A physical change is a change in the state of matter without changing the composition of the substance. During liquefaction, a gas turns into a liquid, but the chemical composition of the gas remains the same. Liquefaction of liquid occurs when water-saturated sediments temporarily lose strength and act like a fluid. 

Liquefaction occurs when the physical conditions of a gas change, such as temperature, pressure, or volume. For example, increasing the pressure on a gas forces its molecules closer together until they form a liquid. Decreasing the temperature of a gas causes its molecules to lose kinetic energy and move more slowly, allowing them to clump together into a liquid.