Fractional Distillation

Introduction

An example of a simple distillation is the separation of a solution of salt and water into two separate pure substances. When the salt water solution is heated to boiling, water vapor from the mixture reaches the condenser and the cold water circulating around the inside tube causes condensation of water vapor into droplets of liquid water. The liquid water is then collected at the lower end of the condenser. The non-volatile salt remains in the flask.

In this experiment, the initial mixture you distill contains two volatile liquids: ethanol and water. In this distillation, both of the liquids will evaporate from the boiling solution. Ethanol and water have normal boiling temperatures of 79°C and 100°C, respectively. One objective of the experiment is to observe what happens when a liquid-liquid mixture is heated and allowed to boil over a period of time. Throughout the distillation, volumes of distillate, called fractions, will be collected. The percent composition of ethanol and water in each fraction will be determined from its density. Water has a density of 1.00 g/cm³ (at 20°C) and ethanol has a density of 0.79 g/cm³ (at 20°C). The fractions you collect will have densities in this range.

Objectives

In this experiment, you will

• Observe what happens when a liquid-liquid mixture is heated and allowed to boil over a period of time.
• Determine percent composition of ethanol and water in the fraction from its density.