How Ionic, Polar and Non-polar Substances Dissolve

Why does salt vanish in water but not in oil?

Drop a pinch of salt into a glass of water and stir: it disappears. Drop the same salt into vegetable oil: it sits on the bottom forever. Yet oil dissolves the wax from a crayon, and water does not. Dissolving is a contest of attractions. In this lesson you will see what wins the contest for different kinds of substances.

The three attractions in dissolving

For a solute to dissolve, three things happen:

1. Solute particles must be pulled apart from each other (this takes energy).
2. Solvent molecules must make room (also takes energy: some hydrogen bonds between water molecules break).
3. Solute and solvent particles attract each other (this releases energy).

A solute dissolves well when the new solute-solvent attractions (step 3) are about as strong as, or stronger than, the attractions that were broken in steps 1 and 2.

Ionic compounds: dissociation

When a crystal of sodium chloride is placed in water:

• the δ⁻ oxygen ends of water molecules are attracted to Na⁺ ions at the crystal surface,
• the δ⁺ hydrogen ends are attracted to Cl⁻ ions.

These ion-dipole attractions pull ions off the crystal one by one. Each ion ends up surrounded by a shell of water molecules: it is hydrated. This process of separating an ionic compound into its ions is called dissociation:

NaCl(s) → Na⁺(aq) + Cl⁻(aq)

The ions were already there in the solid; water just separates them. Count ions carefully when the formula has subscripts:

• CaCl₂(s) → Ca²⁺(aq) + 2 Cl⁻(aq)
• Al₂(SO₄)₃(s) → 2 Al³⁺(aq) + 3 SO₄²⁻(aq) (polyatomic ions stay together)

Not every ionic compound dissolves well. In calcium carbonate (limestone), the attraction between Ca²⁺ and CO₃²⁻ ions in the crystal is stronger than what water can offer, so very little dissolves.

Polar molecular compounds: dissolving as molecules

Sugar (sucrose) molecules have many O–H groups. These form hydrogen bonds with water. Water molecules pull sugar molecules off the crystal, and each sugar molecule becomes surrounded by water. But the sugar molecules stay whole: they do not break into ions.

C₁₂H₂₂O₁₁(s) → C₁₂H₂₂O₁₁(aq)

Ethanol (C₂H₅OH) also has an O–H group and is miscible with water for the same reason.

Ionization: when molecules make new ions

A few molecular compounds react with water to form ions that were not there before. Hydrogen chloride is a molecular gas, but in water it forms ions:

HCl(g) → H⁺(aq) + Cl⁻(aq)

This is ionization: neutral molecules become ions. (In the lesson on Arrhenius acids you will see that the H⁺ actually attaches to a water molecule to form H₃O⁺.)

• Dissociation: ions that already existed separate (ionic compounds).
• Ionization: new ions are formed from molecules (acids).

Non-polar substances: like dissolves like

Vegetable oil, gasoline, wax and iodine (I₂) are non-polar. Their molecules attract each other only weakly. If oil molecules squeezed in between water molecules, they would break water's hydrogen bonds but could not offer anything as strong in return. So water molecules "stick to each other" and push the oil out: the oil forms a separate layer.

In a non-polar solvent such as hexane, a non-polar solute mixes easily: both have similar weak attractions, so nothing strong is lost. This gives the rule "like dissolves like":

• polar and ionic solutes dissolve in polar solvents (water),
• non-polar solutes dissolve in non-polar solvents (hexane, mineral oil).

Soap is the clever exception: a soap particle has a long non-polar tail that dissolves in grease and an ionic head that dissolves in water, so it can carry grease into rinse water.

Electrolytes and non-electrolytes

A solution conducts electricity only if it contains charged particles that can move: ions.

• NaCl(aq), CaCl₂(aq) and HCl(aq) contain ions, so they conduct. The solutes are electrolytes.
• Sugar and ethanol dissolve as neutral molecules, so their solutions do not conduct. They are non-electrolytes.
• Pure water conducts extremely poorly. Tap water conducts a little, because it contains small amounts of dissolved ions such as Ca²⁺ and HCO₃⁻.

A conductivity tester (two electrodes, a battery and a bulb or LED) is a quick way to tell an electrolyte from a non-electrolyte in the lab.