Year 8 Science · Unit 2 · Lesson 05
Challenge Worksheet
Learning Goals
True or false? Fix the false ones
Circle T or F. If a statement is false, correct it on the line.
1. Any substance that is written with a formula must be a compound.
Correct it:
2. A sample that looks the same all the way through must be a pure substance.
Correct it:
3. The atoms in a compound are always in the same fixed ratio, so its formula does not change.
Correct it:
4. A particle diagram is still useful even though its particles are not drawn to scale.
Correct it:
5. If you can name a substance’s classification correctly, you have shown that you understand it.
Correct it:
Scenario: a seasoning blend
A food scientist at a Sydney flavour laboratory stirs together three ingredients: table salt, sucrose (a sugar) and a small amount of vanillin (a flavour compound). No chemical reaction happens: each ingredient keeps its own chemical identity.
| Ingredient | Formula | Elements present | Atoms in one particle |
|---|---|---|---|
| Table salt | NaCl | sodium, chlorine | 1 Na : 1 Cl (a ratio, not one small particle) |
| Sucrose | C₁₂H₂₂O₁₁ | ||
| Vanillin | C₈H₈O₃ |
Data: accepted formulas for these three food ingredients.
1. Complete Table 1. Show the addition you used to find the total number of atoms in one sucrose particle and in one vanillin particle.
Answer: sucrose atoms; vanillin atoms
Find the errors
This lab summary contains exactly three errors. Underline each one and write the correction below.
A trainee’s summary
The seasoning blend is a compound, because it is made of three substances. Sucrose, C₁₂H₂₂O₁₁, is a compound of three elements: carbon, hydrogen and oxygen. Vanillin is an element, because it has only one name. Table salt, NaCl, contains sodium and chlorine in a 1:1 ratio. Sucrose has exactly twice as many hydrogen atoms as oxygen atoms, so each sucrose particle must contain water molecules.
2. Write the three corrections, each with a reason.
3. The scientist doubles the amount of salt in the next batch. Predict whether (a) the formula NaCl changes, and (b) the blend is still the same kind of matter. Justify both answers with particle reasoning.
4. Consider five everyday substances: table salt, water, air, aluminium foil and sugar. A scientist claims: “All five can be fully described using only two of the four tools, classification and particle diagrams, so symbols and models are unnecessary.” Do you agree or disagree? Justify your position using at least two of the five substances as evidence.
5. A student says: “I don’t need to understand particles. I can just memorise the definitions of element, compound and mixture for the test.” Evaluate this approach using a Claim, Evidence and Reasoning structure. Use at least one substance from this sheet as evidence, and explain why particle understanding matters for the next part of the unit (what atoms are made of).
True or false
1. False: elements can have formulas too, for example O₂ and Cl₂; a compound needs different elements bonded together. 2. False: a mixture can look uniform, for example salt water or air. 3. True. 4. True. 5. False: you must be able to justify the classification with particle evidence; a name alone can be memorised without understanding.
Your Turn
1. Sucrose: carbon, hydrogen, oxygen; 12 + 22 + 11 = 45 atoms. Vanillin: carbon, hydrogen, oxygen; 8 + 8 + 3 = 19 atoms. [3 marks: 1 elements for both, 1 each total]
2. (i) The blend is a mixture: the ingredients are physically combined, not bonded, and each keeps its identity. (ii) Vanillin is a compound: C₈H₈O₃ shows three elements bonded together; one name says nothing about classification. (iii) Sucrose contains no water molecules: its H and O atoms are bonded into each sucrose particle; a 2:1 ratio does not mean H₂O is present. [3 marks: 1 per correction with reason]
Go Deeper
3. (a) No: salt is still sodium and chlorine bonded in a fixed 1:1 ratio, whatever amount is used. (b) Yes, still a mixture: the ingredients are not bonded to each other, so their amounts can change freely. [2 marks]
4. Expect disagreement. 1 clear position; 1 symbols/formulas state exactly which atoms and in what ratio (NaCl 1:1, H₂O 2:1, Al one element); 1 models explain behaviour, e.g. why aluminium foil conducts and bends; 1 two named substances used as evidence. Accept a reasoned “partly agree”. [4 marks]
Show What You Know
5. Marking guide (points expected): 1 claim: ineffective (works only for simple recall); 1 evidence: a shortcut fails, e.g. vanillin has one name but is a compound, salt water looks pure but is a mixture; 1 reasoning: particle evidence explains why a sample fits a category, so new substances can be classified; 1 scientists must justify classifications with particles, symbols and models; 1 the next block (protons, neutrons, electrons) builds on knowing what an atom and an element are; 1 clear CER structure with a judgement. [6 marks]
Common error to watch for: in the error hunt, reading the H:O ratio of 2:1 in sucrose as “water inside sugar”. A formula counts atoms bonded into one particle; it does not list smaller molecules hidden inside.