Year 7 Science · Unit 2 · Lesson 5
Foundation Worksheet
Learning Goals
Fill the gap
Choose the correct word from the word bank. Each word is used at most once. Two words are not needed: cross them out.
A substance made of only one type of atom is called an . When two or more elements are chemically together in a fixed , the result is a . For example, water always has two hydrogen atoms for every one oxygen atom, and its , H₂O, shows that fixed pattern. Air, on the other hand, is a , because its gases are only physically combined and the amounts can change.
1. Write the chemical symbols for iron and gold. Then explain why Co and CO are two different substances.
Sort it
Write each substance in the correct box. The boxes do not hold the same number of items.
Element
Compound
Mixture
2. A student says: “O₂ has two atoms, so it must be a compound, just like CO₂.” Explain why O₂ is an element but CO₂ is a compound.
Read the data
Scenario: five particle diagrams
A teacher draws five jars of particles on the board. In each drawing H stands for a hydrogen atom, O for an oxygen atom and Fe for an iron atom. Joined atoms are drawn touching, like H–O–H. Table 1 describes each drawing in words.
| Jar | What the drawing shows | Element, compound or mixture? |
|---|---|---|
| 1 | 8 single Fe atoms, all the same, packed in rows | |
| 2 | 5 particles, each made of two O atoms joined together (O–O) | |
| 3 | 6 particles, each one O atom joined to two H atoms (H–O–H) | |
| 4 | 6 H–H particles and 3 O–O particles, moving about separately and not joined to each other | |
| 5 | 4 H–O–H particles and 3 O–O particles, moving about separately and not joined to each other |
Source: made-up drawings for this worksheet.
3. Complete the last column of Table 1: write element, compound or mixture for each jar.
4. Write the chemical formula for one particle in Jar 2 and for one particle in Jar 3.
5. Jar 5 contains water particles, and water is a compound. Explain why the whole of Jar 5 is still a mixture.
6. Jess says: “Salt water is a compound, because the salt and the water are joined together.” Explain whether Jess is right. Use the ideas of bonding, fixed ratio and separating in your answer.
Fill the gap
element; bonded; ratio; compound; formula; mixture. Not used: atom, dissolve.
Question 1
Iron Fe, gold Au (1). Co is the element cobalt; CO is the compound carbon monoxide, carbon and oxygen bonded (1). [2 marks]
Sort it
Element: O₂; Cu; Fe. Compound: NaCl; H₂O; CO₂. Mixture: air; soil; seawater; granite.
Question 2
O₂ has only one type of atom (two oxygen atoms), so it is an element (1). CO₂ has two different elements, carbon and oxygen, bonded in a fixed ratio, so it is a compound (1). [2 marks]
Question 3
Jar 1 element; Jar 2 element; Jar 3 compound; Jar 4 mixture; Jar 5 mixture. All five correct = 3; four correct = 2; two or three correct = 1. [3 marks]
Question 4
Jar 2: O₂ (1). Jar 3: H₂O (1). [2 marks]
Question 5
It holds two different substances, water and oxygen (1). The water particles and oxygen particles are not bonded to each other; they are only mixed (1). They could be in any amounts, and each keeps its own properties (1). [3 marks]
Question 6
Marking guide: 1 position (Jess is wrong: salt water is a mixture); 1 the salt particles spread among the water particles but are not chemically bonded; 1 no fixed ratio: add more or less salt and it is still salt water; 1 it can be separated physically: evaporate the water and the salt is left; 1 contrast: in NaCl itself sodium and chlorine are bonded in a fixed 1 : 1 ratio (or: the salt in salt water still tastes salty, so it keeps its own properties). [5 marks]
Model answer: Jess is wrong; salt water is a mixture. The salt spreads out among the water particles but is not bonded to them. You can add a little salt or a lot and it is still salt water, so there is no fixed ratio. Evaporating the water leaves the salt behind. In a real compound such as NaCl, sodium and chlorine are bonded 1 : 1 every time.
Common error: calling O₂ a compound because it has two atoms.