GRADE 8 SCIENCE · MATTER IN MOTION

Investigate the Evidence

Use data to build a claim and explain its limits.

Science investigation

Can the evidence support your claim?

Explore three classroom-style investigations. The data are invented for practice, not measurements from a real experiment.

Stirring and dissolving

Does stirring change how quickly a fixed amount dissolves?

A student compares identical portions of sugar and water at the same starting temperature. Each time is the number of seconds until no solid is visible. The amounts, containers and temperature are held constant.

Stirring and dissolving · sample data
Trial	Still water (s)	Stirred water (s)
1	92	45
2	88	42
3	94	48

Calculated from the invented practice data.

Your task: Which condition was changed? What was measured? Calculate each mean time, then describe what this evidence supports.

Your explanation

Compare with worked reasoning

Stirring is the changed variable; dissolving time is measured. The means are 91.3 s for still water and 45 s for stirred water, rounded to one decimal. Under these conditions stirring was associated with a shorter dissolving time. The data do not establish whether equilibrium solubility changed.

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Compare flow rates

Does fluid B flow more slowly than fluid A under the same test conditions?

A class times equal 60 mL portions through the same opening at the same temperature. The recorded times are seconds.

Compare flow rates · sample data
Trial	Fluid A (s)	Fluid B (s)
1	20	30
2	19	31
3	21	29

Calculated from the invented practice data.

Your task: Calculate each mean time and the average flow rate using 60 mL divided by that mean time. Explain what else must be held constant.

Your explanation

Compare with worked reasoning

Both means are 20 s for A and 30 s for B. Using the mean times, the rates are 3 mL/s and 2 mL/s. The same opening, volume, temperature and procedure make the comparison more meaningful. The result supports a comparison of flow under these conditions; it is not a direct measurement of every fluid property.

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Mass, volume and density

Do two different-sized samples behave like the same uniform material?

A student measures samples without their containers. Each sample is assumed uniform and measured at the same conditions.

Mass, volume and density · sample data
Sample	Mass (g)	Volume (mL)
A	36	30
B	72	60
C	60	75

Calculated from the invented practice data.

Your task: Calculate density for each sample. Which two have the same measured density, and what can you infer?

Your explanation

Compare with worked reasoning

A: 36 ÷ 30 = 1.2 g/mL. B: 72 ÷ 60 = 1.2 g/mL. C: 60 ÷ 75 = 0.8 g/mL. A and B have the same measured density, consistent with the same uniform material, but density alone cannot prove they are identical.

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