Water is a good solvent for dissolving ionic compounds because

Correct — C, it has a high dipole moment. Water is a bent molecule: oxygen pulls electron density away from the two hydrogens, and because the two bonds are set at about 104.5 degrees their polarities do not cancel. The molecule therefore carries a permanent dipole of about 1.85 debye, with a partial negative charge on oxygen and partial positive charges on the hydrogens. When an ionic solid is placed in water, those charged ends surround each ion — oxygen towards the cations, hydrogens towards the anions — and the energy released by this hydration is enough to overcome the lattice energy holding the crystal together. The other three options list real properties of water, but none of them has anything to do with pulling ions apart.

• (a)It has a high specific heat — Water does have an unusually high specific heat, about 4,186 joules per kilogram per kelvin, which is why it moderates coastal climates. That property concerns how much energy it takes to warm water, not how it dissolves salts.
• (b)It has no colour — Being colourless is an optical property with no bearing on solvent power. Bromine is deeply coloured and dissolves in water; liquid ammonia is colourless and dissolves ionic solids well, but so does colourless liquid hydrogen fluoride, and colourless hexane dissolves none of them.
• (d)It has a high boiling point — Water's boiling point of 100 degrees Celsius is high for a molecule of its size, and that too comes from hydrogen bonding, but a high boiling point does not dissolve anything. Molten sodium chloride boils far higher and is not a solvent for ionic solids at room temperature.

Dissolution is a competition between two energies. Breaking the crystal apart costs the lattice energy; surrounding the separated ions with solvent molecules releases the solvation energy. A polar solvent releases a large solvation energy because its dipoles can orient around each ion, so it can pay the lattice bill. Water is also helped by its high dielectric constant of about 80, which weakens the attraction between separated ions in solution by a factor of eighty, and by hydrogen bonding. Non-polar solvents such as petrol offer none of this, which is why 'like dissolves like'.

Three of the four options describe genuine and famous properties of water, which is what makes the item tempting. The discipline is to ask which property could physically do the job named in the stem. Dissolving an ionic solid means separating charges, so the answer must be the option that involves charge. Strictly the more complete reason is water's high dielectric constant, and the dipole moment is the molecular property underlying it; among the four printed choices, only the dipole moment names anything in that family.

• The water molecule is bent at about 104.5 degrees, so its two bond dipoles do not cancel; the net dipole moment is about 1.85 debye.
• Water's dielectric constant is about 80, which weakens the force between separated ions by that factor.
• Dissolving an ionic solid works when hydration energy is comparable to or greater than the lattice energy.
• Water's specific heat of about 4,186 J/kg/K and its boiling point of 100 degrees Celsius both come from hydrogen bonding.
• Non-polar solvents such as petrol and benzene cannot dissolve ionic solids, which is the basis of 'like dissolves like'.

• Picking a famous property of water without asking whether it could do the job described.
• Assuming a high boiling point implies a strong solvent.
• Forgetting that carbon dioxide, with two polar bonds, has no net dipole because it is linear.

A cause-and-effect item where all four options are true statements about water and only one is the relevant cause.

• practice — not a real PYQ

  Which one of the following molecules is non-polar in spite of having polar bonds?

  • (a)Water
  • (b)Carbon dioxide
  • (c)Ammonia
  • (d)Hydrogen chloride

  Answer(b) Carbon dioxide — it is linear, so the two bond dipoles cancel exactly.

• practice — not a real PYQ

  Common salt dissolves readily in water but not in petrol chiefly because petrol

  • (a)is less dense
  • (b)is non-polar
  • (c)has a lower boiling point
  • (d)is inflammable

  Answer(b) is non-polar — it cannot solvate ions and so cannot pay the lattice energy.