GCSE Chemistry revision

Properties of small molecules

How bonding and structure are related to the properties of substances

AQA 4.2.2.4 Foundation & Higher

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Simple distillation is a technique used to separate a liquid from a solution, for example separating pure water from seawater.
When the solution is heated, the desired liquid evaporates, travels through a condenser, cools back into liquid form, and is collected in a beaker.
Simple distillation works well when there is only one volatile liquid in the mixture, such as water in seawater, leaving behind dissolved solids like salt.

1. Introduction AQA

Simple distillation is a technique used to separate a liquid from a solution, for example separating pure water from seawater.
When the solution is heated, the desired liquid evaporates, travels through a condenser, cools back into liquid form, and is collected in a beaker.
Simple distillation works well when there is only one volatile liquid in the mixture, such as water in seawater, leaving behind dissolved solids like salt.

A flask holds the solution and is sealed with a bung to prevent gases from escaping, with a thermometer inserted to monitor temperature.
A condenser consists of an inner pipe surrounded by a water jacket through which cold water continuously flows, entering at the bottom and exiting at the top.
A heating device such as a Bunsen burner is placed beneath the flask to heat the mixture, and a beaker is positioned at the end of the condenser to collect the purified liquid.

2. Equipment for Simple Distillation AQA

A flask holds the solution and is sealed with a bung to prevent gases from escaping, with a thermometer inserted to monitor temperature.
A condenser consists of an inner pipe surrounded by a water jacket through which cold water continuously flows, entering at the bottom and exiting at the top.
A heating device such as a Bunsen burner is placed beneath the flask to heat the mixture, and a beaker is positioned at the end of the condenser to collect the purified liquid.

Simple distillation cannot effectively separate mixtures of liquids that have similar boiling points, such as methanol, ethanol, and propanol.
When multiple liquids have similar boiling points, they evaporate at the same time, meaning the collected liquid will not be a pure substance.
In cases where liquids have similar boiling points, fractional distillation must be used instead.

3. Limitations of Simple Distillation AQA

Simple distillation cannot effectively separate mixtures of liquids that have similar boiling points, such as methanol, ethanol, and propanol.
When multiple liquids have similar boiling points, they evaporate at the same time, meaning the collected liquid will not be a pure substance.
In cases where liquids have similar boiling points, fractional distillation must be used instead.

Fractional distillation is the main technique used to separate mixtures of liquids that have similar boiling points.
The key difference from simple distillation is the addition of a fractionating column positioned between the flask and the condenser.
The fractionating column is packed with glass beads or rods, providing a large surface area, and is cooler at the top than at the bottom.

4. What is Fractional Distillation? AQA

Fractional distillation is the main technique used to separate mixtures of liquids that have similar boiling points.
The key difference from simple distillation is the addition of a fractionating column positioned between the flask and the condenser.
The fractionating column is packed with glass beads or rods, providing a large surface area, and is cooler at the top than at the bottom.

As vapours rise up the fractionating column, liquids with higher boiling points condense on the cool glass rods and fall back into the flask before reaching the condenser.
Only the liquid with the lowest boiling point remains as a vapour all the way to the top of the column and passes into the condenser to be collected.
This process ensures that only one pure liquid is collected at a time, achieving effective separation of the mixture.

5. How the Fractionating Column Works AQA

As vapours rise up the fractionating column, liquids with higher boiling points condense on the cool glass rods and fall back into the flask before reaching the condenser.
Only the liquid with the lowest boiling point remains as a vapour all the way to the top of the column and passes into the condenser to be collected.
This process ensures that only one pure liquid is collected at a time, achieving effective separation of the mixture.

To separate methanol, ethanol, and propanol, the mixture is first heated to approximately 65°C so that methanol, which has the lowest boiling point, evaporates and is collected as a pure liquid.
The temperature is then raised to approximately 78°C to evaporate the ethanol, which passes through the fractionating column and is collected separately.
Finally, the temperature is raised further to boil off the propanol, leaving each component collected as a pure substance.

6. Fractional Distillation Step by Step AQA

To separate methanol, ethanol, and propanol, the mixture is first heated to approximately 65°C so that methanol, which has the lowest boiling point, evaporates and is collected as a pure liquid.
The temperature is then raised to approximately 78°C to evaporate the ethanol, which passes through the fractionating column and is collected separately.
Finally, the temperature is raised further to boil off the propanol, leaving each component collected as a pure substance.

Simple distillation is suitable for separating a liquid from a solution containing a dissolved solid, whereas fractional distillation is used for separating mixtures of liquids.
Fractional distillation requires a fractionating column in addition to the standard distillation apparatus, which allows liquids with similar boiling points to be separated.
Both techniques rely on the principles of evaporation and condensation, but fractional distillation provides greater precision when boiling points are close together.

7. Key Differences Between Simple and Fractional Distillation AQA

Simple distillation is suitable for separating a liquid from a solution containing a dissolved solid, whereas fractional distillation is used for separating mixtures of liquids.
Fractional distillation requires a fractionating column in addition to the standard distillation apparatus, which allows liquids with similar boiling points to be separated.
Both techniques rely on the principles of evaporation and condensation, but fractional distillation provides greater precision when boiling points are close together.