Identify the metal.

Learning goal and prerequisites

Students use dry mass and displaced water volume to calculate density, repeat the measurement three times, and identify the closest candidate among aluminum, steel, and copper. They distinguish mass from volume and explain why density is more useful than mass alone for comparing materials.

Prerequisites: grams, milliliters, subtracting volume readings, mass ÷ volume, and the meaning of a mean. Introduce the bottom of the meniscus at eye level. One milliliter equals one cubic centimeter; g/mL and g/cm³ have equal numerical values. Allow about 15–25 minutes plus discussion as a planning estimate.

Procedure and what the app supports

Predict why displacement can help with an irregular solid. Assemble balance, cylinder, unknown sample, forceps, and water.
Tare the empty balance. Move the dry sample onto the pan; mass is read automatically.
Fill the cylinder with fresh water and record the initial meniscus.
Move the same sample into the cylinder. It is fully submerged without bubbles or water loss in this model.
Record the final meniscus. The app calculates final − initial volume, then dry mass ÷ displaced volume.
Recover and dry the same sample, reset the cylinder, and repeat twice. Recovery and drying are automatic supported steps, not a test of the student’s drying technique.
Use the mean density, observed range, and reference candidates to support an identification and a limitation.

The cylinder picture illustrates the procedure; use the numeric readings for calculations. All repeats measure the same piece of metal, not independent specimens. The unknown stays unchanged across three trials. A fresh investigation may randomly select the same candidate again.

Expected results

The model selects one fixed candidate density: aluminum 2.70, steel 7.85, or copper 8.96 g/cm³. It gives the sample a true volume between 8 and 12 mL and calculates its true mass from density × volume. Measurement noise makes the observed estimates vary slightly. A density match is evidence among these candidates, not proof of a unique composition.

Example: dry mass 78.50 g; initial water 30.0 mL; final water 40.0 mL. Displacement is 10.0 mL, so estimated density is 7.85 g/cm³. Do not divide by the entire final water reading. Candidate reference values are set for the activity; real steels and other alloys vary.

Assess the explanation

A claim names the closest reference candidate.
Evidence cites the measured mean with g/cm³ and compares it with a reference value; repeats/range are described accurately.
Reasoning explains displacement and mass per unit volume, rather than relying on color or total mass.
A limitation explains a relevant source of bias or why density alone does not uniquely identify an unknown.

Use the teacher overview’s shared review checklist. Automatic checks evaluate selected choices and the optional calculation, not the quality of the student’s scientific writing.

Questions for discussion

Would a larger piece of the same uniform metal have the same mass? Would it have the same density?
Why does a 40.0 mL final reading not mean the solid’s volume is 40.0 mL?
If an attached air bubble displaced extra water, would the calculated density be too high or too low? Explain the denominator.
How would water left on the sample affect its measured mass?
Can three close repeats reveal a balance that is consistently miscalibrated?
Why could another material or alloy have a similar density? What additional evidence would help?

Explicit measurement model

Balance display adds independent noise within ±0.005 g, rounded to 0.01 g. Each repeat’s true initial water volume is 30 ±1 mL; its final true volume equals that baseline plus the fixed sample volume. Independent initial/final reading errors are within ±0.05 mL, rounded to 0.1 mL. Density uses the difference of the two displayed readings. The mean is the arithmetic mean of the three individual density estimates.

The sample is homogeneous, nonporous, insoluble, and fully submerged at 20°C. There are no bubbles, absorption, dissolution, splashes, water loss, residual surface water, or thermal expansion. Taring, drying, and appropriate immersion are guided steps. Observed ranges are descriptive, not confidence intervals. Repeats show the stated random variation but do not test systematic error or variation among multiple specimens.

Download the text report or open the printable report from Lab options or results. Add optional student details and print/save as PDF. Progress stays in the same browser and device. Real glassware, heavy samples, water, and balances require classroom procedures and teacher supervision.