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In chemistry, a condenser is laboratory apparatus used to condense vapors – that is, turn them into liquids – by cooling them down. Condensers are routinely used in laboratory operations such as distillation, reflux, and extraction. In distillation, a mixture is heated until the more volatile components boil off, the vapors are condensed, and collected in a separate container. In reflux, a reaction involving volatile liquids is carried out at their boiling point, to speed it up; and the vapors that inevitably come off are condensed and returned to the reaction vessel. In Soxhlet extraction, a hot solvent is infused onto some powdered material, such as ground seeds, to leach out some poorly soluble component; the solvent is then automatically distilled out of the resulting solution, condensed, and infused again. Many different types of condensers have been developed for different applications and processing volumes. The simplest and oldest condenser is just a long tube through which the vapors are directed, with the outside air providing the cooling. More commonly, a condenser has a separate tube or outer chamber through which water is circulated, to provide a more effective cooling. Laboratory condensers are usually made of glass for chemical resistance, for ease of cleaning, and to allow visual monitoring of the operation; specifically, borosilicate glass to resist thermal shock and uneven heating by the condensing vapor. Some condensers for dedicated operations may be made of metal. In professional laboratories, condensers usually have ground glass joints for airtight connection to the vapor source and the liquid receptacle; however, flexible tubing of an appropriate material is often used instead. The condenser may also be fused to a boiling flask as a single glassware item, as in the old retort and in devices for microscale distillation.