Year 7 Science · Unit 2 · Lesson 4
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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 mixture that looks the same all the way through is called a mixture. The start of that word, homo-, means “”. Salt water and are both examples, even though you cannot see their separate parts. A mixture in which you can see the separate parts is called a mixture, such as a bowl of . To tell the two kinds apart, take a scoop from several places in the mixture. If the scoops are all , the mixture is homogeneous.
1. Write the three questions, in the correct order, that you ask to classify a new sample. Say what each answer tells you.
Classify each sample
Decide which of the three categories each one belongs to, and give a reason.
| Sample | Category | Reason |
|---|---|---|
| a glass of cordial mixed with water | ||
| a bowl of fried rice | ||
| a bar of pure silver | ||
| a bucket of muddy puddle water |
2. Raw eucalyptus oil collected from steam distillation is oil and water together in one jar. Explain why this is a mixture, say whether it is homogeneous or heterogeneous, and suggest how you could separate the two parts.
Read the data
Scenario: two clear liquids
Lily has two unlabelled bottles, A and B, of clear, colourless liquid. One is distilled water and one is salt water. She heats each on a hotplate and reads the thermometer every minute from the moment the liquid starts to boil.
| Time since boiling began (min) | 0 | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|---|
| Liquid A | 101.2 | 101.5 | 101.9 | 102.4 | 103.0 | 103.7 |
| Liquid B | 100.0 | 100.0 | 100.0 | 100.0 | 100.0 | 100.0 |
Source: sample class results.
3. Plot both liquids on the grid, with time running across the bottom. Join each liquid’s points and label the two lines A and B. Label each axis with its unit (°C, min), and give the graph a title.
4. Which bottle holds the distilled water? Use the shape of each line as evidence, and explain why the other liquid’s temperature keeps rising as it boils.
5. Ava fills a glass from a bottle of spring water and says: “I can’t see anything in it, so it’s pure water. Anything without lumps is pure.” Evaluate Ava’s claim. Use the spring water and one other example, and describe a lab test she could use to check.
Fill the gap
homogeneous; same; air; heterogeneous; muesli; teaspoon; identical. Not used: pure, different.
Question 1
One kind of particle? Yes: pure (1). Two or more substances physically combined? Yes: mixture (1). Separate parts visible? Yes: heterogeneous; no: homogeneous (1). [3 marks]
Classify each sample
Cordial: homogeneous, two substances but even (1). Fried rice: heterogeneous, separate pieces visible (1). Pure silver: pure, only silver atoms (1). Muddy water: heterogeneous, mud visible, scoops differ (1). No mark without a sensible reason. [4 marks]
Question 2
Two substances physically combined, not one kind of particle (1). Heterogeneous: two visible layers (1). Let the layers separate by density and pour off the oil (1). [3 marks]
Question 3
Axes labelled with units (°C, min) and a sensible scale; the temperature axis need not start at 0 (1). All 12 points correct (1). Lines joined and labelled A and B, with a title (1). [3 marks]
Question 4
B (1): its line is flat at 100 °C, a sharp fixed boiling point (1). A climbs because water leaves as steam, so the rest gets saltier and needs more heat (1). [3 marks]
Question 5
Marking guide: 1 position (Ava is wrong); 1 spring water is water plus dissolved minerals (calcium, magnesium), a homogeneous mixture; 1 a second even-looking mixture and its parts (salt water, air); 1 pure means one kind of particle, which looks cannot show; 1 a test: boil and read the temperature, or evaporate some; 1 both results: spring water leaves a solid (or does not stay at 100 °C), pure water leaves nothing (or stays at 100 °C). [6 marks]
Model answer: Ava is wrong. Spring water has dissolved minerals, so it is a homogeneous mixture; salt water also looks clear but is salt plus water. Pure means one kind of particle, which you cannot see. If she evaporates some, spring water leaves a thin solid and pure water leaves nothing.
Common error: starting the temperature axis at 0, which squashes A’s rise flat.