Evaporate, condense, repeat

Water evaporates (goes from a liquid to a gas when heated) from lakes, streams, oceans, trees, and even humans. As water evaporates, it leaves behind any contaminates that it may have accumulated. (That s where the salt comes from on your sweatband and cap.) This process of evaporation is one of nature s ways of purif3dng water. 

The water vapor may then travel many miles, or it may stay relatively local, depending on the prevailing winds. Sooner or later, the vapor condenses (goes from a gas to a liquid when cooled) and falls back to the earth as rain, snow, or sleet. 

Water may fall to earth and collect in a lake or stream. If it does, it eventually finds its way back to the sea. If it falls onto the land, it can form runoff dsid eventually enter a lake or stream, or it can soak into the ground and become groundwater. The porous layer of soil and rock that holds the groundwater forms a zone called an aquifer. This zone provides us with a good source of groundwater. We tap into these aquifers by using wells. 

Human activities can affect this water cycle. Cutting vegetation can increase the rate of runoff, causing less water to become absorbed into the soil. Man-made dams and reservoirs increase the surface area available for water evaporation. Using more groundwater than can be replenished may deplete the aquifers and lead to water shortages. And society can contaminate the water in a wide variety of ways that 1 discuss in this chapter. 

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Cool the solution in an ice bath and very slowly add dry methanol (5 mL) by pipette down the condenser, to quench the excess borane. After the effervescence has ceased, add hydrochloric acid (1 M, 5 mL) and stir the mixture for 10 min. Evaporate the solvents using a rotary evaporator and to the residue add methanol (10 mL) and evaporate again. Repeat this procedure three times to aid removal of trimethyl borate, which forms a low-boiling azeotrope with methanol, b.p. 55°C. 

The ratio of the amount extracted to the amount remaining is called the distribution ratio. There are many situations in which the distribution ratio is low, which would require large amounts of solvent if separatory funnels were used exclusively. This can be very expensive in both chemical and labor costs, and the additional cost of solvent disposal is now often prohibitive. Continuous extractors, in which a small volume of solvent is used to extract a portion of the compound, then evaporated, condensed, and used again, are an ideal solution. This process can be repeated for days if necessary and at the end, there is only a small volume of solvent to remove and dispose of Continuous extractors that involve solvents both heavier and lighter than water are discussed in Chapter 10. A widely used apparatus for continuously extracting components from solids is the batch extractor developed by Soxhiet. The sample is placed in a porous paper thimble and then placed in a horizontal tube with a closed bottom. The extraction solvent is dripped onto the top of the solid, percolates through it, and siphons off after a short time, the process is repeated as often as necessary. Soxhiet extraction is covered in Chapter 10. 

The vapor mixture, generated in a thin-layer or falling film evaporator, flows countercurrently in the fractionator, with the downward liquid reflux at the heated inner wall of the evaporator. By repeatedly vaporizing and condensing some of the... 

Mix 50 ml. of formalin, containing about 37 per cent, of formaldehyde, with 40 ml. of concentrated ammonia solution (sp. gr. 0- 88) in a 200 ml. round-bottomed flask. Insert a two-holed cork or rubber stopper carrying a capillary tube drawn out at the lower end (as for vacuum distillation) and reaching almost to the bottom of the flask, and also a short outlet tube connected through a filter flask to a water pump. Evaporate the contents of the flask as far as possible on a water bath under reduced pressure. Add a further 40 ml. of concentrated ammonia solution and repeat the evaporation. Attach a reflux condenser to the flask, add sufficient absolute ethyl alcohol (about 100 ml.) in small portions to dissolve most of the residue, heat under reflux for a few minutes and filter the hot alcoholic extract, preferably through a hot water fuimel (all flames in the vicinity must be extinguished). When cold, filter the hexamine, wash it with a little absolute alcohol, and dry in the air. The yield is 10 g. Treat the filtrate with an equal volume of dry ether and cool in ice. A fiulher 2 g. of hexamine is obtained. 

From assumed feed temperature (forward feed) or feed flow (backward feed) to the first effect and assumed steam flow, calculate evaporation in the first effect. Repeat for each succeeding effect, checking intermediate assumptions as the calculation proceeds. Heat input from condensate flash can be incorporated easily since the condensate flow from the preceding effects will have already been determined. 

Take 1 g (to nearest 0.001 g) air-dry, sieved soil and place in a 250-mL straight wall, no pore spout beaker. Add 10 mL of 1 1 diluted concentrated nitric acid cover the beaker top with a ribbed watch glass. Heat the sample to 95°C for 15 minutes. The sample should not boil, but the condensate should condense on the watch glass and drip back into the beaker. Allow the sample to cool, then add 5 mL concentrated nitric add and heat for 30 minutes. The sample should not boil, but the condensate should condense on the watch glass and drip back into the beaker. If brown fumes appear, repeat the step until there are no brown fumes. Maintaining the temperature (95°C), allow the sample to evaporate to 5 mL. 

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