= Freeze-drying
= Lyophilization
{synonym}
\b[Freeze-drying], also called <lyophilization>, removes water from a frozen material chiefly by <sublimation>: solid <water> passes directly into vapor. Reduced pressure makes that route useful without requiring the whole sample to pass through a liquid-water stage. <Primary drying> concerns removal of ice by <sublimation>; <secondary drying> concerns removal of residual sorbed water by <desorption>.<Lyophilization of Parenteral (7 93) by the Food and Drug Administration>[Introduction and Glossary, historical July1993 guide: phase-change definition and named stages.]{ref}
A hypothetical mass balance separates water removal from the measurement convention. Suppose a $1.000\,\mathrm g$ specimen contains $0.200\,\mathrm g$ of dry solids and $0.800\,\mathrm g$ of water. If its final mass is $0.210\,\mathrm g$, with no solids lost or chemical change, the residual water is $0.010\,\mathrm g$. Its wet-mass <mass fraction> is
$$
\frac{0.010}{0.210}=\frac1{21}\approx4.76\%,
$$
whereas water relative to dry solids is $0.010/0.200=5\%$.
Both percentages describe the same imagined sample with different denominators. The mass change establishes water removal under the assumptions, but does not independently prove that <sublimation> was the mechanism. That claim requires information about the material's phase and the physical process.
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