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Difficulty: EasyChemical Reactions and Compounds

A technician observes several material transformations while working in a maintenance shop. Which of the following observations indicates that a chemical reaction has occurred to form a new compound?

  1. A
    Solid solder wire melting into a liquid puddle when touched with a hot iron
  2. An iron bracket developing a reddish-brown layer of rust after exposure to humid airAnswer
  3. C
    Rubbing alcohol evaporating completely from a metal surface into vapor
  4. D
    A sheet of brass being hammered and shaped into a curved shield

Answer

An iron bracket developing a reddish-brown layer of rust after exposure to humid air
The formation of rust occurs when elemental iron reacts with oxygen in the presence of water to synthesize iron oxide (Fe2O3Fe_2O_3), a completely new chemical compound with distinct chemical and physical properties from the original metallic iron.

Step-by-Step Solution

1
Differentiate between physical transformations and chemical changes
Physical changes alter state, shape, or appearance without changing molecular structure, whereas chemical changes form entirely new substances with distinct chemical bonds and properties.
Understanding the fundamental definition of a chemical reaction is required to evaluate each scenario.
2
Analyze the formation of rust on iron
Iron (FeFe) combines with oxygen (O2O_2) and moisture from the environment to synthesize iron(III) oxide (Fe2O3Fe_2O_3), which is a brand-new compound with different physical and chemical properties.
This transformation demonstrates a true chemical reaction (oxidation/synthesis).
3
Evaluate phase changes and mechanical shaping
Melting solder, evaporating alcohol, and shaping brass preserve the original chemical identities of the materials.
Eliminating physical state changes confirms the only chemical change.

Key Concept

Identification of Chemical Reactions and Compound Formation versus Physical Changes
Estimated Time:45s
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Topics

Chemical Reactions and Compounds

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