For a main-idea question involving several experiments, separate the observations by purpose and timescale before evaluating the choices. Here, distinguish evidence about which structure forms first from evidence about which structure persists. Reject answers that explain only one experiment or confuse initial formation with long-term stability.
Hints
- Hint 1
Track the two timescales
Compare what happens within minutes or hours with what happens after several weeks.
- Hint 2
Consider each condition’s role
Ask whether water and seed crystals change the compound itself, its long-term stability, or the structure that initially forms.
- Hint 3
Seek a comprehensive answer
Choose an option that accounts for both the ability to produce either polymorph initially and the one-directional change observed later.
Step-by-step
- Step 1
Distinguish initial formation from stability
The beta polymorph has the lower calculated energy, but alpha forms first in dry solutions. Therefore, the form that appears initially is not necessarily the form favored over time.
- Step 2
Interpret the experimental changes
Trace water and added seed crystals change which polymorph forms first. Because the water does not chemically alter the compound, these conditions affect how crystal formation begins rather than creating a different substance.
- Step 3
Use the long-term evidence
Over several weeks, alpha changes into beta while beta remains unchanged. This result supports the calculation that beta is the more stable form over time.
- Step 4
Identify the complete main idea
The best answer must combine both findings: energy calculations reflect long-term stability, while the conditions present during early crystal formation can determine which structure appears first.