The OCR classifies solid, liquid, and gas by the competition between kinetic energy and interaction energy: solids have $K>U$ with maximum freedom of motion.

• Use this when the question asks why shape, volume, compressibility, and molecular freedom differ across the three states.
• The comparison table is not just memory work; each property follows from the balance between binding and thermal motion.
• Trap: students memorise 'solid definite shape' without linking it to strong intermolecular binding and vibration about mean positions.

Crystalline solids show regular three-dimensional arrangement, anisotropy, and a sharp melting point, while amorphous solids lack long-range order, are isotropic, and do not melt sharply.

• Apply this in classification questions involving examples like NaCl or diamond versus glass or rubber.
• The OCR uses long-range order and directional dependence as the cleanest differentiators, not only the examples list.
• Trap: students think every rigid solid is crystalline; the table clearly keeps glassy or amorphous solids separate.