Year 9 Science · Unit 2 · Lesson 12
Challenge Worksheet
Learning Goals
1. Explain what makes petroleum a fossil fuel, and why it is non-renewable.
Find the mistakes: a student’s description
(a) Crude oil is heated to about 400 °C, so most of it turns to vapour.
(b) The column is hottest at the top, so the vapour keeps rising until it reaches the top.
(c) The furnace breaks the long molecules into shorter ones, and that is how petrol is made.
(d) Fractional distillation is a chemical change, because it makes new products such as petrol and diesel.
2. Three of the four sentences are wrong. Identify each wrong sentence and rewrite it so it is correct.
Read the data
Cut-off temperatures in the lesson’s column: LPG below 40 °C; petrol 40–175 °C; kerosene 175–250 °C; diesel 250–350 °C. Kerosene spans C₁₀–C₁₆ and diesel C₁₄–C₂₀.
| Alkane | Carbon atoms | Boiling point (°C) |
|---|---|---|
| Butane | 4 | −1 |
| Heptane | 7 | 98 |
| Nonane | 9 | 151 |
| Undecane | 11 | 196 |
| Hexadecane | 16 | 287 |
| Octadecane | 18 | 316 |
Source: a chemistry data book.
3. Name the fraction each alkane in Table 1 is collected in.
4. By carbon count, hexadecane could belong to two fractions. Which fraction does it actually collect in? Explain why the column sorts molecules by boiling point, not by counting carbon atoms.
5. To make more jet fuel, a refinery raises the cut-off between kerosene and diesel from 250 °C to 290 °C. Which alkane in Table 1 changes fraction? Predict what happens to the amounts of kerosene and diesel the refinery makes.
Plan a short investigation
Question: does the fraction affect how easily it catches fire? The teacher will run the test in a fume cupboard, using small samples of four fractions from a school distillation. Plan it for her.
Prediction
If …, then …, because …
Variables
Which result would disprove it?
One limitation, other than safety
6. A scientist claims that fractional distillation can separate any liquid mixture. Evaluate this claim. Use these three mixtures as evidence: hexane (boils at 69 °C) with hexadecane (287 °C); two liquids that boil at 98 °C and 99 °C; and sugar dissolved in water (sugar breaks down before it can boil).