Distillation under reduced pressure, also called vacuum distillation, is used for liquids that decompose at or below their normal boiling points, such as glycerol. The boiling point of a liquid is the temperature at which its vapour pressure equals the external pressure. By lowering the external pressure using a vacuum pump, the liquid attains the required vapour pressure at a much lower temperature, so it boils and distils well below the temperature at which decomposition would occur. Glycerol is therefore distilled at reduced pressure to avoid charring. Fractional distillation at 1 atm is wrong because the problem is not closely spaced boiling points but thermal decomposition, and operating at atmospheric pressure still overheats the liquid. Simple distillation at 1 atm likewise heats glycerol to its decomposing temperature. Sublimation is meant for solids that pass directly into vapour, which does not apply to a liquid like glycerol. This is the classic NCERT example used to illustrate vacuum distillation, also applied industrially in concentrating sugarcane juice. Plausibility check: lowering pressure lowers boiling temperature, which is exactly what protects a heat-sensitive liquid.