<P> A mixture of reactants and solvent is placed in a suitable vessel, such as a round bottom flask . This vessel is connected to a water - cooled Liebig or Vigreux condenser, which is typically open to the atmosphere at the top . The reaction vessel is heated in order to boil the reaction mixture; vapours produced from the mixture are condensed by the condenser, and return to the vessel through gravity . The purpose is to thermally accelerate the reaction by conducting it at an elevated, controlled temperature (i.e. the solvent's boiling point) and ambient pressure . </P> <P> The diagram shows a typical reflux apparatus . It includes a water bath to indirectly heat the mixture . As many solvents used are flammable, direct heating with a Bunsen burner is not generally suitable, and alternatives such as a water bath, oil bath, sand bath, electric hot plate or heating mantle are employed . </P> <P> The apparatus shown in the diagram represents a batch distillation as opposed to a continuous distillation . The liquid feed mixture to be distilled is placed into the round - bottomed flask along with a few anti-bumping granules, and the fractionating column is fitted into the top . As the mixture is heated and boils, vapor rises up the column . The vapor condenses on the glass platforms (known as plates or trays) inside the column and runs back down into the liquid below, thereby refluxing the upflowing distillate vapor . The hottest tray is at the bottom of the column and the coolest tray is at the top . At steady state conditions, the vapor and liquid on each tray is at equilibrium . Only the most volatile of the vapors stays in gaseous form all the way to the top . The vapor at the top of the column then passes into the condenser, where it cools until it condenses into a liquid . The separation can be enhanced with the addition of more trays (to a practical limitation of heat, flow, etc .). The process continues until all the most volatile components in the liquid feed boil out of the mixture . This point can be recognized by the rise in temperature shown on the thermometer . For continuous distillation, the feed mixture enters in the middle of the column . </P> <P> By controlling the temperature of the condenser, often called a dephlegmator, a reflux still may be used to ensure that higher boiling point components are returned to the flask while lighter elements are passed out to a secondary condenser . This is useful in producing high quality alcoholic beverages, while ensuring that less desirable components (such as fusel alcohols) are returned to the primary flask . For high quality neutral spirits (such as vodka), or post distillation flavored spirits (gin, absinthe), a process of multiple distillations or charcoal filtering may be applied to obtain a product lacking in any suggestion of its original source material for fermentation . The geometry of the still also plays a role in determining how much reflux occurs . In a pot still, if the tube leading from the boiler to the condenser, the lyne arm, is angled upward, more liquid will have a chance to condense and flow back into the boiler leading to increased reflux . Typical results can increase production as high as 50% over the basic worm type condenser . The addition of a copper "boiling ball" in the path creates an area where expansion of gasses into the ball causes cooling and subsequent condensation and reflux . In a column still, the addition of inert materials in the column (e.g., packing) creates surfaces for early condensation and leads to increased reflux . </P>

What is the primary purpose of using sand in the still pot heating bath