Question Video: Determining the Location of the Purest Ethanol Vapor in the Fractional Distillation of an Ethanol-Water Solution | Nagwa Question Video: Determining the Location of the Purest Ethanol Vapor in the Fractional Distillation of an Ethanol-Water Solution | Nagwa

Question Video: Determining the Location of the Purest Ethanol Vapor in the Fractional Distillation of an Ethanol-Water Solution Chemistry • First Year of Secondary School

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A mixture of ethanol and water is separated by fractional distillation. The thermometer reading initially rises and then stays steady at around 78°C, the boiling point of ethanol, while the ethanol in the mixture boils. During this phase of the distillation, where would you expect to find the purest ethanol vapor? [A] Toward the top of the column [B] In the middle of the column [C] At the bottom of the column [D] Spread equally throughout the column

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Video Transcript

A mixture of ethanol and water is separated by fractional distillation. The thermometer reading initially rises and then stays steady at around 78 degrees Celsius, the boiling point of ethanol, while the ethanol in the mixture boils. During this phase of the distillation, where would you expect to find the purest ethanol vapor? (A) Toward the top of the column. (B) In the middle of the column. (C) At the bottom of the column. (D) Spread equally throughout the column.

In general, distillation is a technique used to separate mixtures into their components based on their boiling points. Boiling is the physical change that converts a liquid into a gas. During heating the liquid with the lower boiling point will boil first. It will turn into a gas and can be condensed back into a liquid and collected. The mixture in this question contains ethanol and water. Ethanol’s boiling point is 78 degrees Celsius, and water’s boiling point is 100 degrees Celsius. Therefore, during distillation, ethanol will boil and turn into a gas first.

The difference between the boiling points of ethanol and water is pretty small. To separate mixtures in which the components have a small difference in boiling point, a type of distillation called fractional distillation should be used. Fractional distillation is a type of distillation where the parts of a mixture are separated in sequence by heating the mixture, passing the vapors through a fractionating column, condensing the vapors, and then collecting the condensed liquid.

In the setup shown here, the ethanol water solution is heated in the distillation flask. A fractionating column is attached to the top of the distillation flask, and a thermometer is placed at the very top of the fractionating column with the bulb just in front of the opening to the condenser.

We are told in the question that once heating of the mixture has begun, the temperature reading of the thermometer initially rises. We also know that eventually the thermometer reading reaches 78 degrees Celsius, which is the boiling point of ethanol. The temperature remains constant at 78 degrees Celsius for a while. During this phase, the ethanol is boiling in the mixture and the vapors are rising up from the distillation flask and into the fractionating column.

As the vapors rise into the column, they condense on the glass beads and form droplets of liquid. As more vapors move into the column, the droplets of liquid heat up and turn into vapor again. This condensation and evaporation cycle will repeat over and over as the vapors continue to move up the fractionating column.

Each time evaporation repeats, the vapors are more pure and contain more ethanol until at the very top of the fractionating column, the vapors will be the purest. These ethanol vapors then pass into the condenser where they are cooled, at which point they fully condense and are collected as a liquid in the receiving flask.

We have learned that while the temperature remained constant at 78 degrees Celsius, the ethanol in the mixture was boiling and its vapors were passing through the fractionating column. As the ethanol vapors condensed and reevaporated, moving up the fractionating column, the more pure they became. Therefore, when looking at the answer choices, answer choice (A) is correct. During the phase of distillation, in which the temperature remains constant at 78 degrees Celsius, the purest ethanol vapor would be found toward the top of the column.

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