The short answer
Ionic, covalent and metallic bonds — and the weaker forces between molecules that set melting points and solubility.
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.
Why is water a liquid but a gas at room temperature? Why does salt dissolve in water but not in oil? The answers come from bonding — both the strong bonds within molecules and the weaker forces between them.
Within vs between
Bonds within a molecule — ionic (a metal transfers electrons to a non-metal) and covalent (non-metals share electrons) — are strong. The forces between separate molecules (intermolecular forces, IMFs) are much weaker, but they're what you break when you melt or boil a molecular substance.
Ionic vs covalent
Ionic
- Metal + non-metal
- Electrons transferred → ions
- Form lattices; high melting points
- Conduct when molten/dissolved
Covalent
- Non-metal + non-metal
- Electrons shared
- Often discrete molecules
- Polarity depends on electronegativity difference
The third kind: metallic
In a metal, the atoms give up their outer electrons into a shared sea of delocalised electrons flowing around a lattice of positive ions. Two properties fall straight out of that picture: metals conduct (the electrons are free to move) and they are malleable (the layers of ions can slide past each other without any directional bond snapping — unlike an ionic lattice, which shatters when like charges are forced together).
Rank the intermolecular forces
From strongest to weakest: hydrogen bonding > dipole–dipole > London dispersion. Hydrogen bonds only form when H is bonded to N, O or F. Bigger molecules have more electrons, so stronger London forces — which is why boiling point tends to rise down a group of similar molecules.
"Like dissolves like" — and why salt ignores oil
A solvent can only dissolve a solute if the new solute–solvent attractions pay for the old ones you break. Water is polar, so its δ⁻ oxygen surrounds Na⁺ and its δ⁺ hydrogens surround Cl⁻ — those ion–dipole attractions repay the energy cost of pulling the lattice apart. Oil is non-polar: it has nothing to offer an ion, so the lattice stays put. The same logic runs the other way — non-polar solutes dissolve happily in non-polar solvents, because only weak dispersion forces are being traded for weak dispersion forces.
Worked example
Water (, molar mass 18) boils at 100 °C, while methane (, molar mass 16) boils at −161 °C. They're almost the same size — why the enormous difference?
Check yourself
Which substance can form hydrogen bonds between its molecules?
Key takeaways
- Ionic = electron transfer (metal + non-metal); covalent = electron sharing (non-metals); metallic = a sea of delocalised electrons.
- Intermolecular forces are weaker than bonds, but they set melting/boiling points.
- IMF strength: hydrogen bonding > dipole–dipole > London dispersion.
- Hydrogen bonds need H bonded to N, O or F.
- 'Like dissolves like': polar dissolves polar, non-polar dissolves non-polar.
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