What term describes the process where materials absorb water and expand, changing their structure?

The process described in the question is termed hydration. Hydration involves the absorption of water by certain materials, which leads to a physical and often chemical transformation resulting in the expansion of these materials. This process is significant in geology and soil science, as it can affect the structure and stability of various materials, including minerals and soils.

During hydration, water molecules interact with the material, causing it to swell. This expansion can change the physical properties of the material, such as its density and structural integrity. For example, clay minerals undergo considerable expansion when they absorb water, which can influence the behavior of soils in construction and erosion contexts.

Other options, while related to processes involving water, do not specifically focus on the absorption and expansion of materials. Weathering refers to the breakdown of rocks and minerals through various environmental processes, hydrolysis is a chemical reaction involving water that can lead to the decomposition of materials, and oxidation involves the reaction of a substance with oxygen, which can alter the materials but not necessarily through water absorption and structural expansion. Therefore, hydration is the most appropriate term for the process described in the question.