Separation of a Mixture of Solids

Read the entire experiment and organize time, materials, and work space before beginning.

Remember to review the safety sections and wear goggles when appropriate.

Objective: To become familiar with the separation of mixtures of solids.

Materials: Student Provides: Distilled water

2 Coffee cups 2 Small paper or Styrofoam® cups

2 Sheets of paper Small spoon or stirrer

Small saucer Piece of plastic wrap or a plastic baggie

Crushed ice Paper towels

From LabPaq: Goggles

Magnet 100-mL Beaker

Funnel Burner stand

Graduated cylinder Burner fuel

Digital scale

From Experiment 4 Bag: Mixture of solids

Circular filter paper Plastic weighing dish

Discussion and Review: Many materials are actually mixtures of pure substances.

How to separate mixtures into their component substances is a frequent problem for

chemists. The essential distinction between mixtures and “pure” substances is whether

or not they can be separated by physical means. Physical means of separation are

those techniques that utilize the physical properties of a substance such as melting

point and solubility.

In this experiment you will separate a mixture of four substances: sodium chloride

(NaCl, table salt); benzoic acid (C6H5COOH, a common food preservative); silicon

dioxide (SiO2, sand); and iron (Fe, powder and/or filings) into pure substances. The

separation will be accomplished by utilizing the unique properties of each material and

their differences in water solubility.

Solubility is defined as the amount of the solute that will dissolve in a given amount of

solvent. The extent to which a substance dissolves depends mainly upon the physical

properties of the solvent and of the solute and to some extent upon the solvent’s

temperature. Sodium chloride (table salt) is an ionic substance that dissolves readily in

cold water. Benzoic acid is a polar covalent compound that is only slightly soluble in

cold water but is very soluble in hot water. The Handbook of Chemistry and Physics

reflects that the solubility of benzoic acid in water is 6.8 g/100ml at 95°C and only 0.2

g/100ml at 10°C.

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Table of Solubility of NaCl in Water at Various Temperatures:

Temp in degrees C 0 10 20 30 40 50 60 70 80 90 100

Grams/100 mL H20 35.7 35.8 36.0 36.3 36.6 37.0 37.1 37.5 38 38.5 39.2

From the above information and data, it is apparent that sodium chloride and benzoic

acid can be dissolved at different temperatures in water. Further, we know that sand is

not soluble in water. That leaves the iron filings or iron powder, but we also know that

iron filings can be easily removed by a magnet. With this information we can now devise

a plan or flow chart on how to separate the mixture.

PROCEDURES: Some sections of this experiment may require several days for

drying and evaporation. Plan your time accordingly. Remember to completely read all

instructions and assemble all equipment and supplies before beginning your work.

Before you start the actual separation, challenge yourself. Think about and prepare a

flow chart on how the four substances might be separated. Then read the instructions

and compare your proposed procedure or flow chart to the one presented here.

1. Separating out the Iron:

A. Use your digital scale to determine the mass of your weighing dish.

B. Empty the entire mixture of solids from the plastic bag into the weighing dish and

determine the gross mass of the total mixture and weighing dish. Compute the

net mass of the mixture: this is equal to the gross mass of the weighing dish with

the mixture less the mass of just the weighing dish determined in 1-A.

C. Spread the mixture into a very thin layer over a full sized piece of paper.

D. Cut a second piece of paper into a 10-cm square. Weigh and record its mass and

set it aside.

E. Wrap a small square of clear plastic over the magnet. Remove the iron

powder/filings by passing the magnet closely over the surface of the entire

mixture. Repeat several times to make sure you’ve collected all the iron.

F. Holding the magnet over the 10-cm square of paper, carefully remove the plastic

and allow all the iron to fall onto the paper. Weigh and determine the net mass of

the iron powder/filings.

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2. Separating out the Sand:

A. Put the remaining mixture, containing sand, benzoic acid, and table salt into your

beaker and add 50 mL of distilled water.

B. Set up the beaker stand and burner fuel and heat the beaker of solids and water

to near boiling. Stir the mixture to make sure all soluble material dissolves. At this

point, the benzoic acid and the sodium chloride should have dissolved and been

extracted from the insoluble sand.

C. Decant (pour) the liquid while it is hot into a small paper or Styrofoam® cup.

D. Pour another 10 to 15 mL of distilled water into the beaker containing the sand,

bring the mixture to a boil, and decant again into the same cup used in 2-C. This

assures that any remaining salt and benzoic acid is removed from the sand.

E. Make an ice bath by placing a small amount of crushed ice and tap water into a

coffee cup or similar container that is large enough to hold your paper cup of

benzoic acid and salt solution. Make sure the ice bath level is higher than the

solution level but low enough so that no additional water can pour into the

solution cup.

F. Place the cup containing the water solution of benzoic acid and salt into the ice

bath. Observe the benzoic acid crystallizing out of the solution as it cools. Set

this water bath assembly aside until the next section.

G. Heat the sand in the beaker over low heat until the sand is completely dry. Sand

has a tendency to splatter if heated too rapidly. The possibility of sample loss can

be reduced by covering the beaker with a small saucer and heating it very slowly.

You might accomplish this also by placing the beaker in a warm oven.

Alternatively, you may dump the wet sand onto a double layer of paper towels

and let it air-dry.

H. When the sand is completely dry allow the beaker to cool to room temperature.

I. After the sand and any paper towels used are completely dry transfer the sand to

a weighing dish of known mass and determine the net mass of the sand.

3. Separating out the Benzoic Acid:

A. The benzoic acid crystals from Step 2-F above can be separated out by filtration.

Use the following instructions to set up a filtration assembly:

1) Weigh a paper cup and record the weight (mass).

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2) Set the paper cup inside a slightly larger coffee cup or

similar container to give the paper cup support and

prevent it from tipping over when you add a funnel.

3) Fold a sheet of filter paper in half and then in half again

as illustrated. Weigh it.

4) Open one section of the folded filter paper as shown in

the bottom illustration.

5) Place the opened filter paper into the funnel and the

funnel into the paper cup supported by the coffee cup.

B. Remove the paper cup of salt and benzoic acid crystals

from Step2-F from its ice bath. Fill a graduated cylinder with about 5 mL of

distilled water and place the cylinder in the ice bath to chill the distilled water.

C. Swirl the cup containing the salt and benzoic acid crystals to dislodge any

crystals from the sides. Then, while holding the filter paper in place and open,

pour the contents of this cup into the filter paper-lined funnel.

D. After the sodium chloride solution has fully drained through the filter paper, slowly

pour 2 to 5 mL of chilled distilled water around the inside surfaces of the filter

paper-lined funnel to make sure all the sodium chloride has been removed from

the benzoic acid crystals.

E. After all the liquid has drained from the funnel lay the filter paper containing the

benzoic acid crystals on folded layers of paper towels and put this someplace

where it will not be disturbed while the filter paper and its contents air dry.

Depending upon the humidity in your area this can take several hours or days.

F. When the filter paper containing the benzoic acid crystals is completely dry,

weigh it and subtract the weight of the filter paper to obtain the net weight of the

benzoic acid crystals.

NOTE: A very small amount of benzoic acid may not have precipitated but rather

may have remained in the salt solution and have passed through the filter paper.

This will cause a very small experimental error in your final results.

4. Separating out the Salt:

A. Remove the funnel from the above filtration assembly and set the paper cup of

sodium chloride solution someplace where it will not be disturbed while the water

evaporates. Depending upon the humidity in your area this might take several

days. When all the water has completely evaporated only sodium chloride will be

left in the paper cup.

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B. Weigh the paper cup with the dried salt crystals inside and then subtract the

weight of the cup to get the net weight of the table salt.

C. Prepare a data table listing the various components of the mixture and record

both their masses in grams to at least 1 decimal place, (i.e., .1 or 1/10th of a

gram) and their percentage of the total mixture.

Sample data table:

Grams Percent of mixture

Iron filings .8 .8/4 * 100 = 20 %

Sand 1.4 1.4/4 * 100 = 35 %

Table salt 1.2 1.2/4 * 100 = 30 %

Benzoic acid .6 .6/4 * 100 = 15 %

Total 4.0 100 %

Questions:

A. How did your proposed procedures or flow charts at the beginning of this experiment

compare to the actual procedures of this lab exercise?

B. Discuss potential advantages or disadvantages of your proposed procedure

compared to the one actually used.

C. What were potential sources of error in this experiment?

Cleanup: Thoroughly clean, rinse, and dry all equipment and return it to the LabPaq.

Throw all used paper cups and paper towels in the trash.