without changing temperature. Energy input at a phase change increases potential energy of the system by separating particles rather than increasing kinetic energy, which produces temperature plateaus on heating curves. The magnitude of latent heat depends on the strength of the attractions that must be overcome between particles.
Key notes
Important points to keep in mind