Final answer:
Compressive stress refers to a type of stress where forces push the molecules of a material closer together, which can be difficult due to the strong electrostatic forces in solids or the increased collisions under pressure in gases.
Step-by-step explanation:
A stress that forces the molecules of a material closer together is called compressive stress. This is a type of stress where forces compress an object, thereby deforming it by making its molecules more tightly packed. In a solid, for example, atoms and molecules are closely bound in a lattice structure, making the substance resistant to all kinds of stress due to the force that keeps the atoms in a relatively fixed distance. Under compressive stress, these atoms are pushed closer together, which is difficult due to the strong electrostatic forces that resist such changes.
In contrast to tensile stress, which elongates an object, compressive stress seeks to shorten it. Similarly, when gas particles in a container are squeezed under higher pressure, there is an increase in collisions between particles and the walls of the container, resulting in greater force and hence more pressure exerted by the gas. In the context of surface tension, molecules at the surface of a liquid experience attractive forces that pull them closer together, minimizing surface area. This macroscopic effect is driven by the intermolecular forces at the submicroscopic level.