Question

Background In this lab, we are going to learn how to use a powerful separation technique called chromatography which can separate compounds that are quite similar to one another. Chromatography is a general type of separation in which a mixture of compounds passes through a stationary phase. Different compounds have different relative affinities for the mixture traveling along the stationary phase, or the mobile phase, and the stationary phase, causing these compounds to separate from one another. More specifically, we’re going to use a technique called thin layer chromatography (TLC), which separates components of liquid mixtures based on polarity. Polarity refers to the separation of charge in a molecule – in other words, how unequally the electrons in a molecule are shared among the different atoms. Why do you think this might be important in chromatography? For this experiment, the stationary phase will be a coffee filter, while the mobile phase will be rubbing alcohol. Why do you think we are using rubbing alcohol and not water? The dyes from the coatings of M&M’s and Skittles will be carried varying distances along the coffee filter by the rubbing alcohol, allowing us to visualize the many different dyes that are used on the surface of each piece of candy. For this TLC experiment, the more polar the compound, the greater attraction it will have for the mobile phase and the farther up it will be carried by the mobile phase. Conversely, the more nonpolar the compound, the weaker the attraction it will have with the rubbing alcohol. Before we start, we should talk about what makes the mobile phase move in the first place. In TLC, the mobile phase moves uphill due to a phenomenon called capillary action, which we see in everyday life when a paper towel absorbs a liquid. Have you ever heard of the term capillary action? Capillary action occurs when a liquid moves uphill against gravity due to interactions between the stationary and mobile phases.

Questions:

1. What colors from the candy do you expect to see after you perform this experiment? Explain your answer.
2. Why do you think we are using rubbing alcohol as the mobile phase?
3. Why do you think you are using a pencil and not a pen when drawing on the coffee filter?

Answer 1

Answer 1:

You may have seen the colours according to what type of candy you have used.

But still I can say that you may see blue food coloring and on the other hand, has blue and red dyes; red food coloring has two different red dyes; and the yellow dye may be covered by the blue dye, making it appear green and all of this can depend on the exact food colorings used.  

Answer2:

Rubbing alcohol has a polarity which is almost relative to the stationary phase and mobile phase. It has polarity which is sufficient to carry some of the dyes along with coffee filter. But the polarity is not so high that it carry all the dyes rise along the filter paper.In this way, rubbing alcohol help to distinguish the various dyes.

Answer 3:

Pencil write with graphite.On the other hand, Pen writes with Ink, which is also a kind of dye. If we use pen in drawing on coffee filters,the ink of the pen would also rise along with the sample.Therefore, we will not be able to see the actual spot and and it leads to measure the elution distance. Because of this reason, pen is not used for drawing on coffee filters.