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Difficulty: MediumFormulating and Modifying Hypotheses

A student proposed the following hypothesis regarding the corrosion of iron:

*Hypothesis*: The mass of rust that forms on an iron nail submerged in a sodium chloride (NaClNaCl) solution for 7 days will increase continuously as the concentration of NaClNaCl in the solution increases from 0% to 10%.

To test this hypothesis, the student submerged identical iron nails in 5 different NaClNaCl solutions for 7 days and recorded the mass of the rust that formed on each nail. The results are shown in the table below:

NaClNaCl concentration (% by mass)Mass of rust formed (mg)
0%1.2
1%3.5
3%5.8
5%4.2
10%2.1

Based on these results, how should the student modify their hypothesis?

  1. The student should modify the hypothesis to state that the mass of rust formed increases with NaClNaCl concentration up to approximately 3%, but decreases at higher concentrations.Answer
  2. B
    The student should modify the hypothesis to state that the mass of rust formed is inversely proportional to the NaClNaCl concentration across all tested levels.
  3. C
    The student should modify the hypothesis to state that the mass of rust formed remains constant at all NaClNaCl concentrations greater than 3%.
  4. D
    The student should retain the hypothesis without modification because the nail in the 10% solution produced more rust than the nail in the 0% solution.

Answer

The student should modify the hypothesis to state that the mass of rust formed increases with NaClNaCl concentration up to approximately 3%, but decreases at higher concentrations.
The correct option accurately reflects the empirical data, which shows a rise in rust mass up to 3% NaClNaCl concentration followed by a decline at higher concentrations. This requires a modification of the hypothesis from a continuous increase to a peaked relationship.

Step-by-Step Solution

1
Analyze the original hypothesis to identify the predicted trend.
The original hypothesis predicts that the mass of rust will continuously increase as the NaClNaCl concentration increases from 0% to 10%.
This establishes the baseline expectation that must be compared against the actual data.
2
Examine the experimental data in the table to determine the actual trend.
As the concentration increases from 0% to 3%, the mass of rust increases from 1.2 mg to 5.8 mg. However, as the concentration increases further from 3% to 10%, the mass of rust decreases from 5.8 mg to 2.1 mg.
This identifies the pattern of the empirical evidence.
3
Compare the actual trend to the original hypothesis and determine the necessary modification.
Since the mass does not increase continuously but rather peaks at 3% and then decreases, the hypothesis must be modified to reflect this non-linear relationship (increasing up to a point, then decreasing).
This directly matches the learning objective of modifying a hypothesis based on new experimental data.

Key Concept

Formulating and Modifying Hypotheses
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Topics

Formulating and Modifying Hypotheses

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