Year 8 Science · Unit 2 · Lesson 3
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Learning Goals
Match each substance to its evidence
Write the matching letter in the middle column. Each letter is used once.
| Substance | Letter | Evidence |
|---|---|---|
| 24-carat (pure) gold sample | A. Only one type of atom, and every particle is an identical pair of those atoms. | |
| Carbon dioxide (CO₂) | B. A new substance formed when iron reacts with oxygen; a brittle, reddish-brown solid. | |
| Air | C. Only one type of atom, packed as single atoms rather than joined pairs. | |
| Rust (iron oxide) | D. Water, ethanol, sugars and acids together; sediment can be filtered out. | |
| Hydrogen gas (H₂) | E. Nitrogen, oxygen, argon and other gases together, not bonded, in no fixed ratio. | |
| Wine | F. Every particle is one carbon atom bonded to two oxygen atoms. |
1. Which two substances in the match table are elements? State the feature that makes each one an element.
Cause and effect
Scenario: iron and sulfur
A student stirs grey iron filings into yellow sulfur powder. The mixture looks grey with yellow specks. She then passes a magnet over the dish.
Complete the chain. Each box causes the next.
| Gas | Formula | Share of dry air (%) |
|---|---|---|
| Nitrogen | N₂ | 78.1 |
| Oxygen | O₂ | 20.9 |
| Argon | Ar | 0.93 |
| Carbon dioxide | CO₂ | 0.04 |
Source: standard values for dry air near sea level, rounded. Real air also holds a varying amount of water vapour.
2. A classroom holds 200 000 L of dry air. Calculate the volume of oxygen in the room.
Answer L
3. Air contains both elements and a compound. Use Table 1 to name the elements and the compound. Then explain why air as a whole is still a mixture, not a compound.
4. A student says: “This particle diagram is a mixture because it has two different colours in it.” Evaluate the student’s rule using Claim, Evidence and Reasoning. Describe one diagram where the rule works and one where it fails, then write a better rule.
Match each substance to its evidence
24-carat (pure) gold sample → C; Carbon dioxide (CO₂) → F; Air → E; Rust (iron oxide) → B; Hydrogen gas (H₂) → A; Wine → D.
1. Pure gold and hydrogen gas. Each contains only one type of atom (gold atoms only; hydrogen atoms only, joined in pairs). [2 marks: 1 both named, 1 one type of atom]
Cause and effect
Box 2: the iron and sulfur particles sit side by side but are not chemically bonded (a mixture). Box 3: the iron keeps its own properties, so it is still attracted to a magnet.
Go Deeper
2. 20.9 ÷ 100 × 200 000 = 41 800 L. [2 marks: 1 method, 1 answer]
3. Elements: nitrogen, oxygen, argon. Compound: carbon dioxide. Air is a mixture because these substances are only present together; they are not bonded to each other, and their shares are not fixed (water vapour and carbon dioxide vary). A compound would be one new substance with its atoms in a fixed ratio. [4 marks: 1 elements, 1 compound, 1 not bonded, 1 no fixed ratio]
Show What You Know
4. Marking guide: 1 claim: the rule is unreliable; 1 a case where it works (e.g. separate pairs of white atoms and single black atoms, not joined: a mixture); 1 a case where it fails (e.g. water: one black atom bonded to two white atoms, every particle identical: a compound); 2 reasoning: what decides the group is whether different atoms are bonded in identical repeating units (one substance) or whether separate substances are just present together; 1 better rule, e.g. “If every particle is the same and contains different atoms joined together, it is a compound; if there is more than one kind of particle, it is a mixture.” [6 marks]
Common error to watch for: in Q3, calling air a compound because it “contains carbon dioxide”. A mixture can contain compounds; what matters is that the gases are not bonded to each other.