Year 8 Science · Unit 2 · Lesson 02
Challenge Worksheet
Learning Goals
Find the errors
This lab report contains exactly three errors in symbols or formulas. Underline each one and write the correction below.
A student’s lab report
We heated a strip of magnesium (MG) in air. It burned with a bright white flame and left a white powder, magnesium oxide (MgO). The formula MgO shows one magnesium atom and two oxygen atoms. We then tested how well a piece of copper wire (Cu) and a rod of carbon (Ca) conduct electricity.
1. Write the three corrections, and give the rule or reason for each.
| Substance | Where you meet it | Formula |
|---|---|---|
| Caffeine | coffee and tea | C₈H₁₀N₄O₂ |
| Vitamin C | oranges and capsicum | C₆H₈O₆ |
| Ethanol | hand sanitiser | C₂H₆O |
| Methane | natural gas for cooking | CH₄ |
Source: accepted molecular formulas.
2. How many atoms are shown in one caffeine molecule? What fraction of them are carbon atoms? Give the fraction in its simplest form.
Answer atoms; fraction
3. Which contains more hydrogen atoms: 3 caffeine molecules or 4 vitamin C molecules? Show every step.
Answer
4. Glucose is C₆H₁₂O₆. A student notices that one glucose molecule and one caffeine molecule have the same total number of atoms, and says they must be the same substance. Use the formulas to explain why the student is wrong.
Evaluate the proposal
A letter to a science magazine
“Symbols like Fe, Na and Au are old-fashioned and confusing. English is the main language of science today, so we should change them to match English names: I for iron, S for sodium and G for gold.”
5. Two of the new symbols would cause an immediate clash. Name them and explain the problem.
6. Give one fair point in the letter. Then give two reasons why the change would make science worse overall, and reach a judgement.
7. A student asks: “When a chemist writes O, does it mean one oxygen atom, the element oxygen, or a jar of oxygen gas?” Write a full answer. Explain what O, O₂ and a jar of oxygen gas each represent, and what a symbol on its own does not tell you.
Find the errors
1. (i) MG should be Mg: the second letter of a symbol is lowercase. (ii) MgO shows one magnesium atom and one oxygen atom: with no subscript, there is one of each. (iii) Carbon is C; Ca is calcium, a different element. [3 marks: 1 per correction with reason]
Your Turn
2. 8 + 10 + 4 + 2 = 24 atoms. Carbon: 8 out of 24 = 1/3. [2 marks]
3. Caffeine: 3 × 10 = 30 hydrogen atoms. Vitamin C: 4 × 8 = 32 hydrogen atoms. 4 vitamin C molecules contain more (by 2). [3 marks: 1 each count, 1 conclusion]
4. Both have 24 atoms, but caffeine contains nitrogen (4 N atoms) and glucose has none, and the numbers of C, H and O atoms are different (C₈H₁₀N₄O₂ versus C₆H₁₂O₆). A formula identifies a substance by which elements and how many of each, not by the total. [2 marks]
Evaluate the proposal
5. I is already the symbol for iodine, and S is already the symbol for sulfur. Every formula using I or S would become unclear: a reader could not tell iron from iodine or sodium from sulfur. [2 marks: 1 both clashes named, 1 problem]
6. Fair point: English-matching symbols might be easier for English-speaking beginners to learn. Reasons against (any two): symbols are designed to be the same in every language, so English-based symbols would favour one language and break that; every existing book, label, safety sheet and paper would become out of date or confusing; new clashes with existing symbols would cause dangerous mistakes; the old symbols carry history (ferrum, natrium, aurum). Judgement: the change would cause more confusion than it solves, so keep the current symbols. [4 marks: 1 fair point; 2 reasons; 1 judgement]
Show What You Know
7. Marking guide (points expected): 1 O can stand for the element oxygen; 1 O on its own can also represent one oxygen atom; 1 O₂ is a formula: the subscript 2 shows two oxygen atoms bonded together (an oxygen molecule), which is how oxygen gas exists; 1 a jar of oxygen gas is a real sample, actual matter made of an enormous number of O₂ molecules; 1 and 1 for any two things a symbol does not tell you: how much of the substance there is, its state (solid, liquid or gas), what it looks like, or what the atom looks like. [6 marks]
Common error to watch for: in Q3, multiplying the total number of atoms (24 and 20) instead of only the hydrogen atoms. Students must read the subscript that belongs to H in each formula.