Most materials in our world are mixtures. Very few materials are pure substances. The art of separating mixtures is important because it enables us to isolate pure substances. Mixtures are either homogeneous or heterogeneous. Homogeneous mixtures are uniform in composition. Heterogeneous mixtures are not. Salt water is a mixture of water and NaCl and is homogeneous if thoroughly mixed, with all the salt dissolved. Oil in water is a heterogeneous mixture. Both types of mixtures can be separated into their component parts by physical means. A salt water mixture can be separated by distilling or evaporating the water and collecting the salt residue. An oil and water mixture will separate into an oil layer and a water layer because the materials are not attracted to one another and gravity "pulls" the denser water beneath the less dense oil. Settling, filtration, chromatography, and manual methods are all means of separating the components of a mixture. Choice of method depends on the type of mixture and the characteristics of its components.

A heterogeneous mixture of solid and liquid or solid and gas is usually fairly easy to separate because of the 2 different physical phases. The solid may settle out, allowing you to pour off the liquid. Or, maybe the liquid can be evaporated, leaving the solid behind. Or the mixture can be poured through a filter, catching the solid on the filter and allowing the liquid or gas to pass through. We use filtration frequently--in our coffee makers, automobile fuel lines, automobile air cleaners to name only a few examples.

A mixture of two or more solids is usually separated by utilizing the different chemical or physical properties of the substances. For example, a heterogeneous mixture of red M&M's and yellow jellybeans can be separated using the different colors or the different shapes of the solids. The parts of the mixture are large enough to be separated manually. A mixture of black peppercorns and white table salt might be separated this way as well. But what could be done with a mixture of sand and sugar? True, you could get a magnifying glass and tweezers and try picking out the grains of sand, but is there an easier way? Is there some property that sugar has that sand does not (or vice versa)? Could this be used to separate sand and sugar? If you said that sugar dissolves in water and sand does not, you are on the right track.

Homogeneous mixtures of a solvent and one or more solutes (dissolved substances) are often separated by chromatography. Chromatography works to separate a mixture because the components of a mixture distribute themselves differently when they are in contact with a "two phase system". One phase is stationary and the other is moving or mobile. The stationary phase may be a solid packed in a tube or a piece of paper. The mobile phase may be liquid of gaseous. Food colorings are one example, a homogeneous mixture of a solvent and a single dye or combination of selected dyes that produce the desired color.

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Chromatography

Paper chromatography is a modern method used separate mixtures. Paper chromatography uses paper as the stationary phase and a liquid solvent as the mobile phase.

You will use paper chromatography to test food colorings to see if the color results from a single dye or mixture of dyes. If you cannot get food coloring you can use colored felt pens or other water soluble coloring pens. Your exercise is simple but uses very essential principles. An optional part of the activity is to check a household product for the presence of the same dyes that are in food coloring.

The technique relies on the idea that the solvent and the paper both have an attraction for the components in a mixture. The solvent creeps along the surface of the paper. If a material is placed on one spot on the paper and is soluble in the liquid solvent, the material will be dissolved when the solvent moves over it. The material will move along with the solvent. Each compound in a mixture will have its own characteristic balance of attractions to solvent and to paper, so all will not move at the same speed. Eventually this difference in speed will separate the compounds.

In paper chromatography when the conditions are kept constant, a particular compound always travels a fixed percentage of the distance traveled by the solvent front. The ratio of the distance the compound travels to the distance the solvent travels is called the Rf value. The symbol Rf stands for "retardation factor" or "ratio-to-front". It is expressed as a decimal fraction. When the conditions are duplicated, the same average relative positions will turn up for the solvent and solute; thus the Rf value is a constant for a given compound. The Rf value is a physical property for that compound. The Rf value is useful in identifying compounds, but other properties should be used in combination with the Rf value to confirm compound identification. Since it is difficult for different laboratories to exactly duplicate conditions for a chromatography experiment, Rf values are more useful for comparisons within one lab than for comparisons of data from different labs.

Substances that are not colored can also be separated by chromatography. They are detected using ultraviolet or black light. These substances appear to glow in the dark.

This animation ( be sure to watch animation) shows what happens to a mixture of two dyes. The green and yellow dyes can be separated. The green dye moves along with the solvent better than the yellow dye. The mixture is separated into two "spots". There are two substances in the mixture. The R_f for the yellow dye is about 0.5 while the R_f for the green dye is about 0.8.The yellow dye moves half as far as the solvent front. Water can be used as a solvent but so can other liquids if need be. return to top of page

 

The illustration below shows the progress of the solvent and a solute containing only one dye. The solvent moves along the paper and the solute typically trails behind. The solute spot in the illustration appears to travel about half the distance of the solvent. The R_f value would be about 0.5 return to top of page

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Sample R_f determination

The location of the solvent frontabove is clear from the definition of the leading edge of the wet area marked by the solvent, 66 mm. The position of the solute is not so simple, especially when there is "tailing" or a spread of the solute spot. The position of the solute is taken to be where the "center or gravity" of the solute spot falls.The red dye moved faster. The distance traveled is 57 mm. The "center of gravity" marks the average position of the solute molecules. Even for the same compound, some molecules move faster and some slower. The distance moved by the slow dye is 32 mm.

The R_f value has a maximim value . The ratio of the distance travelled by the solute divided by the distance travelled by the solvent can NEVER exceed a value of "1".

The SOLUTE CAN NEVER LEAP AHEAD of the solvent.

 

Make a start line with pencil mark at half an inch from one end of the paper strip; this will be the bottom. Do this with each strip.

Components of a mixture can also be separated using methods of filtration and/or evaporation. If the evaporation is carried out so that the solvent is collected as well, the method is called distillation. In this activity you will make a mixture of salt and ground pepper. You will dissolve the salt portion of the mixture in water, filter to recover the pepper, and then evaporate the water to recover the salt. These methods are probably more familiar to you than is chromatography since we often filter liquid mixtures to remove undissolved particles. Can you name two examples of filtration (other than the ones already mentioned in this lab) that you encounter in your activities? List these on the Report Sheet, along with a brief description of each.

1/2 teaspoon ground black pepper

1/2 teaspoon table salt

2 jars or glasses, able to hold 2 cups of liquid each

white cone coffee filter

a funnel or the top you cut off the 2-L plastic bottle

spoon or other item for stirring

 Food color

 Colors observed in chromatogram for each dye(on paper)

 Blue

___________________________________________

 Yellow

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 Green

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 Red

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 Food color

 Mixture

Yes/ No

 How many dyes are in the food color?

 Blue

________

________________________

 Yellow

________

________________________

 Green

________

________________________

 Red

________

________________________

List the food colors that is/are mixtures of dye molecules?

Explain your choices using the results of your chromatography experiments.

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Do you think that other substances like vegetable dyes or inks could be tested using this chromatography method?

Justify your answer.

Explain briefly what would have to be done if the dyes or inks would not dissolve in water? Could chromatography still be used to separate the mixture?

Would a pure substance show more than one color or SPOT in a chromatogram? Explain

Data and calculations for Rf valuesreturn to top of page

If a food color contains only 1 dye, enter data for "fast dye" only.

If a food color contains only 2 dyes, enter data for "fast dye" and "slow dye" only.

 Blue food color solvent front distance mm

Yellow food color solvent front distance

Green food color solvent front distance

 Red food color solvent front distance

Distance traveled by Solvent Front all distances in mm

___________

_________

___________

___________

 Data for the "Fast Dye" in food coloring (colorin dye that travelled greatest distance)

 Blue food color

Yellow food color

Green food color

 Red food color

Color

Distance traveled in mm

___________

_________

___________

___________

 Rf value

___________

_________

___________

___________

Data for the "Middle Dye"in food coloring (colorin dye that travelled intermediate distance)

"Middle Dye"

Blue food color

Yellow food color

 Green food color

Red food color

 Color

Distance traveled

 Rf value

Data for the "Slow Dye" in food coloring (color in dye that travelled shortest distance)

 Color

Distance traveled

 Rf value

How does the Rf value for your "Fastest Dye" in this mixture compare to the Rf value for the same color dye molecule obtained when you tested the individual food colors? Give a numerical comparison. Does the Rf value seem to change? Are the very different or very similar?

What color is the solid that remains on your filter?

What do you think is the dominant substance in this material?

Was the filtration a physical or a chemical change? How can you tell the difference? Justify your answer.

Approximately how many teaspoons of dry solid are in your saucepan at the end of this experiment?

What is the most likely identity of this solid?

What happened to the water that you added to the salt and pepper mixture?

Was this a physical or a chemical change? Was this reversible? Justify your answer.

Name and describe two other examples of filtration.