One molecule, many solids
Here is one of the most surprising facts in pharmaceutics: the very same drug molecule can pack itself into more than one kind of crystal. This is [[polymorphism|polymorphism]], and each arrangement is a *polymorph*. The molecules are identical; only their three-dimensional packing differs — like the same bricks stacked into two different walls. Yet that packing changes nearly everything a formulator cares about: melting point, solubility, dissolution rate, hardness, and stability.
Usually one polymorph is the most stable form — lowest energy, hardest to dissolve — while others are *metastable*, dissolving faster but tending to convert toward the stable form over time. That tension is central to development. A metastable form might give better absorption, but if it slowly turns into the stable form in the bottle, the medicine's behaviour drifts. When water joins the crystal lattice you get a hydrate; when other solvents do, the broader family is called a pseudopolymorph.
Habit: the shape, not the packing
Do not confuse polymorph with crystal habit. Habit is the external *shape* a crystal grows into — needles, plates, prisms, blocks — even though the internal packing is the same polymorph. Habit is set partly by how the crystal is grown (the solvent, the cooling rate). It matters less to solubility but a great deal to handling: needles flow poorly and can break during compression, while compact blocks pour and pack neatly.
The amorphous wildcard
Now the wildcard. A crystalline solid has long-range, repeating order. An amorphous solid has none — its molecules are frozen in disarray, like a liquid that stopped moving. Because amorphous material skips the energy cost of melting an orderly lattice, it usually dissolves faster and reaches higher apparent solubility. For a poorly soluble drug, that is tempting. But disorder is higher-energy and restless: amorphous solids tend to crystallize over time, especially above their glass transition temperature, the point where the rigid glass softens into a mobile state.
So preformulation must identify which solids exist for a drug and how each behaves. Two workhorse tools do most of the talking. X-ray powder diffraction shines X-rays through the powder: a crystalline form gives a fingerprint of sharp peaks at characteristic angles, while an amorphous solid gives only a broad, featureless hump. Differential scanning calorimetry heats a tiny sample and measures heat flow, revealing melting points, the glass transition, and any form-to-form conversion as the sample warms.