Distillation (AQA GCSE Chemistry Combined Science): Revision Notes
Distillation
Distillation is a method used to separate liquids from mixtures. It works by heating the mixture until one liquid turns into vapour, then cooling the vapour back into a liquid to collect it separately.
What happens during distillation
When you heat a mixture, the liquid with the lowest boiling point will turn into vapour first. This vapour then travels through a condenser, where it cools down and turns back into liquid. This collected liquid is called the distillate.
The key to distillation is that different liquids have different boiling points. By carefully controlling the temperature, we can separate them one by one as they turn into vapour at their specific boiling temperatures.
Simple distillation
Simple distillation separates two different substances. You can use it to:
- Separate two liquids with different boiling points
- Separate a liquid from a solid dissolved in it (like getting pure water from salt water)
Worked Example: Separating Salt Water
Step 1: Heat the salt water in the distillation flask Step 2: Water vapour (boiling point 100°C) rises first, leaving salt behind Step 3: Water vapour travels through the condenser and cools back to liquid water Step 4: Collect the pure water (distillate) in a separate container
How the equipment works
The condenser has two tubes - one inside the other. Cold water flows through the space between these tubes. This keeps the condenser cold so the vapour can turn back into liquid. The cooling water doesn't mix with the substance being separated.
The condenser is essential because it provides the cool surface needed to turn vapour back into liquid. Without proper cooling, the vapour would just escape into the air.
Key point to remember
In any mixture, the liquid with the lower boiling point will always come out first as the distillate.
Fractional distillation
Fractional distillation separates mixtures of more than two liquids. It uses similar equipment to simple distillation, but with an extra piece called a fractionating column.
How it works differently
The fractionating column is packed with glass beads. As vapours rise up through this column, they cool down at different heights. This means different liquids condense and can be collected separately.
The different liquids leave the top of the column in order of increasing boiling point - so the liquid with the lowest boiling point comes out first, then the next lowest, and so on.
Worked Example: Separating Crude Oil
Step 1: Heat the crude oil mixture in the distillation flask Step 2: Vapours rise through the fractionating column packed with glass beads Step 3: Different fractions condense at different heights:
- Petrol (lowest boiling point) - collected first
- Kerosene (medium boiling point) - collected second
- Diesel (higher boiling point) - collected last Step 4: Each fraction is collected separately as pure liquids
Safety considerations
Critical Safety Requirements:
When distilling flammable substances like ethanol, always use an electrical heater instead of a Bunsen burner. This prevents the risk of fire or explosion.
You should also add anti-bumping granules to the mixture to prevent violent boiling that could cause the apparatus to break.
Key differences
| Simple distillation | Fractional distillation |
|---|---|
| Separates 2 substances | Separates more than 2 liquids |
| No fractionating column | Has a fractionating column with glass beads |
| Simpler setup | More complex setup |
Key Points to Remember:
- Distillation separates liquids based on their different boiling points
- The liquid with the lowest boiling point always comes out first
- Simple distillation is for separating two substances
- Fractional distillation is for separating more than two liquids using a special column
- Always use electrical heating with flammable liquids for safety
- The condenser cools vapour back into liquid using cold water flowing between two tubes