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Question

Difficulty: MediumProperties of Matter

During a routine vehicle maintenance check, a technician records four distinct observations about an engine and its fluid systems. Which of the following observations describes a physical change rather than a chemical change?

  1. A
    Diesel fuel burning to form carbon dioxide, water vapor, and exhaust soot
  2. Engine oil decreasing in viscosity and flowing more freely as operating temperature risesAnswer
  3. C
    Acidic combustion byproducts reacting with and corroding the steel exhaust manifold
  4. D
    A basic fuel additive neutralizing sulfuric acid present in fuel residues

Answer

Engine oil decreasing in viscosity and flowing more freely as operating temperature rises
The observation describing engine oil thinning as temperature increases represents a physical change. Viscosity is a physical property governed by intermolecular forces; heating increases molecular motion and reduces resistance to flow without altering the chemical composition of the oil.

Step-by-Step Solution

1
Distinguish between physical and chemical changes
A physical change alters the form, state, or appearance of a substance without changing its chemical identity. A chemical change produces one or more new substances through bond breaking and formation.
Correct classification requires evaluating whether the molecular structure of the substance is modified.
2
Evaluate the fluid behavior under temperature change
When engine oil is heated, thermal energy reduces intermolecular attractions, causing the fluid to thin (lower viscosity). The chemical structure of the hydrocarbon base oil remains identical.
Since no new substance is formed and the process is reversible upon cooling, the change in flow properties is purely physical.

Key Concept

Physical versus chemical changes in matter
Estimated Time:45s
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Topics

Properties of Matter

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