Separation Techniques

Methods of Separating Mixtures Magnet Filter Decant Evaporation Distillation

Magnet Useful for separating a metal from a mixture Example: extract iron filings from mushed up cereal mixture containing iron

Filtration -separates a liquid from a solid Mixture of solid and liquid Stirring rod Filtrate (liquid component of the mixture) Filter paper traps solid Funnel Filtration -separates a liquid from a solid Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 40

Filtration Separates a liquid from a solid Use proper size of filter paper for the funnel Fold filter paper properly Use distilled water to help keep filter paper open in funnel Make sure mixture is well stirred Use glass rod to guide liquid while pouring Useful for quantitative analysis

Decanting Also useful for separating a solid from a liquid Simply pouring the top layer of liquid into another container and leaving the solid behind To avoid losing any solid some liquid may need to be left behind Useful when quantitative study is NOT required

Evaporation Same concept as distillation but simpler set up Simply use evaporating dish to heat the solution until only the solid remains Be careful not to over heat and cook the solidd

Setup to heat a solution Ring stand Beaker Wire gauze Ring Bunsen burner Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 42

Distillation Distillation is a process of boiling a liquid and condensing and collecting the vapor. The liquid collected is the distillate. The usual purpose of distillation is purification or separation of the components of a mixture. This is possible because the composition of the vapor is usually different from that of liquid mixture from which it is obtained. Gasoline, kerosene, fuel oil, and lubricating oil are produced from petroleum by distillation. Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.

A Distillation Apparatus liquid with a solid dissolved in it thermometer condenser tube distilling flask pure liquid receiving hose connected to cold water faucet “A Distillation Apparatus”   Description: This transparency shows an apparatus used for laboratory distillations. Basic Concepts The separation of substances by distillation occurs because the boiling point of the substances differ. The substance with the lowest boiling point boils away first. Distillation is based on the principle that a substance will change from a liquid to a vapor during heating and from a vapor back to a liquid while cooling. Teaching Strategies Use this slide to explain to students how a distillation apparatus is used to purify liquids and to separate the components of liquid mixtures. Begin by reviewing the phases of matter, stressing the processes of evaporation and condensation. Remind students that different liquids have different boiling points. Point out that substances that are solids at room temperature usually have higher boiling points than do substances that are liquids at room temperature. Questions If the distilling flask shown in the diagram were open to the air (rather than being connected to the condensor), what would happen to each component of the mixture in the flask as boiling continued? Explain your answer to question 1 in terms of the boiling points of the liquid and the dissolved solid. In the distillation apparatus, the vaporized liquid must enter the condenser. The condenser consists of a long tube within another tube (the jacket). The substance in the central tube and the jacket cannot mix. Cold water enters at the bottom of the jacket and exits at the top. What is the function of the condenser jacket? How would changing the length of the condenser jacket affect how well it performs this function? Explain why the liquid entering the receiving flask is pure. To separate a mixture of methanol and water, you set up a distillation apparatus and heat the mixture to the boiling point of methanol, 65 oC. What substance would you expect to collect at the receiving flask? When you finish the experiment, you discover that the substance in the receiving flask is not pure. It still contains both water and methanol. Can you explain why? (Hint: Think about vapor pressures.) Before automobiles were invented, crude petroleum was used mainly as a source of kerosene. The petroleum was heated in a device called a still. Gasoline, then considered to be useless, evaporated first into the air. Kerosene boiled off next and was condensed and collected. A tar-like residue remained in the still. From this information, what can you deduce about the boiling points of gasoline, kerosene, and the residue? How will you know when all of one component has been separated? Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 282

The solution is boiled and steam is driven off. Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 39

Salt remains after all water is boiled off. Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 39

No chemical change occurs when salt water is distilled. Saltwater solution (homogeneous mixture) Distillation (physical method) Salt Pure water Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 40

Electrolysis

Water Molecules Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 8

The decomposition of two water molecules. Diatomic Diatomic oxygen molecule hydrogen molecules + Electric current 2 H2O  O2 + 2 H2

Electrolysis H2O(l) O2 (g) + 2 H2 (g) “electro” = electricity Water Hydrogen gas forms Oxygen Electrode Source of direct current “electro” = electricity “lysis” = to split *H1+ H2O(l) O2 (g) + 2 H2 (g) water oxygen hydrogen This demonstration needs to be done using DC current (to produce a 2:1 H:O ratio). Use of AC current will produce equal amounts of hydrogen and oxygen gas. *Must add acid catalyst to conduct electricity Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 32

Electrolysis of Water Half reaction at the cathode (reduction): hydrogen gas cathode oxygen anode D.C. power source water Half reaction at the cathode (reduction): 4 H2O + 4 e -  2 H2 + 4 OH 1- Half reaction at the anode (oxidation): 2 H2O  O2 + 4 H 1+ + 4 e -

Steps to Separate a mixture Begin with a mixture of sand and salt What are the steps you would follow to separate the sand and the salt You want to end up with pure dry salt and dry sand with no salt left in it

Separation of a sand-saltwater mixture. Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 40

Separation of Sand from Salt Gently break up your salt-crusted sand with a plastic spoon. Follow this flowchart to make a complete separation. Salt- crusted sand. Weigh the mixture. Pour into heat-resistant container. Fill with water. Stir and let settle 1 minute. Dry sand. Calculate weight of salt. Weigh sand. Decant clear liquid. No How does this flow chart insure a complete separation? Wet sand. Evaporate to dryness. Repeat 3 times? Yes