Year 7 Science · Unit 2 · Lesson 5
Challenge Worksheet
Learning Goals
1. Explain what the formula CO₂ tells you about one particle of carbon dioxide, including what the small number means.
Find the errors
A student wrote four revision notes. Three of them contain an error and one is correct. Write the number of each wrong note, and rewrite it so it is correct.
Revision notes
1. Air has the formula N₂O, because it is mostly nitrogen with some oxygen.
2. O₂ is an element, because it contains only one type of atom.
3. One particle of methane, CH₄, contains 4 atoms: 1 carbon and 4 hydrogen.
4. Sodium chloride can be separated back into sodium and chlorine by filtering.
2. A student claims: “If I mix hydrogen gas and oxygen gas in a container, I’ve made water.” Identify what is wrong with this claim and explain what would actually need to happen to make water from hydrogen and oxygen.
3. Table salt (NaCl) is safe to eat, even though sodium catches fire in water and chlorine is a poisonous gas. Explain what this tells us about the properties of a compound compared with the elements it is made from. Use the word “bonded” in your answer.
Read the data
Scenario: what is it made of?
A lab analyses three samples each of pure sodium chloride, pure water and salt water, recording how each sample’s mass is shared between its parts.
| Substance | Sample 1 | Sample 2 | Sample 3 |
|---|---|---|---|
| Pure sodium chloride (NaCl) | 39% sodium, 61% chlorine | 39% sodium, 61% chlorine | 39% sodium, 61% chlorine |
| Pure water (H₂O) | 11% hydrogen, 89% oxygen | 11% hydrogen, 89% oxygen | 11% hydrogen, 89% oxygen |
| Salt water | 2 g salt in every 100 g | 7 g salt in every 100 g | 4 g salt in every 100 g |
Source: made-up samples; the NaCl and water percentages are real values, rounded.
4. Use Table 1 to find the mass of sodium in 50 g of pure NaCl.
Answer g
5. Which substances in Table 1 have a fixed make-up? What does this show about compounds and mixtures?
6. Explain it to a Year 5 student. Your cousin says: “Water must be a mixture. It’s made of hydrogen AND oxygen, so there are two different things in it!” Explain why water is a compound. You may start with: “The basic idea of a compound is…”, “A mixture is different because…”, “In water, the hydrogen and oxygen atoms are…”, “People often confuse the two because…”
Question 1
1 carbon and 2 oxygen atoms bonded, 3 atoms in all (1). The subscript counts the atom before it; no number means one (1). [2 marks]
Find the errors
Note 2 is correct. 1 each: Note 1: air is a mixture with no fixed ratio, so it has no formula (N₂O is a compound). Note 3: CH₄ has 5 atoms. Note 4: NaCl is a compound, so filtering cannot split it. [3 marks]
Question 2
Mixing gives only a mixture, in any ratio (1). Nothing has bonded (1). Energy such as a spark must start a reaction so the atoms bond as H₂O, fixed 2 : 1 (1). [3 marks]
Question 3
A compound’s properties are new, not its elements’ (1). In NaCl sodium and chlorine are bonded, not mixed (1). Bonding uses up what made them dangerous (sodium’s loose outer electron has gone to chlorine), or the compound keeps neither element’s dangerous properties, so salt is safe to eat (1). [3 marks]
Question 4
39% of 50 g = 0.39 × 50 (1) = 19.5 g (1). [2 marks]
Question 5
NaCl and water are the same in every sample; salt water varies (2, 7, 4 g per 100 g) (1). Compounds have elements bonded in a fixed ratio (1). A mixture’s parts are not bonded, so any amounts are possible (1). [3 marks]
Question 6
Marking guide: 1 compound: elements chemically bonded in a fixed ratio; 1 mixture: physically combined, not bonded, any amounts; 1 every water particle is 2 H bonded to 1 O, always; 1 new properties: hydrogen explodes, oxygen helps burning, water puts fires out; 1 an everyday mixture used as a contrast; 1 confusion: both have more than one element, but only a compound’s atoms are bonded. [6 marks]
Model answer: A compound is different elements’ atoms joined in the same pattern every time. A mixture’s parts sit together without joining, in any amounts, like cordial in water. In water every bit is 2 hydrogen joined to 1 oxygen, always, so it has new properties: hydrogen explodes and oxygen helps fires burn, but water puts fires out. People confuse them because both have more than one element; only a compound’s atoms are joined.
Common error: counting CH₄ as 4 atoms; the carbon counts too.