2002ExpQ - page 6 of 21

6 of 21

Q-6

The difference in solubility characteristics allows for the separation of halogens and halides. If an aqueous

solution that contains both halogens and halides, is mixed with a less polar solvent (e.g. dichloromethane),

then the halide ions will be found in the aqueous layer and the halogens will be found primarily in the

nonpolar or less polar layer.

Some organic solvents, like dichloromethane, are not miscible with water. Water and CH2Cl2 form two

layers. You will observe these two layers in the test tube when you perform Part II of this experiment.

The upper layer is an aqueous layer because water has a lower density than dichloromethane. The bottom

layer is the dichloromethane layer.

Each halogen has a distinctive color in CH2Cl2. Specifically, iodine is purple, (pink if dilute), bromine is

orange, (yellow if dilute) and chlorine is almost colorless (very pale yellow if concentrated) in CH2Cl2.

The charged halide ions, are not soluble in CH2Cl2, and remain in the aqueous layer. Halides are

essentially colorless in aqueous solution. Consequently, if a reaction of a halogen or halide ion takes place

in aqueous solution, and the solution is then shaken with CH2Cl2, any free halogen will dissolve in the

CH2Cl2 but the halide will not. The presence of a halogen can be detected by observing the color of the

CH2Cl2 layer. Since each halogen has a distinct color in CH2Cl2, comparison of the color of the CH2Cl2

layer after reagents are allowed to react will allow for the identification of any halogen in the

dichloromethane. For instance, if I

(aq)

and Br

2(aq)

are allowed to react, two possible results may be

expected - orange CH2Cl2 layer or purple CH2Cl2 layer. If the CH2Cl2 layer is orange, this suggests that

no reaction occurred (i.e. Br2 is a reactant and remains unchanged after time for reaction has been

allowed). If the CH2Cl2 layer is purple, this suggests the formation of I2. Since I

(aq)

is a reactant, and I2 is

detected as a reaction product, a redox reaction is assumed to have occurred (2I

I2 + 2 e

). This

procedure will be used to determine the order of substitution among the halogens, and to place them in the

displacement series obtained by voltaic cell measurements.