Year 7 Science · Unit 2 · Lesson 8
Foundation Worksheet
Learning Goals
Match each material to what it is made of
Write the letter of the correct description next to each material. Each letter is used once.
| Material | Letter | What it is made of |
|---|---|---|
| Seawater | A. Iron and a small amount of carbon | |
| Air | B. Copper and zinc | |
| Bronze | C. H₂O only, with nothing dissolved in it | |
| Steel | D. About 78% nitrogen and 21% oxygen, plus argon and carbon dioxide | |
| Distilled water | E. Copper and tin | |
| Brass | F. Water with about 35 g of dissolved salts in every litre |
True or false? Fix the false ones
Circle T or F. If a statement is false, correct it on the line.
1. Tap water is a mixture, because it has minerals dissolved in it.
Correct it:
2. Air is mostly oxygen.
Correct it:
3. Pure substances are easy to find in nature, for example in soil, rocks and the ocean.
Correct it:
4. The amount of each gas in the air of a crowded, closed room can change.
Correct it:
5. An alloy is a compound, because its metals are chemically joined.
Correct it:
Sort it
Ask: “Is there only one type of particle in it?” Then write each material in the correct box. The boxes do not hold the same number of items.
Pure substance
Mixture
1. Engineers use steel instead of pure iron to build bridges. Give one reason steel is the better choice, and explain why steel is still a mixture.
Read the data
Scenario: what is left behind?
A class heats 1 litre of four different water samples until all the water has evaporated. Then they weigh the solid left behind in each dish.
| Sample | Solid left behind (g) |
|---|---|
| Seawater | 35 |
| Mineral water | 0.4 |
| Tap water | 0.2 |
| Distilled water | 0 |
Source: sample class results (the seawater figure is from the lesson).
2. How much solid would be left if the class evaporated 3 litres of seawater? Show your working.
Answer g
3. Which sample in Table 1 is only H₂O, with nothing dissolved in it? Explain how the table shows this.
4. Mia says: “The tap water at school is pure water. It is clear, and it looks the same all the way through.” Is Mia right? Use Table 1 and the idea of particles to explain.
Match each material to what it is made of
Seawater → F; Air → D; Bronze → E; Steel → A; Distilled water → C; Brass → B.
True or false
1. True. 2. False: air is mostly nitrogen (about 78%); oxygen is about 21%. 3. False: almost everything in nature is a mixture; pure substances usually have to be separated out by people. 4. True. 5. False: an alloy is a mixture; the metal atoms are mixed, not chemically joined.
Sort it
Pure substance: distilled water; a pure copper wire; pure oxygen gas; salt purified in a lab. Mixture: tap water; a bronze medal; soil; seawater; milk; steel.
Question 1
Steel is much stronger and harder than pure iron (1). It is still a mixture because the iron and carbon atoms are mixed together, not chemically joined (1). [2 marks]
Read the data
2. 3 × 35 (1) = 105 g (1). [2 marks]
3. Distilled water (1). No solid is left when it evaporates (0 g), so nothing was dissolved in it: it is a pure substance (1). [2 marks]
Question 4
Marking guide: 1 Mia is wrong; 1 a mixture can look the same all the way through (homogeneous), like air or seawater; 1 Table 1: 1 litre of tap water leaves 0.2 g of solid, so it has substances dissolved in it; 1 a pure substance has only one type of particle; 1 tap water has water particles plus dissolved mineral particles, so it is a mixture; only distilled water is pure. [5 marks]
Model answer: Mia is wrong. Many mixtures, such as air and seawater, look the same all the way through. Table 1 shows that 1 litre of tap water leaves 0.2 g of solid when it evaporates, so minerals were dissolved in it. A pure substance has only one type of particle; tap water has water particles and mineral particles, so it is a mixture. Only distilled water, which left 0 g, is pure.
Common error: “If it looks like one substance, it must be pure.” The test is the particles, not the look.