The vapours with the highest boiling point will condense in the lower-most portion of a fractional distillation column.
This is because the lower portions of the column are cooler. As the vapours rise up the column, the higher boiling fractions will condense first at the lower, cooler portions of the column. The lower boiling fractions will continue up to the higher, warmer portions of the column before they condense.
So the correct answer is B: In the lower-most portion.
The reasoning is:
1) Fractional distillation separates petroleum into fractions based on boiling point.
2) The temperature decreases going down the distillation column. The higher up the column, the warmer the temperatures. The lower down the column, the cooler the temperatures.
3) Vapours will condense when they reach temperatures at which they can condense. Higher boiling vapours require lower (cooler) temperatures to condense.
4) Therefore, the higher boiling vapours will condense at the lower portions of the column where temperatures are cooler. The lower boiling vapours will continue up to warmer portions of the column to condense.
5) So the highest boiling point vapours will condense in the lower-most portion of the fractional distillation column.
The other options can be ruled out:
A) The upper-most portion is the warmest, so the highest boiling fractions would not condense there.
- C) The middle portion would still be too warm for the highest boiling fractions to condense.
- D) We can determine the answer based on the reasoning above.