When placed under pressure, what do certain crystalline substances generate?

Certain crystalline substances are known to generate minute potential differences when placed under pressure due to a phenomenon called piezoelectricity. This effect occurs in materials that possess a crystalline structure, where the application of mechanical stress can create an imbalance in charge distribution within the crystal lattice. As a result, this pressure induces a small electrical voltage across the material.

Piezoelectric materials have important applications in various fields, including sensors, actuators, and electronic devices, because they can convert mechanical energy into electrical energy and vice versa. This phenomenon is not dependent on thermal or chemical changes but rather on the physical characteristics of the crystalline structure under stress.

While the other options relate to electrical phenomena or properties, they don't accurately describe what happens in this context with crystalline substances under pressure. High voltage and electrical resistance are characteristics that relate more to the behavior of materials in electric fields or circuits rather than the direct result of mechanical pressure on crystalline structures. Chemical reactions generally involve the breaking and forming of bonds, which is not applicable to the immediate effect of pressure on crystalline materials.