Year 7 Science · Unit 2 · Lesson 4
Foundation Worksheet
Learning Goals
Match each term to its meaning
Write the letter of the correct meaning next to each term. Each letter is used once.
| Term | Letter | Meaning |
|---|---|---|
| Pure substance | A. Two or more substances physically combined, each keeping its own properties. | |
| Mixture | B. A mixture in which you can see the separate parts. | |
| Homogeneous | C. A homogeneous mixture in which one substance is dissolved in another. | |
| Heterogeneous | D. Matter made of only one kind of particle. | |
| Solution | E. A mixture that looks the same all the way through. |
1. A carton says “100% pure orange juice”. Would a chemist call the juice a pure substance? Explain, using the chemistry meaning of “pure”.
Sort it
Write each item in the correct box. The boxes do not hold the same number of items.
Pure substance
Homogeneous mixture
Heterogeneous mixture
2. Seawater is water with salt dissolved in it. How could you get the salt back? Name the method and say what happens to each part.
Read the data
Scenario: four mystery samples
A lab technician finds four unlabelled containers. She looks at each sample with a magnifying glass, takes a teaspoon scoop from the top and from the bottom, and evaporates a few drops of each liquid on a watch glass. Table 1 shows what she saw.
| Sample | What it looks like | Top and bottom scoops | After a few drops are evaporated |
|---|---|---|---|
| W | clear, colourless liquid | identical | nothing is left |
| X | clear, colourless liquid | identical | a white crust is left |
| Y | grey powder; black and white grains are visible | the bottom scoop has far more black grains | not tested (already dry) |
| Z | clear, pale yellow liquid | identical | a sticky brown solid is left |
Source: made-up samples for this worksheet.
3. Classify samples X, Y and Z as a pure substance, a homogeneous mixture or a heterogeneous mixture. Give one reason from Table 1 for each.
4. The technician thinks sample W is pure water, but looking at it cannot prove this. Name a lab test that would check, and describe the result she should get if W really is pure water.
5. Your friend says: “Air is a pure substance. It’s just one invisible gas, and it looks the same everywhere.” Explain whether your friend is right. Use the words mixture, homogeneous and particle.
Match each term to its meaning
Pure substance → D; Mixture → A; Homogeneous → E; Heterogeneous → B; Solution → C.
Question 1
No: it is water, sugar, fibre, vitamins and acids, so a mixture (1). Chemists call a substance pure only if it has one kind of particle; the label means nothing was added (1). [2 marks]
Sort it
Pure substance: distilled water; a bar of pure gold; oxygen from a lab cylinder; table sugar from a chemical supplier. Homogeneous mixture: brass; salt water; air. Heterogeneous mixture: trail mix; beach sand; muesli.
Question 2
Evaporation (1). The water leaves as water vapour and the salt is left behind as solid crystals (1). [2 marks]
Question 3
X: homogeneous mixture: even, but a white solid is left, so something was dissolved (1). Y: heterogeneous: separate grains visible and scoops differ (1). Z: homogeneous: identical scoops, but a solid is left (1). [3 marks]
Question 4
Heat it to boiling and measure its temperature (or measure its melting point) (1). Pure water boils at 100 °C and stays at 100 °C the whole time it boils: one sharp, fixed value (1). [2 marks]
Question 5
Marking guide: 1 clear position (the friend is wrong); 1 names three substances in air (nitrogen, oxygen, argon, carbon dioxide, water vapour); 1 air holds more than one kind of particle, so it is a mixture; 1 the gases are spread evenly, so every sample is the same and it is homogeneous; 1 looking even (or being invisible) does not prove purity: pure means one kind of particle, like the oxygen in a lab cylinder. [5 marks]
Model answer: My friend is wrong. Air is nitrogen, oxygen, argon, carbon dioxide and water vapour, so it has several kinds of particle and is a mixture. The gases are spread evenly, so it is a homogeneous mixture. Looking the same does not make it pure; a pure substance has one kind of particle.
Common error: calling any clear or invisible sample pure.