TLC is a laboratory technique commonly used to separate components of a mixture. Mixtures are placed on the TLC plate (stationary phase), which is then transferred to a jar containing the solvent. The solvent travels through the plate and carries components of the mixture along with it. Based on its properties, each component is dragged to different distances on the plate. The relative distances travelled by each component can be used for separation and identification.

It is important to place a lid on the jar because the solvent will be a volatile substance. An open system will allow for the solvent to evaporate from the TLC plate and reduce the amount of solvent travelling through the plate. The solvent in the solution will evaporate, but it is negligible and inconsequential to the data collected on the TLC plate. 

Thin layer chromatography (TLC) is used to separate components in a mixture. Components are separated on a TLC plate because each component travels a different distance. The distance travelled depends on several factors. One of those factors is polarity; therefore, TLC can used to determine polarity of substances.

TLC is not useful for identifying substances. Other techniques such as NMR and IR spectroscopy are useful for identifying individual components. The R_f value of a compound can help identify it in a broad sense, but this technique provides only very rudimentary identification capabilities.

To solve this problem we need to recall the definition of R_f.

The solvent is typically the most mobile substance on a TLC plate; therefore, it travels the farthest distance. This means that R_f is always less than one because the numerator of R_f is always smaller than the denominator. The question states that the R_f value is 2. Since this is greater than 1, the results don’t seem valid. This value indicates that the solute compounds were able to teravel farther on the TLC plate than the solvent (mobile phase).

Compounds A and B are in the same mixture; therefore, they will be separated using the same TLC plate. Recall the definition of R_f.

Since the same TLC plate was used, the denominator for both compound A and B will be the same (the total distance travelled by solvent alone is the same for a TLC plate). To separate them effectively, compounds must have travelled different distances along the plate. The best separation occurs when the difference between the distances travelled by the compounds is greatest.

Let’s assume the denominator to be 1 for both compounds. This means that the numerator is equal to the R_f value. If we compare the given R_f values with each other we realize that R_f values of 0.4 and 0.2 will give the biggest difference between the distances travelled by compounds A and B (difference is ); therefore, this will have the best separation.