Imagine a giant tower of jars filled with different kinds of liquid. When you heat the bottom jar, the heaviest liquids stay there, while the lighter ones rise to the top. This is exactly what happens in a fractionating column used to separate crude oil into useful fuels.
At the bottom of the column you find the longest hydrocarbon chains, like the long spaghetti strands in a bowl of pasta. These are often alkanes with the formula \$C{n}H{2n+2}\$ where n is large (e.g., C20+). As you move up, the chains get shorter, just like a stack of pancakes that gets thinner from bottom to top.
Volatility is how easily a liquid turns into gas. The heavier, longer‑chain liquids at the bottom are less volatile – they need more heat to evaporate. The lighter, shorter‑chain liquids at the top are highly volatile – they boil off quickly, like a cup of hot tea that steams right away. Think of