Limitations of the Kinetic Theory

The kinetic theory of matter has limitations when it comes to the condensed state of matter:

Liquid state - kinetic theory does not fully explain the properties of liquids. This is due to the fact that in liquids, molecules are close together, but still retain certain degree of constant, random motion. Thus, liquids have no definite shape, and can diffuse (e.g. a drop of ink in water spreads until it is uniformly distributed throughout the volume of water).

The molecules of liquids are relatively close to one another because the attractive forces between them is strong enough to hold them close together. Therefore, liquids have fixed volumes and are practically incompressible. In liquids, the kinetic energy of the molecules is proportional to the absolute temperature. The rise in temperature increases the molecular motion in the liquid and consequently weakens the intermolecular forces.

Therefore, the volume of liquids increase with increasing temperature, but not as much as in gases. Note: the properties of liquids : indefinite shape, diffusion and increase in volume with increase in temperature can be explained using the kinetic theory; while the properties: fixed volumes and incompressibility cannot be explained by the kinetic theory.

Solid state - in the solid state, the molecules are in fixed positions. I.e., they are unable to move over and beyond one another to assume new positions. Therefore, the kinetic theory does not explain the properties of solids. Solids have definite shapes and fixed volumes, and cannot diffuse.

There are four categories to which solids can be classified based on the nature of the particles that composed them and the forces holding their particles together:

1. Metallic solids - in a metallic solid the lattice points are occupied by positive ions which are held together by more or less mobile electrons. E.g. Al, Fe, Mg and Cu.

2. Molecular solids - molecular solids are crystalline substances consisting of molecules held together by hydrogen bonding (for polar molecules, such as water, ethanol and sugar) or Van der waals forces (for non polar molecules, such as carbon tetrachloride, camphor, and naphthalene. Hence, they are soft and of low melting point. They are generally volatile.

3. Covalent solids (also called network solids or atomic crystals) - covalent solids consist of atoms held together by covalent bonds into a network extending throughout the crystal. They are normally hard, possessing high melting points, and are involatile. E.g. silica (sand) and diamond and aluminium nitride, AlN.

4. Ionic crystals - in these, the lattice points are occupied by positive and negative ions held together by strong electrostatic forces which operate between positive and negative ions. They are hard, brittle and have high melting points. E.g., MgCl₂, NaCl and MgSO₄.

Note: A liquid substance contains mainly molecules, and sometimes ions. This is because most liquids at room temperature are covalent compounds.

E.g., CCl₄, CHCl₃, and CS₂ – their melting points are below room temperature. Liquids containing ions (ionic liquids) are formed when ionic solids are melted at high temperatures (e.g., KCl and NaCl).

Water is a covalent liquid, however, it can ionize slightly.