Gas chromatography involves first vaporizing the sample, while liquid chromatography typically involves making a solution or suspension of the mixture to be separated. In gas chromatography all of the molecules in the vaporized sample will have the same average kinetic energy, but the different chemical species will each have different average velocities which are determined by their molecular weight. Heavier molecules will move with slower average velocities than lighter ones. This is how separation of different chemical components is achieved in a gas sample. In liquid samples, the mixture is dissolved in a solution that flows over a stationary phase, which interacts differentially with different chemical species in the sample. These different interactions with the stationary phase cause some compounds to move faster than others through the chromatography column, and this is the basis for separation via liquid chromatography.