Year 7 Science · Unit 2 · Lesson 5
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Learning Goals
Compare two
Complete both columns for each feature.
| Feature | Compound (e.g. NaCl) | Mixture (e.g. salt water) |
|---|---|---|
| Are the parts chemically bonded? | ||
| Is the ratio of the parts fixed? | ||
| Can it be separated by physical methods? | ||
| New properties, or does each part keep its own? | ||
| What does its particle diagram look like? |
Decode the formula
Use each formula to complete its row. Remember: no number means one atom.
| Substance and formula | Atoms of each element, and the total | Element or compound? |
|---|---|---|
| ammonia, NH₃ | ||
| nitrogen gas, N₂ | ||
| glucose, C₆H₁₂O₆ | ||
| hydrogen peroxide, H₂O₂ |
1. Each carbon dioxide particle (CO₂) is 1 carbon atom bonded to 2 oxygen atoms. Explain why carbon dioxide is a compound and not a mixture of carbon and oxygen.
Read the data
Scenario: iron and sulfur, two ways
Ruby stirs 7 g of grey iron filings into 4 g of yellow sulfur powder in Tube A. She makes Tube B in exactly the same way, then her teacher heats Tube B strongly in a fume cupboard. It glows, and when it cools a new black solid, iron sulfide, is left. Table 1 shows Ruby’s tests.
| Test | Tube A (stirred) | Tube B (heated) |
|---|---|---|
| What it looks like | grey specks and yellow powder, both visible | one grey-black solid; no yellow powder or separate specks |
| Magnet held near it | the grey specks jump to the magnet; the yellow powder stays | the solid is not attracted |
| Amounts that can be used | any amounts of iron and sulfur can be stirred together | iron and sulfur always combine in the ratio 7 g of iron to 4 g of sulfur |
Source: typical results of a teacher demonstration.
2. Explain why Ruby can stir any amounts of iron and sulfur into Tube A, but in Tube B they always combine in the same ratio.
3. Ben heats 14 g of iron with 10 g of sulfur. Use the ratio in Table 1 to work out how much sulfur is left over that has not combined.
Answer g
4. Ruby says: “Tube B is just Tube A with the bits squashed closer together by the heat. It’s still iron and sulfur.” Evaluate Ruby’s claim. Use at least two results from Table 1 and what you know about compounds and mixtures.
Compare two
1 mark per row with both cells correct. Bonded: yes, chemically / no, just mixed. Fixed ratio: yes, always the same / no, any ratio. Physical separation: no, it needs a chemical reaction / yes (e.g. evaporation). Properties: completely new / each part keeps its own. Particle diagram: different atoms joined in a repeating pattern / different particles moving freely, not joined. [5 marks]
Decode the formula
1 mark per row with both cells correct. NH₃: 1 N, 3 H, total 4; compound. N₂: 2 N, total 2; element. C₆H₁₂O₆: 6 C, 12 H, 6 O, total 24; compound. H₂O₂: 2 H, 2 O, total 4; compound. [4 marks]
Question 1
The atoms are chemically bonded, not just mixed (1), always in the fixed ratio 1 C : 2 O; a mixture could have any ratio (1). [2 marks]
Question 2
In Tube A the iron and sulfur are only mixed, not bonded, so any ratio is possible (1). In Tube B they have bonded into a compound, iron sulfide, and a compound always has a fixed ratio of its elements (1). [2 marks]
Question 3
14 g of iron is 2 × 7 g, so it needs 2 × 4 g = 8 g of sulfur (1). 10 g − 8 g = 2 g of sulfur left over (1). [2 marks]
Question 4
Marking guide: 1 judgement (wrong: Tube B is a new substance, a compound); 1 evidence: yellow powder and grey specks gone, one black solid; 1 evidence: not attracted by the magnet, so the iron lost its properties (or the fixed 7 : 4 ratio); 1 the atoms are now chemically bonded, not just close; 1 so it has new properties and a magnet cannot separate it; 1 stirring only mixes; a compound needs a chemical reaction, which the heat started. [6 marks]
Model answer: Ruby is wrong. Tube B is a new substance, iron sulfide. The yellow powder and grey specks have gone, and the magnet no longer attracts the solid, so the iron has lost its own properties. The atoms are now bonded, not just close, which gives new properties and means a magnet cannot pull them apart. Stirring only mixes; making a compound needs a reaction, which the heat started.
Common error: saying the iron in Tube B is still iron “underneath”; bonded iron no longer behaves like iron.