Why do some substances crystallise while some don't?

Answer 1

Igneous rocks are formed when liquid rock (magma underground, or lava on the surface) cools down and solidifies. If cooling is slow, usually underground, the minerals have time to form large, visible crystals, e.g. in granite. If cooling is faster, on the surface of the Earth, the crystals can usually only be seen with a microscope. Occasionally, the lava cools extremely rapidly, for instance when lava hits cold water, and there is no time for any crystal formation. This results in a volcanic glass such as obsidian.

Answer 2

Substances crystallize when their molecules arrange into repeating patterns, which results in a solid crystalline structure. Factors such as chemical composition, temperature, pressure, and impurities can influence whether a substance will crystallize or remain in an amorphous state. Substances with strong intermolecular forces tend to crystallize, while those with weak intermolecular forces or complex structures may not easily form crystals.

Answer 3

Intermolecular forces, forces between molecules, determine how well molecules "stick" together.