How to Compose a Distillation Curve

How to Compose a Distillation Curve
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When you ferment fruit to make alcohol, you can distill the liquid mixture to isolate parts of it. This method of distillation takes advantage of the different compositions that make up the liquid in a process like fermentation. Chemists make great use of these processes for purifying solvents and other products of liquid reactions, including separating the components of crude oil.

Distillation Apparatus

Distillation graphs show you the quantities measured through distillation experiments that separate constituents of liquids. These experiments use ​fractional distillation columns​ consisting of a column that lets liquid drip into a round-bottom flask with a thermometer at the top of the column to determine the vapor's temperature.

A diagonal liquid chamber connects to a point along the fractional column near the top that extends away from the chamber. This creates a surface area on which the vapor can condense and collect in an external flask.

Through the distillation setup from a simple distillation diagram, a liquid boils into a gas, condenses back into a liquid and continues this process until the liquid you want to distill gathers in the external flask. The apparatus works by heating the liquid that collects in the flask such that the fractional column tells you the vapor pressure of the gas form of the liquid mixture.

The thermometer at the top should read the boiling point of the liquid. The external flask lets the liquid collect that you want to distill and also serves as a vent so that the apparatus doesn't break by overheating.

Control the temperature very carefully by maximizing the contact between the liquid that drips back into the round-bottom flask and the vapor that rises through the fractional column. Sometimes the fractional column has glass beads or levels protruding from the internal sides to maximize the surface area of contact. Keep track of the temperature using the thermometer to figure out the temperature at which this happens. You should end up with the vapor pressures of the liquids in the mixture.

The apparatus setup guarantees that the vapor pressure of the compound with a lower boiling point in the mixture is greater than the vapor pressure of the one with a higher boiling point. This also lets you define boiling point as the temperature at which the vapor pressure equals atmospheric pressure for a liquid in an open container. This is the lowest temperature at which the liquid form of the mixture or compound boils into a gas. These methods of fractional distillation make them useful in industrial settings for manufacturing chemical compounds.

Simple Distillation Graph

You can also use the fraction of the gas that is distilled as a mole fraction to plot a graph of the temperature of the liquid, the liquid-vapor mixture and the vapor itself to determine the boiling point of the two or more components of the compound. Many distillation apparatus setups will automatically measure the temperature throughout the heating of the experiment. This can give you a continuous set of data points over time that can easily be graphed using Excel or some other software.

The curve tells you this because, as the vapor heats up and passes through the fractional column, it should separate into the two separate mixtures of liquids and gases. By recording the temperature throughout the distillation process, you can figure out what the compounds actually are based on the boiling point.

Or you can use the same process to determine the boiling point of a known compound. The process is limited, though, by the temperatures you can achieve with the heat source affecting the round-bottom flask.

Volume vs Temperature

The simple distillation graph should show you a distillation graph of volume versus temperature of the mixture with the points at which the temperature of both or all gases intersect locate the boiling point of each component of the gas. This composition curve lets you figure out the appropriate apparatus setup and temperature to separate the gas or liquid mixture. You can experiment with different types of fractional columns to figure out which one gives you the clearest idea of boiling point for the constituents.

The simple distillation graph follows simple distillation theory. ​Simple distillation​ means the gas condenses into liquid once so you need to perform it on liquids or gases that have boiling points far enough from one another to discern them.

Using multiple steps of condensation is called ​fractional distillation​, and, in this case, you'd use a fractional distillation graph of volume vs. temperature. You can extrapolate to figure out theoretical setups for other liquids and mixtures because having more beads or plates in the setup should theoretically improve the separation method while increasing the time taken to separate the mixture.

Simple Distillation Theory

Mixtures that distill through experiments don't produce pure samples, but result in impurities in the different mixtures that you measure. This means you can use equations to explain experimental results from distillation as well as from predictions based on previously established data about the composition of gases and liquids. Raoult's law and Dalton's law give you ways of measuring these proportions of simple distillation theory.

The precise composition of that vapor that switches between boiling and condensing follows ​Raoult's law​, which states that the vapor pressure of a compound decreases when it's in a solution and can be related to the molar composition. The equation


tells you that the partial pressure of a certain component A ​PA​ is produced for the percent of the component ​PoA and the mole fraction of A "chi" ​χA.

The partial pressure is the pressure that a constituent gas of a mixture would have if it had the entire volume of that mixture at the same temperature. This lets you determine how much of a gas should be present if you know the mole fraction before hand.

You can then, use ​Dalton's law​ which states that the total pressure of a gas mixture equals the sum of the partial pressures that make it up. The theory of how particles of gas move and interact with one another explains this.

You can describe the vapor pressure of a compound using the temperature of the solution and the boiling point of the compound because, when temperature increases, more of the gas molecules will have enough kinetic energy to strike one another in an appropriate orientation to let the reaction occur. They need this to overcome intermolecular forces that would hold the particles together in the liquid phase.

Distillation in Industry

In addition to research on the boiling point and gaseous properties of compounds, distillation finds itself useful in many applications across industry. It's used in studying and forming reactions between oil, water and other components such as methane that are used in fuel. Food scientists and manufacturers can use it to make liquor, beer and different types of wine. Distillation techniques have found practical use in industries of cosmetics, pharmaceutical drugs and other chemical manufacturing methods.

The technique is even used in light bulbs for preventing the tungsten filament from being damaged and providing glow in the light bulbs. They do this by separating the air to produce the gases necessary to manufacture light bulbs. These distillation methods follow theory and experimental methods for separation.