Identify a metal by its density
Weigh an unknown metal painted grey, find its volume by water displacement, and compare the density — with its uncertainty — against handbook values to find out what it is.
Aim Measure the density of an unknown metal sample and decide which metals agree with it within the measurement uncertainty.
Procedure
- Pick an unknown sample. The balance (readability 0.01 g) shows its mass; record it.
- In the “before” lens, level your eye with the bottom of the meniscus and read the water volume V₁ to 0.1 mL.
- With the sample submerged, read V₂ the same way. The sample volume is ΔV = V₂ − V₁.
- Calculate ρ = m/ΔV and see which metals fall inside the ±2u band on the number line.
- Tick “Use half the sample”, repeat, and compare the two densities; finally reveal the sample and check your conclusion.
What to notice
- Half a sample halves both mass and volume but not density — density is an intensive property.
- The smaller the sample, the larger the relative uncertainty in ΔV and the wider the band — it may cover several metals at once.
- Copper (8.93) and nickel (8.91), or zinc (7.13) and tin (7.29), are too close for displacement to separate; you need a second property such as colour or magnetism.
- The volume readings, with only two or three significant figures, limit the density — not the balance.
In a real lab
- Slide the sample down the tilted cylinder wall so no water splashes out and the base does not crack; check that no air bubbles cling to it.
- The sample must be fully submerged and must not dissolve in or react with water; porous or coated samples give a low apparent density.
- A narrower cylinder, a larger sample, or Archimedes’ method (weighing in air and in water) gives a better density.
- Wash your hands after handling lead or other heavy-metal samples, and keep metal fragments out of the drain.