While researching a topic, a student has taken the following notes:Scientists have developed a 'freeze-thaw' battery that can retain 92%...
GMAT Expression of Ideas : (Expression) Questions
While researching a topic, a student has taken the following notes:
- Scientists have developed a 'freeze-thaw' battery that can retain \(92\%\) of its charge after twelve weeks.
- The battery contains molten salt (a type of salt that liquifies when heated and solidifies at room temperature).
- When the salt is in a liquid state, energy flows through the battery.
- When the salt is in a solid state, energy stops flowing and is stored in the battery.
- The stored (frozen) energy can be used by reheating (thawing) the battery.
The student wants to specify how the salt enables energy storage. Which choice most effectively uses relevant information from the notes to accomplish this goal?
Scientists have developed a freeze-thaw battery that contains molten salt, which liquifies when heated and solidifies at room temperature.
The stored energy in a freeze-thaw battery, which contains molten salt, can be used by reheating the battery.
When the molten salt in a freeze-thaw battery solidifies at room temperature, energy stops flowing and can be stored in the battery.
Molten salt allows a freeze-thaw battery to retain 92% of its charge after twelve weeks.
Step 1: Decode and Map the Passage
Part A: Create Passage Analysis Table
| Text from Passage | Analysis |
|---|---|
| "Scientists have developed a 'freeze-thaw' battery that can retain 92% of its charge after twelve weeks." |
|
| "The battery contains molten salt (a type of salt that liquifies when heated and solidifies at room temperature)." |
|
| "When the salt is in a liquid state, energy flows through the battery." |
|
| "When the salt is in a solid state, energy stops flowing and is stored in the battery." |
|
| "The stored (frozen) energy can be used by reheating (thawing) the battery." |
|
Part B: Provide Passage Architecture & Core Elements
Main Point: The freeze-thaw battery uses molten salt's phase changes between liquid and solid states to control energy flow and enable long-term energy storage.
Argument Flow: The notes establish the battery's impressive performance, then explain its key ingredient (molten salt), and finally detail the three-phase operating mechanism where the salt's physical state determines whether energy flows, gets stored, or becomes accessible again.
Step 2: Interpret the Question Precisely
What's being asked? The student wants to specify how the salt enables energy storage - not just what the salt does generally, but specifically the mechanism by which it makes energy storage possible.
What type of answer do we need? We need an explanation that connects the salt's properties or behavior directly to the storage function.
Any limiting keywords? "How the salt enables energy storage" is the key limiting phrase - we're looking for the causal relationship between salt behavior and storage capability.
Step 3: Prethink the Answer
- Looking at our analysis, energy storage happens when the salt changes from liquid to solid state
- The mechanism is clear: liquid salt allows energy to flow through the battery, but when the salt solidifies, energy flow stops and the energy gets trapped/stored in the battery
- This phase change from liquid to solid is what actually enables the storage function
- The right answer should explain this cause-and-effect relationship between the salt solidifying and energy being stored
Scientists have developed a freeze-thaw battery that contains molten salt, which liquifies when heated and solidifies at room temperature.
- Describes what the battery contains and basic salt properties
- Doesn't explain how the salt enables energy storage - just states what it is and does physically
- What trap this represents: Students might think describing the salt's basic properties answers "how it enables storage," but this only gives background information, not the mechanism
The stored energy in a freeze-thaw battery, which contains molten salt, can be used by reheating the battery.
- Explains how stored energy can be accessed (by reheating)
- Focuses on energy retrieval, not on how salt enables the initial storage
- What trap this represents: Students might confuse "using stored energy" with "enabling energy storage" - these are different parts of the process
When the molten salt in a freeze-thaw battery solidifies at room temperature, energy stops flowing and can be stored in the battery.
- Directly explains the storage mechanism: salt solidifies, energy stops flowing, energy gets stored
- Shows the cause-and-effect relationship between salt's phase change and storage capability
- Matches our prethinking perfectly by connecting salt behavior to storage function
Molten salt allows a freeze-thaw battery to retain 92% of its charge after twelve weeks.
- States the battery's performance (92% retention) without explaining how salt achieves this
- Gives an outcome but not the mechanism behind it