1. In Thin Layer Chromatography (TLC), increasing the polarity of the developing solvent (eluent) will make it to adhere more to the adsorbent (silica gel) which lower the Rf values of the compound. This happens because, the distance travelled from the base will be smaller.

2. One way to visualize colorless compounds is by exposing the TLC plate to iodine (I₂) vapor. This technique can be done by placing iodine crystals to a TLC chamber or by adding iodine crystals to a chamber which contains silica or alumina. The TLC plate is placed on the chamber and the iodine sublimes and reacts with the compounds in the plate forming yellow-brown spots. Coloration and formation of the yellow-brown spots can be seen within seconds in some compounds while it would take 10 minutes or longer for other compounds.

3. a. TLC (thin-layer chromatography)

b. TLC (thin-layer chromatography)

c. column chromatography

Step-by-step explanation

1. Remember that low polarity compounds have higher Rf values while higher polarity means that the Rf values are lower because the absorptivity of a compound increases when polarity increases.

2. The iodine is used in visualizing colorless compounds because it causes colorization when reacted with many organic compounds.

3. a. TLC is used more frequently as an analytical tool rather than for purification. It also works best for very small amount of compounds.

b. The solvent front on TLC moves downward because of the gravitational force.

c. Column chromatography is used as preparative method because it can isolate a compound in a quantity and at a purity level that can be used for further experiments. It is commonly used to purify liquids and also solids. TLC is more on analytical methods not on purification.