Boiling Point

Boiling Point: fractional distillation you can watch

This free game is part of our AQA GCSE Chemistry (8462) course for exams in EXAM YEARS 2027-2029.

What is fractional distillation?

Crude oil is a mixture of a very large number of compounds, and most of them are hydrocarbons: molecules made of hydrogen and carbon atoms only. For AQA GCSE Chemistry (spec point 4.7.1.2), the many hydrocarbons in crude oil are separated into fractions by fractional distillation. Each fraction contains molecules with a similar number of carbon atoms, so they have similar boiling points.

In the exam you need to explain how it works in terms of evaporation and condensation. Boiling Point lets you watch it happen. The front of a fractionating column has been cut away, so you can see the trays inside and the hydrocarbon molecules rising through them, drawn as ball-and-stick models billions of times bigger than real life.

Evaporate, rise, cool, condense

The game starts with the furnace. You hold HEAT until the crude oil reaches about 400 °C, so most of it evaporates and the vapour pours into the bottom of the column. The column has a temperature gradient: it is hottest at the bottom and coolest at the top. In the game the trays are at 350 °C, 250 °C, 150 °C and 25 °C, and the inside wall glows warm at the bottom and cool blue at the top.

As each vapour molecule rises, it cools. It passes straight through any tray that is still hotter than its boiling point, and it condenses on the first tray that is cooler than its boiling point. C₈H₁₈, for example, boils at 126 °C: it rises past the 350, 250 and 150 °C trays as a gas and condenses on the 25 °C petrol tray. Big molecules have high boiling points, so they condense low down; small molecules rise higher. Gases such as propane never condense, so they leave from the top, and the biggest molecules never evaporate at all, so they run to the bottom as the residue (bitumen).

Run the draw-off valves

Your job is to run the draw-off valves. Only one valve can be open at a time, so for each molecule you compare its boiling point with the tray temperatures and open the valve where it will condense before it gets there. Catch it and the liquid runs off along the pipe into that fraction’s tank. Miss it and the liquid runs back down to the furnace to be heated again, a missed gas is burnt in the flare, and a card explains exactly where the molecule went and why.

The levels build up one idea at a time: one molecule at a time with Nadia, the control room lead, then two at once, then a single puff of crude oil vapour that splits into fractions in front of you, then a timed stream. Later the furnace starts to cool down, and you have to hold HEAT to keep it hot: if the crude oil isn’t heated above a hydrocarbon’s boiling point, that hydrocarbon doesn’t evaporate and is lost with the residue. The game ends with a 60-second rush.

Fuels and feedstock

When a tank fills it is sent out. The fractions are processed to produce fuels and feedstock for the petrochemical industry. The fuels named in the AQA specification are petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases (LPG): petrol for cars, kerosene for jet aircraft, diesel oil for lorries, buses and trains, heavy fuel oil for ships and power stations, and LPG as bottled gas. Feedstock goes to the petrochemical industry, which makes useful materials such as solvents, lubricants, polymers and detergents.

Every molecule in the game is an alkane (C_nH_2n+2), and each one is a CH₂ unit longer than the last. Carbon atoms can form chains like this, giving families of similar compounds, which is why there is such a vast array of natural and synthetic carbon compounds.

The 6-mark exam answer

Near the end you build a full-mark answer to “Explain how fractional distillation separates crude oil into fractions” from five links: the crude oil is heated so most of it evaporates; the vapour goes into a column that is hot at the bottom and cooler at the top; the vapour rises and cools; each hydrocarbon condenses when it cools to its boiling point, at a different height; and hydrocarbons with similar boiling points are collected together as a fraction. Common wrong answers are there too, such as saying the molecules are cracked in the column or that the top is the hottest part. A game takes about 8 minutes, works on a computer, tablet or phone, and saves your best score.