EXPERIMENT FOUR: SEPARATION OF A SOLID MIXTURE (by Melissa Willis)

In this experiment, you will be given a mixture of silt, sand, and salt in an unknown proportion.  First, you will need to determine the solubility of each these substances to dissolve in water.  From this information, you will then be able to separate and identify each material within the unknown mixture.  

PART ONE: THE SOLUBILITY IN WATER OF EACH SOLID COMPONENT

1.Obtain three 13 x 100 mm test tubes and use a wax pencil to write a number on each tube as follows: 1 for salt, 2 for sand, and 3 for silt.         

 

Locate the sand, silt, and salt reagent bottles.

 

2. Place an appropriate amount of each solid into its test tube. ( Each reagent bottle has a test tube attached to its side that shows you the approximate amount that is needed).

 

3.  To each solid, add a few millimeters (about one inch) of distilled water.  Gently flick side of each test tube with your fingers to thoroughly mix the contents.

 

4. Record your solubility observations in the first RESULTS box.

 

PART TWO: DEVELOPING A SEPARATION SCHEME AND DRAWING A FLOW CHART

Using the solubility results, complete the flow chart in the second RESULTS box.

PART THREE: SEPARATION OF A SOLID MIXTURE

1.  First, record the number of the unknown that you will be using in the third RESULTS box.

2. Weigh a piece of filter paper, a watch glass, an evaporating dish, and ( as shown) a 50ml beaker.  Record the masses in the third RESULTS box.

3. Label two, clean 13 x 100 mm test tubes as #1 and #2 using a wax pencil. 

4. Tare test tube #1 on the balance and then weigh approximately 1 gram ( to the nearest 0.001 gram of the chosen unknown into it.  Record this mass in the third RESULTS box.

To weigh out a sample of the unknown mixture we need to first tare a beaker and a test tube. Taring the beaker and the test tube will allow us to eliminate their masses from any calculations.  Taring is the same as 'zeroing' the mass of these items. (NOTE: To replay the video click on the small arrow on lower left portion of the image.)
 

Having tared the beaker and test tube we are now ready to measure out approximately 1 gram of the unknown mixture. Notice this particular student has measure more than 1 gram. This is no biggy, we only need approximately 1 gram. If it is a little more, as in this case, or a little less we can still complete the experiment.  (NOTE: To replay the video click on the small arrow on lower left portion of the image.)

 

5. Add a few milliliters (about one inch) of distilled water to the unknown.

Shake the test tube well to dissolve the salt and suspend the silt in the solution.

6. Use the disposable pipet to carefully draw off the liquid suspension, leaving the solid portion behind.  Squirt the contents of the pipet into test tube #2.

7. You will need to repeat steps five and six two more times to separate the solid and liquid portions as much as possible.

8. Set up a filtration apparatus as shown.  (Go here to see how to fold the filter paper shown in the filter funnel.  Go here to see how to place and hold the filter paper in the filter funnel.) Carefully pour the liquid suspension in test tube #2 onto the filter paper in the funnel, allowing the filtrate to collect into the 50ml beaker.  Rinse the test tube with as little distilled water as possible ( the more you use, the longer it will take to dry).  Pour the rinsing onto the filter paper as well.

9. Place the 50ml beaker containing the filtrate onto a wire gauze supported by a ring and ring stand.

Use caution when lighting a Bunsen burner.  Always remember to light the match first, then turn on the gas.  Light the flame by approaching the side of the burner, as shown here.

 

Heat the beaker very gently to avoid spattering .  This will evaporate the water from the solution.

 

 

10. Remove the filter paper from the funnel and place it onto the pre-weighed watch glass.  Set the watch glass under a heat lamp and allow it to dry.

11. Using as little distilled water as possible from a wash bottle, completely transfer the solid from test tube #1 into the pre-weighed evaporating dish.

12. Set the evaporating dish onto another wire gauze supported by a ring and ring stand and using a second Bunsen burner, evaporate the water from the dish.

 

13. When the 50mL beaker, the watch glass, and the evaporating dish are completely dry, allow them to cool.

Weigh  the 50mL beaker, the watch glass, and the evaporating dish separately on the balance.  Record the masses in the third RESULTS box.

Steps 14 and 15 involve calculations.  See lab manual.

16. To clean up, put the dried silt and sand in the trash, do not put them in the sink!  Rinse out the disposable pipet with distilled water.  Wash your glassware in the sink with soap and water.  Return the ring stands, rings, and Bunsen burners to the cabinet and make sure the gas valve is shut off.  Return all  other equipment to your locker and securely fasten your lock.  Wipe your bench with a wet paper towel.  Wash your hands and obtain your instructor's signature before you leave.