The molecular mass of a substance, more commonly referred to as molecular weight, is the mass of one molecule of that substance, relative to the unified atomic mass unit (equal to 1/12 the mass of one atom of carbon-12). May be specific mass of this molecule (i.e. the particular isotopes present) or an ensemble mass (i.e.taking account of the natural relative abundance of particular isotopes). Further disambiguation is required. The melting point of a solid is the temperature range at which it changes state from solid to liquid. Although the phrase would suggest a specific temperature and is commonly and incorrectly used as such in most textbooks and literature, most crystalline compounds actually melt over a range of a few degrees or less. At the melting point the solid and liquid phase exist in equilibrium. The boiling point of a liquid is the temperature at which the vapour pressure of the liquid equals the environmental pressure surrounding the liquid. The identifier which is used to refer to this molecule / fragment within a particular document. The identifier which is used to refer to this atom position in the molecule for example; numbers, element symbols or greek characters.