Why oil and vinegar separate
Shake a salad dressing and it soon separates into an oily layer and a watery layer. The vinegar is mostly water, made of polar molecules; the oil is made of non-polar molecules. Polarity decides what mixes with what, what boils easily, and even how a microwave oven heats your leftovers (it makes polar water molecules rotate back and forth).
In this lesson you will combine two skills: bond polarity from electronegativity (Grade 11) and molecular shape from VSEPR (this unit).
Step 1: Bond polarity (review)
Remember from Grade 11: electronegativity (EN) is an atom's ability to attract shared electrons. For a bond, find ΔEN = EN(higher) − EN(lower), using the Pauling values rounded to one decimal place:
- 0 to 0.4: non-polar covalent (C–H, 2.6 − 2.2 = 0.4, is treated as non-polar)
- 0.5 to 1.7: polar covalent: the more electronegative atom is δ−
- more than 1.7: mostly ionic
A polar bond is a bond dipole, drawn as an arrow pointing toward the δ− end, with a crossed tail (+) at the δ+ end.
Step 2: Molecular polarity
A molecule is polar if it has a net dipole: one end slightly positive and the other slightly negative. Bond dipoles are vectors: they have size and direction, and they add like forces in physics.
- If the bond dipoles cancel, the molecule is non-polar, even though its bonds are polar.
- If they do not cancel, the molecule is polar.
Bond dipoles cancel when the molecule is symmetrical: the central atom has identical atoms in a symmetrical VSEPR shape (linear AX₂, trigonal planar AX₃, tetrahedral AX₄, trigonal bipyramidal AX₅, octahedral AX₆). Lone pairs on the central atom usually make a shape lopsided (bent, pyramidal, seesaw), so those molecules are usually polar. The exceptions are AX₂E₃ (linear, like XeF₂) and AX₄E₂ (square planar, like XeF₄), where the lone pairs are placed symmetrically.
How big is a dipole?
The size of a molecule's net dipole is its dipole moment, measured in debyes (D), a unit named after the physicist Peter Debye. Some values: CO₂ 0 D, CH₄ 0 D, HCl about 1.1 D, NH₃ about 1.5 D, H₂O about 1.85 D. Water's large dipole moment is one reason it is such an excellent solvent for ionic and polar substances.
Why polarity matters
- Solubility ("like dissolves like"): polar substances tend to dissolve in polar solvents (sugar in water) and non-polar in non-polar (grease in hexane), as you saw in the Grade 11 Solutions and Solubility unit.
- Boiling and melting points: polar molecules attract each other with dipole-dipole forces, so they usually boil at higher temperatures than non-polar molecules of similar size (next lesson).
- Response to electric fields: polar molecules turn to line up in an electric field. Microwave ovens use a rapidly changing field to make water molecules jostle and heat food.
Hands-on activityBending water with static
You need: a tap, a plastic comb or an inflated balloon, a wool sweater or dry hair, and a dry day if possible. Grown-up note: if you use a balloon, supervise young children and pick up any broken pieces at once.
- Turn on the tap so that a very thin, smooth stream of water runs (thinner than a pencil lead is ideal).
- Charge the comb or balloon by rubbing it quickly on wool or dry hair 10 to 15 times.
- Bring it close to the stream (about 1 to 2 cm away) without touching. Watch the stream.
- Try charging for longer, or bringing it closer, and record how far the stream bends each time.
What to look for: the stream bends toward the charged object. Water molecules are polar, so in the electric field near the comb they turn so that their oppositely charged ends face it and are pulled toward it. This is a classic demonstration of polarity; the full explanation also involves electric charges that collect on the surface of the stream, so treat it as good evidence of how strongly water responds to electric fields rather than as a perfect test for polarity. In a school lab, teachers sometimes compare the water stream with a stream of a non-polar liquid, which bends much less; never try that at home, because those liquids are flammable.