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Difficulty: MediumChemical Reactions and Compounds

A technician working with lead-acid vehicle batteries observes several different processes in a maintenance facility. Which of the following processes represents a chemical reaction rather than a physical change?

  1. A
    Distilled water evaporating from an unsealed battery cell cap on a warm workbench
  2. B
    Solid lead terminal posts melting under high heat during terminal reshaping
  3. Solid lead and lead dioxide reacting with sulfuric acid to produce lead sulfate during battery dischargeAnswer
  4. D
    Water vapor in humid air condensing into liquid droplets on the cool exterior battery casing

Answer

Solid lead and lead dioxide reacting with sulfuric acid to produce lead sulfate during battery discharge
The reaction between lead, lead dioxide, and sulfuric acid during battery discharge is a chemical change because the original substances are chemically altered to form an entirely new compound, lead sulfate (PbSO4PbSO_4), releasing electrical energy through bond reorganization.

Step-by-Step Solution

1
Differentiate between a chemical change and a physical change.
A chemical change involves the rearrangement of atoms via bond breaking and bond formation to yield new chemical substances with distinct properties. A physical change alters only physical form, shape, or state of matter while keeping the chemical composition unchanged.
Clear scientific criteria are required to evaluate the nature of each observed process.
2
Analyze the atomic and molecular changes occurring in each scenario.
Evaporating water (H2O(l)→H2O(g)H_2O_{(l)} \rightarrow H_2O_{(g)}), melting lead (Pb(s)→Pb(l)Pb_{(s)} \rightarrow Pb_{(l)}), and condensing water (H2O(g)→H2O(l)H_2O_{(g)} \rightarrow H_2O_{(l)}) are all phase changes where molecular formulas remain identical. In contrast, lead-acid battery discharge involves lead (PbPb), lead dioxide (PbO2PbO_2), and sulfuric acid (H2SO4H_2SO_4) reacting to create a new chemical substance, lead sulfate (PbSO4PbSO_4).
Determining whether chemical composition is conserved or transformed identifies the presence of a chemical reaction.
3
Identify the process that forms new chemical compounds.
The discharge of the lead-acid battery is the only process where reactants are converted into chemically distinct products.
The generation of lead sulfate (PbSO4PbSO_4) confirms a chemical reaction.

Key Concept

Differentiating chemical reactions (which produce new substances with new chemical properties through bond breaking and formation) from physical phase changes (which preserve molecular composition).
Estimated Time:1m 0s
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Topics

Chemical Reactions and Compounds

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