Changes of State
This topic explains how the arrangement and movement of particles change during melting, boiling, freezing, and condensation. It links the energy required for these state changes directly to the strength of the chemical bonds or intermolecular forces within a substance.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- During a change of state, such as melting or boiling, the temperature of the substance remains constant. The energy being supplied is used to overcome forces between particles, increasing their potential energy, not their kinetic energy.
- Melting involves particles in a solid's fixed, regular lattice absorbing energy to vibrate more intensely until they can break free and slide past one another as a disordered liquid.
- Boiling occurs when liquid particles gain enough energy to overcome all attractive forces holding them together, escaping to form a gas with particles that are far apart and move randomly.
- The energy needed for melting or boiling is a direct measure of the strength of the forces between particles. Stronger forces demand more energy, resulting in higher melting and boiling points.
- Substances with giant structures (ionic, metallic, giant covalent) have very high melting and boiling points because a large amount of energy is needed to overcome strong electrostatic forces or break numerous covalent bonds.
- Simple molecular substances have low melting and boiling points because only the weak intermolecular forces between the molecules need to be overcome, not the strong covalent bonds within the molecules.
Definitions
- Melting
- The phase transition from solid to liquid at a constant temperature (the melting point), where particles gain enough energy to break from their fixed positions and move past each other.
- Boiling
- The phase transition from liquid to gas that occurs throughout the bulk of the liquid at a constant temperature (the boiling point), when particles gain enough energy to overcome all intermolecular forces.
- Condensation
- The phase transition from gas to liquid, where particles lose kinetic energy, slow down, and form attractive forces, releasing energy in the process.
- Intermolecular Forces
- The relatively weak attractive forces that exist between discrete molecules, such as in water or iodine. The strength of these forces dictates the physical properties of simple molecular substances.
Worked example
Consider the substances potassium fluoride (KF), fluorine (F2), and diamond (a form of carbon). Arrange them in order of increasing melting point and provide a justification based on their structure and bonding.
- 1
First, identify the structure and bonding type for each substance.
KF is a giant ionic lattice.
F2 is a simple molecular substance.
Diamond is a giant covalent network solid.
- 2
Next, identify the forces that must be overcome to melt each substance.
For KF, strong electrostatic forces of attraction between K+ and F- ions must be overcome.
For F2, only weak intermolecular forces between F2 molecules need to be broken.
For diamond, many strong covalent bonds linking carbon atoms must be broken.
- 3
Compare the relative strengths of these forces.
The covalent bonds in diamond and the electrostatic forces in KF are vastly stronger than the weak intermolecular forces in F2.
- 4
Relate the force strength to the melting point.
Since F2 has the weakest forces, it will require the least energy to melt and have the lowest melting point.
Diamond requires breaking a vast network of strong covalent bonds, which needs more energy than overcoming the ionic attractions in KF.
Therefore, diamond will have the highest melting point.
- 5
Combine these points to establish the final order.
Answer: The order of increasing melting point is F2 < KF < Diamond.
Common mistakes
- ×Confusing boiling with evaporation. Evaporation is a surface phenomenon that can occur at any temperature below the boiling point, whereas boiling is a bulk process occurring at a specific temperature.
- ×Believing that temperature increases during a phase change. The energy supplied (latent heat) is used to overcome forces between particles, so temperature stays constant until the change is complete.
- ×Incorrectly stating that strong covalent bonds break when simple molecular substances like methane (CH4) or water (H2O) boil. Only the weak intermolecular forces between the molecules are overcome.
No-calculator tips
- ✓When asked to compare melting or boiling points, first classify each substance's structure: is it giant (ionic, metallic, covalent) or simple molecular? This initial step is the key, as giant structures will always have significantly higher melting/boiling points than simple molecular ones.
- ✓Use 'room temperature' (around 20°C) as a mental benchmark. If a substance is a solid at room temperature, its melting point is high, indicating strong forces. If it's a gas, its boiling point is very low, indicating weak forces.