$9.$ Steam distillation is particularly useful for separating compounds that are $\_\_\_\_\_\_$ or decompose at their boiling points by utilizing steam to $\_\_\_\_$ the total pressure and lower the effective boiling points of the $\_\_\_\_\_\_.$

$10.$ The Underwood equations are fundamental in determining the $\_\_\_\_\_\_$ reflux ratio for achieving a desired separation, taking into account the relative volatility of the key components, $\_\_\_\_$ and $\_\_\_\_\_\_.$

$11.$ Total reflux in a distillation column means that all of the condensed vapor is returned as reflux, leading to the $\_\_\_\_\_\_$ number of theoretical stages; this concept is essential for determining the $\_\_\_\_$ and $\_\_\_\_\_\_$ reflux ratios.

$12.$ Multiple feeds in a distillation column can improve the column's flexibility and separation efficiency by introducing different feeds at optimal stages, which is determined by their $\_\_\_\_\_\_,\_\_\_\_,$ and desired $\_\_\_\_\_\_.$