Year 8 Science · Unit 2 · Lesson 3
Challenge Worksheet
Learning Goals
Find the errors
This answer has exactly three errors. Underline each one and correct it below.
A student’s answer
“Water is a mixture, because anything made of two elements must be a mixture. When you boil water, the hydrogen and oxygen separate and escape as two different gases. That is the same as evaporating salt water to get the salt back, so a physical process can split water into its elements.”
1. Write the three corrections.
2. A model kit has 20 white balls (hydrogen atoms) and 12 red balls (oxygen atoms), plus sticks to join them. (a) What is the largest number of water molecules (H₂O) the kit can build, and how many balls of each colour are left over? (b) Instead, a student uses every ball to build a mixture of hydrogen gas (H₂) and oxygen gas (O₂). How many of each particle can she build?
(a) H₂O
(b) H₂ and O₂
3. Sketch about six particles of the water you built in 2(a) and six of the hydrogen and oxygen you built in 2(b), with a key. Then write one sentence explaining why only the water sample was limited by the number of balls of each colour.
Read the data
A class collected water from the open ocean, from an estuary where a river meets the sea, and from a rainwater tank. They evaporated 100 mL of each sample and weighed the solid left behind.
| Sample | Trial 1 (g) | Trial 2 (g) | Average (g) |
|---|---|---|---|
| Open ocean | 3.4 | 3.6 | 3.5 |
| Estuary | 1.3 | 1.7 | 1.5 |
| Rainwater tank | 0.0 | 0.0 | 0.0 |
Source: sample class results.
4. Explain why the ocean and estuary results are different, and what this shows about whether seawater is a compound or a mixture.
A builder claims
“Concrete is a compound. Once it sets, you can never get the sand and gravel back out, so it must be chemically bonded.”
Concrete is made by mixing cement, sand, gravel and water. The cement reacts chemically with the water as the concrete hardens. Builders choose different recipes for different jobs, for example 1 : 2 : 3 or 1 : 3 : 5 parts of cement : sand : gravel.
5. Evaluate the builder’s claim. Identify the part of the reasoning that has some truth, explain the science error, and use the recipes as evidence for your judgement.
Find the errors
1. (i) Water is a compound: the number of elements does not decide the group; its hydrogen and oxygen atoms are chemically bonded in a fixed 2 : 1 ratio. (ii) Boiling does not separate hydrogen and oxygen; it turns liquid water into water vapour, which is still H₂O molecules. (iii) Evaporating salt water separates two substances that were only mixed; the elements in water can only be separated by a chemical reaction such as electrolysis. [3 marks: 1 per correction]
Your Turn
2. (a) Each H₂O needs 2 white and 1 red. 20 white ÷ 2 = 10 molecules, using 10 red; so 10 water molecules, with 0 white and 2 red left over. (b) 20 white ÷ 2 = 10 H₂; 12 red ÷ 2 = 6 O₂. [4 marks: 1 working, 1 ten molecules, 1 leftovers, 1 (b)]
3. Water: every particle is one red ball joined to two white balls, all identical. Mixture: separate white–white pairs and red–red pairs, not joined to each other. Key shows which colour is hydrogen and which is oxygen. Sentence: a compound needs its atoms in a fixed ratio (2 H to 1 O), so the hydrogen ran out first; a mixture has no fixed ratio, so any numbers of H₂ and O₂ can be combined. [4 marks: 1 water diagram, 1 mixture diagram, 1 key, 1 fixed-ratio sentence]
Read the data
4. In the estuary, fresh river water mixes with seawater, so each 100 mL holds less dissolved salt. The amount of salt compared with water changes from place to place, so seawater has no fixed ratio: it is a mixture, not a compound. [3 marks: river water dilutes; ratio varies; so a mixture]
Show What You Know
5. Marking guide: 1 some truth: the cement does react chemically with water, forming new compounds as it hardens; 2 error: being hard to separate does not make something a compound; a compound is one new substance with its atoms bonded in a fixed ratio, while the sand and gravel are only held in place by the set cement, not bonded into a new substance; 1 evidence: the recipes use different ratios (1 : 2 : 3 or 1 : 3 : 5), and a compound has a fixed ratio; 1 evidence: the sand and gravel keep their own properties (you can still see the gravel in broken concrete); 1 judgement: concrete is a mixture that contains compounds (the set cement), so the claim is wrong. [6 marks]
Common error to watch for: in Q1, accepting that boiling splits water. Steam is still water molecules; boiling is a change of state, not a separation of elements.