CHM 105

Separating a Mixture

Exp. 3

This activity is designed to introduce you to a variety of techniques useful for separating mixtures, and the reasons why a particular technique is used for a given mixture.

 INTRODUCTION

 One characteristic that distinguishes a mixture from a pure substance is the ability to separate the mixture into its components by simple physical means. Whether a mixture is homogeneous or heterogeneous, its components will have properties that can be used to affect this separation. Some of the properties that can aid in the isolation of components are: particle size; color; density; solubility in water; ability to sublime; and magnetism. You will take advantage of some of these properties during the procedures below.

 

For each mixture used you are to record the following information in your lab notebook before you begin working with that mixture:

detailed physical description of the original mixture; include whether it is homogeneous (same throughout) or heterogeneous; &approach you might take to separate the mixture based on your visual inspection.  When you have finished the separation you must record a physical description of the separated components.

1.  Separating a Mixture by Decantation

Obtain one small scoop of a mixture  (fine sand, SiO2, and table salt, NaCl) and place it in a medium (25mm x 200mm) test tube. What does Mixture 1 look like?   After completing the visual inspection, add approximately 5 mL of distilled water to the mixture. Agitate the contents to insure complete mixing.  What does this new mixture with the water mixture look like?  Has anything happened to the original mixture? Explain.

After allowing any material to settle, carefully pour off the water collecting it in a small evaporating  dish.  Keep the solid component in the test tube. What do you think is the identity of the solid left in the test tube?

This technique that separates a liquid from an insoluble solid by carefully pouring off the liquid is called decanting or decantation.

Now heat the water solution in the dish to dryness on a hot plate.

What is the purpose of the heating?  What is the identity of the solid remaining in the dish?  Why do you think the solid did not evaporate away?

Record the characteristics of the isolated components in your notebook. You have just done a separation through evaporation of one component.

   

2.  Separating a Mixture by Gravity Filtration

 

Obtain one scoop of a mixture  (fine sand and table salt) and place it in a medium test tube. Add water as before and mix.  Set up a filtration device as shown here. Ask your instructor for directions on how to fold the filter paper for use in the funnel. Pour the entire contents of the tube into the funnel. Collect the liquid passing through the funnel in a small evaporating dish. Rinse the material in the funnel with a few milliliters of distilled water and collect the additional liquid in the same evaporating dish.  Why do you rinse the filter paper while it is in the funnel?

 This separation process is called gravity filtration. The liquid in the evaporating dish is called the filtrate.  What is the composition of the filtrate?

 

 

 

 Gravity Filtration

Place the dish on a hot plate and evaporate off the water. Describe the isolated mixture components as before. Why might you choose to use gravity filtration rather than decantation? The filtrate is also a mixture. Explain why.

 

3.  Separating a Mixture by Sublimation

Take one small scoop of  a mixture (ammonium chloride, table salt, and sand) and place it in a medium test tube. Add approximately 5 mL of distilled water and mix well.  What happens? What does this tell you about the components of this mixture?

Take a small scoop of  a mixture  (ammonium chloride and sand) and place it in a small, DRY 150 mL beaker. Cover the dish with a watch glass. Place the covered beaker on a hot plate and heat on a high setting.

What happens?

Heat until no material is vaporizing from the BOTTOM of the beaker. Carefully remove the covered beaker with "hot hands" and place it on the counter to cool. When cool, remove the watch glass and describe the two newly separated components.  This separation used a process involved a process called sublimation. A substance that converts from a solid directly to a gas at a low temperature is said to sublime.  From your experience, which of the two components of Mixture 3 sublimed?

   

4.  Find Percent Composition of an Unknown Mixture

Record the weight of a clean, dry evaporating dish.  Obtain about 10 grams of the Unknown Mixture (NaCl, SiO2, NH4Cl) and place in evaporating dish and re-weigh to get the weight of the mixture.  Place the evaporating dish on a ring stand and position the dish under a bench improvised fume hood (SEE Demo.). At first, heat the evaporating dish slowly, and then strongly until white fumes of ammonium chloride cease to be evolved (about 15 minutes). Occasional stirring of the sample with a glass stirring rod may aid in the complete removal of the ammonium chloride. Allow the evaporating dish to cool to touch. Weigh the evaporating dish and its contents at room temperature and record.


Add 20 ml of distilled water to the sample in the evaporating dish and stir with a stirring rod for several minutes. Decant the liquid into a clean, dry, previously weighed evaporating dish. Add 10 ml more of distilled water to the solid, stir for several minutes and combine it with the first liquid extract. Add one more 10 ml portion of distilled water to the solid, stir and combine it with the previous water extracts. At this point you should have one evaporating dish with a wet solid and another evaporating dish with about 40 ml of water.

Place the evaporating dish containing the remaining solid on a ring stand. Heat slowly and stir with a stirring rod, breaking up clumps of the solid. Heat the dish to a dull red color for 10 minutes to be certain that all of the water have evaporated from the solid. Cool the evaporating dish to room temperature, weigh and record. While the dish is cooling, place the evaporating dish containing the liquid on the ring stand and heat slowly to minimize boiling the water too quickly. When nearly all of the water has evaporated, reduce the intensity of the flame to avoid spattering.  When the salt appears dry, heat strongly for several minutes. Allow the dish to cool to room temperature, weigh and record.

Calculate the experimental percent composition of each component in the mixture. Obtain the true % of each component in the mixture from your instructor and compute a percent error (accuracy)  for the sand, NaCl, and NH4Cl. 

Always clean up your workstation and surrounding area before leaving lab.