A flask contains a mixture of isohexane and 3-methylpentane. One of the liquids boils at 63$$^\circ$$C while the other boils at 60$$^\circ$$C. What is the best way to separate the two liquids and which one will be distilled out first?

We have two hydrocarbons in the same flask: isohexane and 3-methylpentane. Their normal boiling points are given as $$60^{\circ}\text C$$ and $$63^{\circ}\text C$$ respectively.

First, let us examine the temperature data. Writing the temperatures in an inequality form, we obtain

$$60^{\circ}\text C \; < \; 63^{\circ}\text C.$$

Now we calculate the difference in their boiling points:

$$\Delta T \;=\; 63^{\circ}\text C \;-\; 60^{\circ}\text C \;=\; 3^{\circ}\text C.$$

This small temperature gap is crucial. In laboratory practice, the rule of thumb is that:

If } \Delta T \;\lt\; 25^{\circ}\text C,\; \text{use fractional distillation.

Fractional distillation provides a fractionating column that offers many theoretical plates, giving repeated condensation-evaporation cycles. These cycles allow liquids whose boiling points are very close to be separated effectively. Simple distillation, on the other hand, is suitable only when the boiling-point difference is fairly large (usually $$\ge 25^{\circ}\text C$$), so it would not work efficiently here.

Therefore, the best separation technique is fractional distillation.

Next, we identify which component will come out first. The general statement is:

The liquid with the lower boiling point vaporizes (and hence distills) first.

From the inequality $$60^{\circ}\text C < 63^{\circ}\text C,$$ we see that isohexane (boiling at $$60^{\circ}\text C$$) has the lower boiling point, whereas 3-methylpentane (boiling at $$63^{\circ}\text C$$) has the higher boiling point.

Consequently, isohexane will rise through the fractionating column first and will be collected in the distillate before 3-methylpentane.

Putting both conclusions together, we state: the mixture should be separated by fractional distillation, and isohexane will be the first liquid to distill.

Hence, the correct answer is Option A.