Separation and Purification

The objective of this lab is to demonstrate various techniques used in the purification and separation of components in mixtures. The lab focuses on the use of tools and equipment, safe laboratory practices, understanding of chemical bonding and composition, and recording of experimental results.

The lab introduces six techniques commonly used in the analysis of substances: decantation, filtration, coagulation, distillation, extraction, and sublimation. Each technique is explained along with its purpose and application.

Decantation is a process of separating liquids from unmixed solids by carefully pouring off the liquid while leaving the solids behind. It is useful when the solids have settled at the bottom and can be easily separated from the liquid portion.

Filtration is employed when a solution contains solid matter that does not settle and cannot be separated by decantation. The mixture is passed through a filter paper, which allows the liquid portion to pass through while retaining the solid particles.

Coagulation is used to separate suspended colloidal particles that do not pass through filter paper. A gelatinous precipitate is formed in the solution, causing the colloidal particles to settle along with the precipitate.

Distillation is a common technique for separating volatile liquids from non-volatile impurities. It involves vaporizing the volatile liquid, condensing it back into a liquid, and collecting it in a separate container. The non-volatile impurities remain in the original mixture.

Extraction is employed when a component of a solution cannot be removed by the above methods. An immiscible solvent is added to the solution, and the desired component is extracted into the non-aqueous layer. The layers are separated, and the component of interest is isolated.

Sublimation is a method for separating two solids when one can undergo direct conversion from solid to gas and back. It involves heating the mixture, causing one solid to sublime and form pure crystals on a cooler surface, while the other solid remains behind.

The lab provides step-by-step procedures for each technique. Decantation involves pouring off the liquid portion of a mixture of sand, dirt, salt, and water. Filtration is performed by passing the decanted solution through filter paper. Coagulation is achieved by adding a mixture of saturated Ca(OH)2 and 0.1M Al2(SO4)3 to the filtered solution. Precipitation occurs by adding AgNO3 to a portion of the coagulated solution. Distillation is carried out to collect a distillate from the precipitated solution. Finally, the extraction technique is demonstrated using an iodine/water solution and carbon tetrachloride.

The lab also provides information on the gas burner and its proper usage. Safety precautions are highlighted, such as controlling the gas flow and air inlets to achieve the desired flame for heating purposes.

Through this lab, students gain practical experience in the purification and separation techniques commonly used in the analysis of substances. They learn to utilize specific tools and equipment, practice safe laboratory procedures, and understand the principles behind each technique. The lab emphasizes the importance of proper technique and precise recording of experimental data.

Objective

·         Use the tools and equipment necessary for basic scientific analysis and research

·         Demonstrate safe practices in the Chemical Laboratory

·         Demonstrate an understanding of Electrons in Atoms and the Periodic Table and Chemical Bonding

·         Demonstrate how to isolate the components of a mixture of three compounds.

·         Demonstrate an understanding of Chemical Reaction and quantities in Chemical Reactions

·         Demonstrate an understanding of Molecules, Compounds and Chemical Composition

·         Demonstrate the proper use of exponential notation and significant figures

·         Record the results of investigation through writing

·         Use scientific reasoning to evaluate physical and natural phenomena

·         Identify the unifying themes of the scientific field of study

Materials

·         Decantation

o    Mixed Solution (Sand, Dirt, Salt, Water)

o    Beaker (2x)

o    Glass Stir Rod

·         Filtration

o    Liquid Funnel

o    Ring Stand

o    Graduated Cylinder

o    Filter Paper

·         Coagulation

o    Saturated Ca(OH)2

o    0.1 M Al2(SO4)3

o    Test Tubes

·         Precipitation

o    AgN03

·         Distillation

o    Distillation Flask

o    Round Bottom Flask

o    Condenser

o    Erlenmeyer Flask

o    Ring Stand (2x)

o    Bunsen Burner

o    Boiling Chips

·         Extraction

o    Iodine/Water Solution

o    Carbon Tetrachloride (CCl4)

o    Separatory Funnel

Discussion

In the analysis of a substance, there are four primary steps involved: sample collection, preparation for measurement, measurement itself, and data evaluation. This experiment aims to demonstrate four commonly employed techniques for purifying and preparing a sample for measurement: decantation, filtration, coagulation, and distillation. Additionally, a concise overview of two other frequently utilized methods, extraction and sublimation, is provided below.

It should be noted that the remaining steps in the analysis process, namely sample collection, measurement, and data evaluation, will be addressed in future experiments and discussions.

TECHNIQUES

DECANTATION

Decantation is a simple technique for separating liquids from solids that have not been mixed together. It involves gently pouring off the liquid while leaving the solids behind. To ensure its success, the solids should be given a brief moment to settle. For instance, a mixture of sand and water can be separated using decantation. Figure 6 provides a visual representation of the decantation process.

FILTRATION

Frequently, a solution includes solid matter with a density similar to that of the liquid portion. As a result, the solids do not settle at the bottom, making decantation ineffective for separation. In such cases, filtration becomes a valuable method. Filters consist of insoluble solids with pores that permit the liquid portion to pass while retaining the solid portion. Often, it is the solid part that requires analysis. In either scenario, separation can be achieved. Figure 7 illustrates a typical setup for gravity filtration.

COAGULATION

Filtration can be ineffective in separating suspended particles of colloidal size, as they are capable of passing through filter paper. These colloidal particles remain suspended due to the solvent's kinetic energy and display random movement known as "Brownian" motion. Nevertheless, their settling can be induced by the introduction of a gelatinous precipitate, such as Al(OH)3, into the solution. As this precipitate descends to the container's bottom, it carries down a significant portion of the insoluble matter. This process, called coagulation, serves as one of several purification steps employed in numerous municipal water treatment plants.

DISTILLATION

Distillation is a widely used method for purifying volatile liquids by separating them from unwanted non-volatile impurities. Figure 8 illustrates a standard setup for a basic distillation process. The volatile liquid to be purified is transformed into vapor within a round bottom flask, then condensed back into liquid form through the condenser, and ultimately collected in a receiving flask. On the other hand, the non-volatile impurity remains in the initial mixture. To prevent abrupt boiling, particularly in the presence of solid substances at the flask's bottom, a precautionary measure is taken by adding 2 or 3 small boiling chips to the flask prior to heating. 

EXTRACTION

In certain cases, it becomes necessary to remove a component from a solution that cannot be eliminated using the aforementioned methods. Let's consider a solution containing iodine (I2) dissolved in water (H2O). To extract the iodine (I2), an immiscible solvent like carbon tetrachloride (CCl4) is introduced to the iodine I2/H2O solution and mixed using a separatory funnel (refer to Figure 9). The iodine (I2) exhibits higher solubility in CCl4 compared to water. As a result, two distinct liquid layers form: the aqueous layer containing water and CCl4, and the non-aqueous layer primarily containing the extracted iodine (I2). Due to its higher density, carbon tetrachloride settles at the bottom, while the water layer floats on top, enabling the separation process. Your instructor will provide a demonstration of this technique. 

SUBLIMATION

Sublimation serves as a convenient technique for separating two solids from each other. It involves the direct transition of molecules from a solid to a gaseous state. In this particular scenario, a mixture of sodium chloride (NaCl) and iodine (I2) is placed in a beaker. A watch glass is positioned over the beaker, and a piece of ice is placed on top of the watch glass. The heating of the beaker causes pure iodine crystals to form on the underside of the watch glass. This separation is possible because iodine can undergo direct conversion from a solid to a gaseous state, while sodium chloride cannot. Sublimation is a phenomenon observed in various substances, with solid CO2 or "dry ice" being one of the most commonly known examples.

Procedure

1. Decantation

In the laboratory, there is a solution that needs purification. This solution consists of sand, dirt, salt, and water. Take around 100 ml of this solution and transfer it to a beaker. Let it settle undisturbed for a few minutes. Carefully pour the liquid part into another beaker without disturbing the insoluble sediment. Dispose of the sediment in the provided containers. Keep the decanted solution for the next part of the experiment.

2. Filtration

Take a sheet of filter paper (refer to Figure 7b) and fold it properly. Place the folded filter paper in a funnel. Slowly pour the solution obtained in part 1 through the filter paper, being cautious not to let it overflow. Collect the filtered solution in a graduated cylinder with a volume of 100 ml.

3. Coagulation

In a test tube, mix 8 mL of saturated Ca(OH)2 with 2 mL of 0.1 M Al2(SO4)3 from the provided reagents. Shake the test tube to ensure thorough mixing. Add this solution to the solution obtained in part 2 and allow it to settle. The reaction can be represented by the equation:

Al2(SO4)3 + 3Ca(OH)2 → 2Al(OH)3 + 3CaSO4

4. Decantation and precipitation

Decant approximately 50 mL of the clear solution from part 3 and add a few drops of AgN03 to it, enough to create a white precipitate. The reaction that occurs is:

Ag+ + Cl- → AgCl(s) (white)

The presence of Cl- ions is due to the original solution containing dissolved NaCl.

5. Distillation

Transfer the solution obtained in part 4 to the distillation flask and set up the distillation apparatus as instructed (refer to the discussion at the end of this section). Your instructor will demonstrate the use of the gas Bunsen burner. Proceed to distill the solution until approximately 10 mL of distillate has been collected. Remove the burner and test the distillate (the liquid that has been distilled) with AgNO3 to check for the presence of dissolved chloride ions.

Laboratory Technique

The Gas Burner

The gas Bunsen burner illustrated in Figure 10 will be regularly employed throughout this course. It operates by allowing gas to enter the barrel through the gas inlet, which is regulated by the gas needle valve. The airflow into the barrel can be adjusted by rotating the barrel itself, thus controlling the air inlets.

To achieve the desired combustion mixture, it is crucial to effectively manage the gas needle valve and air inlets. The following steps outline the recommended procedure for igniting the burner. 

To ignite the burner, follow these steps:

1.   Close the gas needle valve and air inlets to ensure they are completely shut.

2.   Turn on the gas supply and slightly open the gas needle valve on the burner, approximately one-half turn. Without delay, use a lit match from the side and position it slightly above the top of the barrel to ignite the burner. Take note of the flame's color.

3.   Gradually open the air inlets until the flame exhibits a distinct blue color, along with a well-defined inner blue cone. The hottest point of the flame is at the top of this inner cone.

4.   In the event that the air or gas supply is excessive, the flame may extinguish, often accompanied by a noise. Should this occur, immediately turn off the gas and start the process anew. When relighting, remember to initially close the air inlets before proceeding.

Review Questions

1.       What is the purpose of decantation in the lab?

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2.       How does filtration differ from decantation in terms of separating mixtures?

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3.       Describe the process of coagulation and its role in purification.

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4.       Explain the concept of distillation and its application in separating volatile liquids.

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5.       What is the purpose of extraction in the lab, and how does it work?

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6.       How does sublimation differ from other separation techniques in terms of physical changes?

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7.       List three safety precautions to follow when using the gas burner in the lab.

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8.       Describe the steps involved in performing decantation.

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9.       What factors determine whether decantation or filtration is more suitable for separating a mixture?

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10.    Explain the principle behind distillation and how it enables the separation of volatile liquids.

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11.    Compare and contrast coagulation and filtration as purification methods.

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12.    What are the key components of a distillation apparatus, and how do they contribute to the separation process?

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13.    Discuss the significance of extracting a component from a solution using an immiscible solvent.

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14.    How can sublimation be used to separate two solids in a mixture?

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15.    Identify two factors that can affect the efficiency of filtration.

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16.    Explain why it is necessary to adjust the air inlets of the gas burner during operation.

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17.    Describe the steps involved in setting up a filtration apparatus.

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18.    Compare the purposes and outcomes of decantation and precipitation in the lab.

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19.    Discuss the potential risks associated with using a gas burner and how to mitigate them.

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20.    Explain the significance of recording and analyzing experimental data in the context of these purification techniques.

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