Which property primarily differentiates a solid from a liquid?

A solid is primarily differentiated from a liquid by its shape. Solids have a definite shape, which means that they maintain their form regardless of the container they are placed in. This is due to the close packing of their particles, which are held together by strong intermolecular forces, allowing them to resist changes in shape. In contrast, liquids do not have a fixed shape; they take the shape of their container as their particles are less tightly packed and can move past one another, allowing for a flowable state.

While the other properties listed, such as the ability to flow, density, and compressibility, can also be relevant in distinguishing between different phases of matter, the defining characteristic that sets solids apart from liquids is their ability to maintain a constant shape. Liquids can flow and adapt to the shape of their containers, which is a clear distinction from solids.