Organic Chemistry (Years 10–11) — Science

The petrol in a car, the candle wax on a birthday cake, the plastic of a drinks bottle — all of them share one element at their heart: carbon. The chemistry of carbon compounds is so vast it has its own name: organic chemistry.

Why this matters: Almost every fuel we burn and every plastic we use comes from carbon compounds in crude oil. Understanding hydrocarbons explains where our energy comes from — and why burning them produces the gases that warm the planet.

Methane (CH₄) — one carbon, four hydrogens

Hydrocarbons: only two elements

A hydrocarbon is a compound made of only hydrogen and carbon — nothing else. The name gives it away: hydro (hydrogen) + carbon. The simplest one is methane, CH₄: one carbon atom joined to four hydrogen atoms. Methane is the main gas in the natural gas that heats homes.

Crude oil and fractional distillation

Small molecules rise and cool high up; big ones stay low — each fraction taps off at its height.

Crude oil is not a single substance — it is a mixture of many different hydrocarbons, some with small molecules and some with very large ones. Because it is a mixture, it can be separated without any chemical reaction, using fractional distillation.

The trick is that different hydrocarbons boil at different temperatures. Think of a tall tower, hot at the bottom and cool at the top. The oil is heated until it evaporates, then rises up the tower. Small molecules stay as gas and rise high before cooling; large molecules cool and turn back to liquid low down. Each level collects a different useful fraction — petrol, diesel, and so on. It works just like a mixed pot of substances separating out because each one condenses at its own height.

Substance	What it is	Separated by
Element	One type of atom	Cannot be split easily
Compound	Atoms bonded chemically	Chemical reaction
Crude oil	A mixture of hydrocarbons	Fractional distillation

Burning fuels: combustion

Complete combustion always gives carbon dioxide + water.

When a hydrocarbon burns, it reacts with oxygen and releases energy. Complete combustion (plenty of oxygen) always gives the same two products: carbon dioxide + water. The carbon in the fuel joins with oxygen to make carbon dioxide; the hydrogen joins with oxygen to make water.

Worked reasoning. "What is made when methane burns completely?" Ask: what atoms are in the fuel? Methane, CH₄, contains carbon and hydrogen. The carbon must go somewhere — it grabs oxygen to become carbon dioxide (CO₂). The hydrogen also grabs oxygen to become water (H₂O). So the products are carbon dioxide and water, every time, for any hydrocarbon in plenty of oxygen. You never have to memorise separate answers — just follow the carbon and the hydrogen.

Common mix-up: Water (H₂O) and carbon dioxide (CO₂) are not hydrocarbons, even though water contains hydrogen and CO₂ contains carbon. A hydrocarbon must contain both hydrogen and carbon and nothing else. Water has no carbon; CO₂ has no hydrogen. Only compounds like CH₄ qualify.

Only CH₄ is a hydrocarbon — it has both C and H and nothing else.

Memory hook: Hydrogen + carbon, nothing more. And complete combustion always gives the "CO₂ and water" pair — follow the carbon, follow the hydrogen.