The terms melting range, melting point, or melting temperature are all used in pharmacopeial contexts. Most substances exhibit a melting transition, spanning the temperatures at which the first detectable change of phase or liquid phase is detected to the temperature at which no solid phase is apparent. The transition may appear instantaneous for a highly pure material, but usually a range is observed from the beginning to the end of the process. Factors influencing this transition include the sample size, the particle size, the efficiency of heat diffusion within the sample, and the heating rate, among other variables, that are controlled by procedure instructions.