Year 8 Science · Unit 2 · Lesson 19
Challenge Worksheet
Learning Goals
1. State two differences between an element and a compound.
2. Carbon burns in air, and oxygen helps things burn. Yet carbon dioxide, made from these two elements, is used to put fires out. Explain how this is possible.
3. Both are colourless liquids. Give one measurement from Table 1 (below) that tells them apart, and the result for each.
Two compounds, the same two elements
| Property | Water | Hydrogen peroxide |
|---|---|---|
| Formula | H₂O | H₂O₂ |
| Boiling point (°C) | 100 | 150 |
| Density (g/cm³) | 1.00 | 1.45 |
| Stability | Stable | Slowly breaks down into water and oxygen gas |
| Uses | Drinking, cooking, washing | Bleaching hair, disinfecting surfaces |
Values are for the pure liquids.
4. Hydrogen peroxide is sold in bottles with a tiny vent in the cap and a label saying “store away from heat”. Use Table 1 to explain why. Why does a bottle of water need neither?
Find the errors
A student’s answer
“Hydrogen is a flammable gas and oxygen helps things burn, so water should be very flammable. Water and hydrogen peroxide are made of the same elements, so they must have the same properties. Hydrogen peroxide has one extra oxygen atom, so it is basically just water with a bit of oxygen gas mixed in.”
5. Identify the three errors in the student’s answer. For each, explain why the reasoning fails, using evidence from Table 1 where you can.
6. “If you know which elements are in a compound, you can predict how the compound will behave.” To what extent do you agree? Use at least three compounds from this lesson as evidence, and say what the elements can tell you.
Warm Up
1. Any two: an element has one type of atom, a compound two or more elements chemically bonded; a compound can be broken down chemically, an element cannot; a compound’s properties differ from its elements’. [2 marks]
2. In carbon dioxide the carbon and oxygen are chemically bonded into a new substance with its own properties: CO₂ does not burn and does not help things burn, and it is denser than air, so it smothers a flame. [2 marks: 1 new substance, 1 its properties]
Your Turn
3. Density: water 1.00 g/cm³, hydrogen peroxide 1.45 g/cm³ (e.g. 10 cm³ has a mass of 10 g vs 14.5 g). (Or boiling point: water boils at 100 °C, hydrogen peroxide at 150 °C.) [2 marks: 1 property, 1 both results]
4. Hydrogen peroxide slowly breaks down into water and oxygen gas; the gas would build up pressure in a sealed bottle, so the vent lets it out (heat speeds the breakdown). Water is stable and gives off no gas. [3 marks: 1 breaks down to oxygen gas, 1 vent releases pressure, 1 water is stable]
Go Deeper
5. (1) “Water should be very flammable”: in water the hydrogen is bonded to oxygen, making a new substance that does not burn. (2) “Same elements, so the same properties”: Table 1 shows different boiling points (100 vs 150 °C), densities (1.00 vs 1.45 g/cm³), stability and uses; the number and arrangement of atoms matter. (3) “Water with oxygen gas mixed in”: the extra oxygen atom is chemically bonded in each molecule, so hydrogen peroxide is a different compound, not a mixture. [6 marks: per error, 1 identified, 1 explained]
Show What You Know
6. Marking guide (points expected): 1 position: mostly disagree; 1 NaCl: sodium explodes in water, chlorine is toxic, salt is safe to eat; 1 H₂O: hydrogen burns, oxygen helps burning, yet water is a liquid that does not burn; 1 a third compound, e.g. CO₂ puts out fires though carbon burns, or H₂O vs H₂O₂ with the same elements but different properties; 1 what elements do tell you: which atoms are present and what you would get back if the compound were broken down; 1 conclusion: the compound’s own properties must be tested or looked up. [6 marks]
Common error to watch for: calling hydrogen peroxide “water plus oxygen”. It is a different compound with a different formula, not a mixture of water and oxygen gas.