1. An old emoji and an even older technique
Have you ever wondered what the odd-looking alembic flask ⚗️ is doing in the emoji keyboard? And how did it get there way before something as ubiquitous as the bubble tea? You are not alone. Indeed, no one uses the flask now. But it was an important equipment in the past that alchemists used for distillation.
They would heat a mixture in the alembic flask to vaporise the liquid inside. The vapour would then flow through a tube, where it would condense and drip into another container. This essentially separates the liquid from the mixture! We call the separated liquid the distillate.
Distillation is a method to separate and purify a liquid from a mixture by a process of boiling and condensation.
2. Modern distillation requires modern apparatus: the condenser
In the old days, alchemists had to manually cool the tube by pouring cold water onto it, such that condensation could continue to take place. This was not fun. The lazy (and rich) ones even hired an attendant to do it.
Now, this cooling has been ‘automated’ by a modern laboratory equipment: the condenser. In school laboratories, the condenser is kept cold by allowing cold tap water to run through it continuously. To maximise heat transfer, tap water should flow in the opposite direction, from the bottom of the condenser to the top.
The cold tap water does not mix with the hot vapour, as they flow through two separate channels in the condenser.
The condenser provides a cold surface to cool and condense the vapour into a liquid distillate.
3. Monitoring distillation with a thermometer
We can use a thermometer to monitor the progress of distillation. This works best when the bulb of the thermometer is placed right beside the opening to the condenser, so that it measures the temperature of the vapour.
Distillation begins when the temperature reaches the boiling point of the liquid. For example, in the separation of water from sea water, water is distilled over and collected when the temperature is 100 °C.
The constant temperature during distillation also shows that the distillate is a pure substance with a fixed boiling point.
4. Distillation works because of a huge gap in boiling point
Distillation works best to separate the solvent of a solution. This is because the liquid solvent has a much lower boiling point than the solid solute, allowing it to be collected first.
A common example is sea water, which is a mixture of water and sodium chloride. Water boils at 100 °C while sodium chloride only melts at 801 °C and boils at 1465 °C. Therefore, when we heat sea water, only water will boil to form a pure vapour. Sodium chloride will remain behind in the round-bottom flask as a solid.
What if the mixture contains two liquids with very similar boiling points? This will require a modified form of distillation called fractional distillation.
5. Not fake news: desalination of seawater by distillation
Using distillation to separate pure water seems ironic, since we waste a lot of tap water to cool the condenser.
To solve this problem, some industrial desalination plants use sea water itself as the coolant. The sea water is pumped through the condenser to cool it down. This transfers the heat from the vapour to the sea water. The warmer sea water then returns to the vacuum chamber where it vaporises easily under low pressure.
