The short answer
Same formula, different molecule — structural vs stereoisomers, chirality, and why it matters.
Written and checked by GAMSAT tutors — not AI-generated.
Try the reasoning style
We treat forgetting as a failure — a lapse to be patched with reminders and records. Yet a mind that kept everything could not think; it would drown in the undifferentiated noise of every moment it had ever lived. To forget is not so much to lose information as to decide, mostly without our noticing, what was never worth keeping.
The author's argument relies most directly on which unstated assumption?
Pick an option to see how the tutor reasons to the answer — not just whether you were right.
Not quite — the answer is B.
Work backwards from the conclusion: a mind that ‘kept everything’ supposedly ‘could not think.’ That only follows if thinking means leaving most of experience out — so B is the premise the argument quietly rests on. A raises reliability, which the passage never weighs; C contradicts ‘mostly without our noticing’; D smuggles in a claim about intellect the passage never makes. The question rewards finding the hidden premise, not recalling a fact.
Isomers share a molecular formula but differ as molecules. It's not a technicality: one arrangement can be a life-saving drug while its mirror image is useless or harmful. The GAMSAT tests whether you can classify the relationship between two structures.
The first split
Constitutional (structural) isomers have the same formula but different connectivity — the atoms are bonded in a different order. Stereoisomers have the same connectivity but a different 3D arrangement in space.
Enantiomers vs diastereomers
Enantiomers
- Non-superimposable MIRROR images
- Identical physical properties (except with polarised light / other chiral things)
- Like your left and right hands
Diastereomers
- Stereoisomers that are NOT mirror images
- Different physical properties (melting point, etc.)
- Includes cis/trans (geometric) isomers
Spotting a chiral centre
A carbon is a chiral centre when it's bonded to four different groups. Such a carbon makes the molecule chiral, giving a pair of non-superimposable mirror images (enantiomers). If two of the four groups are the same, it's not chiral.
Worked example
Two molecules have the same formula and the same connectivity, but one is the non-superimposable mirror image of the other. What is their relationship, and how do their boiling points compare?
Check yourself
cis- and trans-2-butene (which differ in the geometry around a C=C double bond) are best classified as:
Key takeaways
- Isomers: same formula, different molecule.
- Constitutional isomers differ in connectivity; stereoisomers differ in 3D arrangement.
- Enantiomers = non-superimposable mirror images (need a chiral centre).
- Diastereomers = stereoisomers that aren't mirror images (includes cis/trans).
- A chiral carbon has four DIFFERENT groups.
Practise this with real GAMSAT-style questions
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