kiintoaine

Kiintoaine, in Finnish terminology, refers to solid matter, one of the fundamental states of matter alongside liquids and gases. In a solid, matter has a definite shape and volume, and its particles are closely packed.

At the microscopic level, the particles in a solid are held together by relatively strong intermolecular or interatomic forces, which constrain movement to small vibrations around fixed positions. This arrangement gives solids their rigidity and mechanical stability. Diffusion and flow are slow compared with liquids and gases, and solids typically have higher densities.

Solids are not a single uniform category. They can be crystalline, with long-range order and a well-defined lattice structure, or amorphous, lacking such order. Crystalline solids often exhibit sharp melting points, while amorphous solids (such as glass) soften over a range of temperatures rather than melting abruptly. The properties of a solid—such as hardness, elasticity, brittleness, and thermal expansion—depend on its composition, structure, temperature, and pressure, as well as the presence of defects.

Phase transitions connect kiintoaine to other states. Heating a solid can cause it to melt into a liquid; under certain conditions, solids can sublimate directly into a gas. Compression can alter densities and defect arrangements but does not in itself define the phase unless accompanied by a phase change.

Examples include metals, minerals, ice, ceramics, and many polymers in their solid form. Kiintoaine is central to materials science, geology, and engineering, where understanding its properties informs design and analysis of countless everyday and industrial materials.