Year 7 Science · Unit 2 · Lesson 8
Challenge Worksheet
Learning Goals
1. Four materials from around the house and the lab are listed below. Decide whether each is a mixture, an element or a compound, and give one piece of evidence.
| Material | Type | Reason |
|---|---|---|
| Clear mineral water | ||
| Salt (sodium chloride) purified in a lab | ||
| A pure copper wire | ||
| Blood |
2. Salt pans on the coast collect salt from the sea using only sunlight and wind. No amount of filtering or evaporating will get aluminium out of bauxite ore. Explain the difference.
3. A sealed bottle is labelled “pure spring water: nothing added”. Its label also lists calcium, magnesium and sodium. Is the water a pure substance? Explain what the word “pure” means to a scientist.
Read the data
Scenario: at the jeweller’s
Jewellers rarely make rings and chains from pure gold or pure silver. Table 1 compares the pure metals with the metals most jewellery is made from.
| Metal | What it is made of | How easily it scratches and dents |
|---|---|---|
| Fine gold (24 carat) | Gold only (99.9%) | Very easily |
| 18 carat gold | 75% gold, 25% silver and copper | Much less easily than fine gold |
| Fine silver | Silver only (99.9%) | Very easily |
| Sterling silver | 92.5% silver, 7.5% copper | Much less easily than fine silver |
Source: typical figures used by jewellers.
4. Which two metals in Table 1 are alloys? Use the table to explain how you know.
5. Sterling silver is only 7.5% copper, yet it dents much less easily than fine silver. Explain why, using what happens to the atoms.
6. A shop advertises an 18 carat gold ring as “pure gold”. Use Table 1 to judge this claim, and suggest why the jeweller chose 18 carat gold for a ring that will be worn every day.
7. Someone claims: “Pure substances are always better than mixtures, so we should use pure metals and pure water wherever we can.” Evaluate this claim. Use at least two examples from this worksheet or the lesson.
Question 1
Mineral water: mixture; minerals dissolved in water. Lab salt: compound; sodium and chlorine joined (NaCl). Copper wire: element; copper atoms only. Blood: mixture; plasma, cells and platelets. [4 marks: 1 per row, both columns correct]
Question 2
In seawater the salt is only physically mixed with the water (1), so evaporation, a physical process, leaves it behind (1). In bauxite the aluminium is joined in a compound, so freeing it needs a chemical process and more energy (1). [3 marks]
Question 3
No: calcium, magnesium and sodium are dissolved in it, so it is a mixture (1). To a scientist, pure means only one type of particle, like distilled water (1). [2 marks]
Read the data
4. 18 carat gold and sterling silver (1). Each is made of more than one metal mixed together (1). [2 marks]
5. In fine silver the atoms are all the same size and sit in tidy layers that slide easily (1). Copper atoms are a different size and sit among them (1). The layers snag on them, so the metal resists denting (1). [3 marks]
6. The claim is wrong: 18 carat gold is 25% silver and copper, so it is a mixture, not pure gold (1). It dents much less easily than fine gold, so the ring survives daily wear (1). [2 marks]
Question 7
Marking guide: 1 position (wrong, or only sometimes true); 1 a job needing a pure substance, with a reason (e.g. distilled water in labs, where nothing dissolved in it can interfere with a test); 1 a job where a mixture is better (steel, bronze, 18 carat gold); 1 reason from data or atoms (layers snag, so the alloy is harder); 1 pure substances are rare and take work and energy to refine; 1 conclusion: choose by the properties the job needs. [6 marks]
Model answer: The claim is wrong. Pure substances suit some jobs: labs use distilled water, because nothing dissolved in it can interfere with a test. But steel is much stronger than pure iron, and 18 carat gold dents far less than fine gold, because different-sized atoms stop the layers sliding. Pure substances are also rare and take work to refine. The best choice is the one with the properties the job needs.
Common error: reading “pure” on a label as the scientific meaning.