Feststoff

Feststoff is the term used in science to describe a substance that exists in the solid state, possessing a definite shape and volume. In a solid, particles are arranged in a fixed lattice or network, which gives rigidity and relatively high density. Solids contrast with liquids, which take the shape of their container, and gases, which have neither fixed shape nor volume under ambient conditions.

Solids can be broadly categorized as crystalline or amorphous. Crystalline solids exhibit long-range order and defined repeating unit cells; common types include ionic solids (for example, sodium chloride), covalent-network solids (such as quartz), metallic solids (like iron), and molecular solids (for instance, solid carbon dioxide). Amorphous solids lack long-range order and include materials such as glass and many polymers. Polymers can also show semi-crystalline regions, combining both ordered and disordered domains.

Key properties of Feststoffe include hardness, rigidity, and incompressibility relative to gases; these properties depend on the type of bonding and structure. Crystalline solids often display anisotropy, where properties vary with direction. Solids have characteristic phase behavior: they melt at a specific melting point when heated, may sublime, or undergo phase transitions under pressure. Thermal and electrical conductivities, optical properties, and solubility in liquids are also important.

Common methods to study solids include X-ray diffraction (to reveal crystal structure), differential scanning calorimetry (to measure melting and other transitions), and microscopy. Solids are central to materials science and engineering, underpinning metals, ceramics, polymers, semiconductors, catalysts, and pharmaceutical active ingredients, with processing techniques such as forging, sintering, annealing, and crystallization shaping their properties and applications.