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An industrial chemist is studying a sample of an unknown metal. Describe two ways he could change the metal physically and two ways he could change the metal chemically to try to identify it.
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Two physical changes (no change in chemical identity)
1) Mechanical deformation (hammer, bend, file or roll)
- What to do: hammer or bend a small piece, or file/scratch it to produce shavings.
- What it shows: malleability/ductility vs brittleness (gold, silver, copper are very malleable; bismuth and some intermetallics are brittle). Filing reveals colour of fresh metal surface (e.g., copper reddish, aluminium silvery-white), and produces powder for further tests.
2) Heat to change phase / determine melting behaviour
- What to do: heat a small sample (in a controlled crucible) to observe softening and measure (approx.) melting point.
- What it shows: approximate melting point is a strong identifier (e.g., mercury liquid at room temp, tin ~232 °C, lead ~327 °C, aluminum ~660 °C). Also the appearance of molten metal and how it oxidises on heating (colour of oxide/flammability) can be diagnostic.
Two chemical changes (permanent chemical reactions)
1) Reaction with dilute hydrochloric acid (HCl)
- What to do: add a little dilute HCl to a small cleaned piece or filings (in a fume hood).
- What it shows: evolution of hydrogen gas (bubbling; testable with a lit splint) indicates an active metal (Mg, Zn, Fe, etc.). Rate of reaction gives a clue to reactivity. Some metals (Cu, Ag, Au) do not react with dilute HCl — that helps rule them out.
2) Displacement / redox test with a copper(II) solution (CuSO4)
- What to do: place the unknown metal into a blue copper(II) sulfate solution.
- What it shows: if the metal is more reactive than copper it will displace Cu2+ → Cu metal plates out and the blue colour of the solution fades (e.g., Zn, Fe). No change suggests the metal is less reactive than copper (e.g., Cu, Ag, Au). This test locates the metal on the reactivity series and can be combined with other acid tests for a tighter ID.
(Other useful chemical tests: reaction with NaOH to reveal amphoteric behaviour — Al and Zn dissolve in NaOH — and treatment with oxidising acids like HNO3, which give characteristic coloured solutions or gases for some metals.)
Safety note: use small samples, eye/skin protection, do reactions in a fume hood and neutralize/dispose of wastes properly.
1) Mechanical deformation (hammer, bend, file or roll)
- What to do: hammer or bend a small piece, or file/scratch it to produce shavings.
- What it shows: malleability/ductility vs brittleness (gold, silver, copper are very malleable; bismuth and some intermetallics are brittle). Filing reveals colour of fresh metal surface (e.g., copper reddish, aluminium silvery-white), and produces powder for further tests.
2) Heat to change phase / determine melting behaviour
- What to do: heat a small sample (in a controlled crucible) to observe softening and measure (approx.) melting point.
- What it shows: approximate melting point is a strong identifier (e.g., mercury liquid at room temp, tin ~232 °C, lead ~327 °C, aluminum ~660 °C). Also the appearance of molten metal and how it oxidises on heating (colour of oxide/flammability) can be diagnostic.
Two chemical changes (permanent chemical reactions)
1) Reaction with dilute hydrochloric acid (HCl)
- What to do: add a little dilute HCl to a small cleaned piece or filings (in a fume hood).
- What it shows: evolution of hydrogen gas (bubbling; testable with a lit splint) indicates an active metal (Mg, Zn, Fe, etc.). Rate of reaction gives a clue to reactivity. Some metals (Cu, Ag, Au) do not react with dilute HCl — that helps rule them out.
2) Displacement / redox test with a copper(II) solution (CuSO4)
- What to do: place the unknown metal into a blue copper(II) sulfate solution.
- What it shows: if the metal is more reactive than copper it will displace Cu2+ → Cu metal plates out and the blue colour of the solution fades (e.g., Zn, Fe). No change suggests the metal is less reactive than copper (e.g., Cu, Ag, Au). This test locates the metal on the reactivity series and can be combined with other acid tests for a tighter ID.
(Other useful chemical tests: reaction with NaOH to reveal amphoteric behaviour — Al and Zn dissolve in NaOH — and treatment with oxidising acids like HNO3, which give characteristic coloured solutions or gases for some metals.)
Safety note: use small samples, eye/skin protection, do reactions in a fume hood and neutralize/dispose of wastes properly.
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