← Bonding, structure, properties and energy changes

Writing a full "compare and contrast" answer

The single most recurring Excellence task in this paper, in three different disguises (melting point, polarity, solubility) — but the same shape underneath every time.

The shape of a full-credit comparison

  • Name the STRUCTURE AND BONDING of substance A, and state what property that produces.
  • Name the STRUCTURE AND BONDING of substance B, and state what property that produces.
  • Make an EXPLICIT comparison between the two — which is bigger/stronger/more soluble, and WHY, tying the difference back to a genuine difference in structure or bonding (not just restating which number is bigger).
  • Never describe only the "winning" substance — the schedule specifically rewards addressing BOTH substances' structure and bonding.
Worked example Comparing the shape and bond angles of two molecules Freon-11 (CCl₃F, bond angle 109.5°) has 4 regions of electron density around carbon; sulfur dioxide (SO₂, bond angle 120°) has 3 regions around sulfur, one of which is a lone pair. Compare and contrast their shapes and bond angles. FREON-11: 4 regions of electron density around the central carbon, all bonding. These arrange for maximum separation, giving a tetrahedral parent geometry. Since all 4 regions are bonding, the final molecular shape IS tetrahedral, with bond angles of 109.5°. SULFUR DIOXIDE: 3 regions of electron density around the central sulfur — 2 bonding, 1 lone pair. These arrange for maximum separation, giving a trigonal planar PARENT geometry with angles of 120°. Because one region is a lone pair (non-bonding), the final molecular SHAPE is bent, not trigonal planar — but the bond angle stays close to the parent 120° since there are only 3 regions total. EXPLICIT COMPARISON: freon-11 has one more region of electron density than SO₂ (4 vs 3), and none of freon-11's regions are lone pairs, whereas SO₂ has one — these two structural differences together explain both the different parent geometries and the different final shapes. ✓ Both substances are correctly described AND an explicit link is drawn between the DIFFERENCE in their electron-region count/type and the difference in shape — that link is what an Excellence-level "compare and contrast" answer requires. ✕

Describing both substances separately is not the same as comparing them

Writing a correct paragraph about substance A, then a correct paragraph about substance B, with no sentence connecting the two, stops short of a genuine comparison. The schedule specifically wants the DIFFERENCE stated and explained — "freon-11 has one more bonding region than SO₂, which is why..." — not simply two accurate but disconnected descriptions placed next to each other.

The same shape applies to melting point AND solubility comparisons, not just shape

  • Melting point comparison: name each substance's particle type and attractive force, THEN say which force is stronger and why that means a higher melting point.
  • Solubility comparison: name each substance's polarity/bonding, THEN say whether new attractions with the solvent can or cannot overcome the original ones, for BOTH substances.
  • The four-part shape (structure A, structure B, explicit link, both addressed) is identical whichever property the question happens to ask about.

Practise just this