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Why does salt dissolve in water?

The GCSE picture: ions, a polar solvent and why the crystal comes apart.

  1. A glass of water10 cm
  2. One grain of salt0.3 mm
  3. The grain’s edge100 nm
  4. The ionsabout 3 nm across
NaCl(s) → Na+(aq) + Cl−(aq)
What the simulation shows, in words

A square slice of a salt crystal sits in water, sodium and chloride ions alternating like a chessboard. Water molecules jostle around it. The ones near an ion turn so that their oxygen end faces a sodium ion or their hydrogen end faces a chloride ion. Ions on the corners and edges, held by fewer neighbours, are pulled away first, each wrapped in a ring of water. Raising the temperature makes everything move faster and ions come away sooner. Stirring also speeds it up. The graph shows the number of ions in solution rising until the crystal has gone.

What you are watching

Salt is a lattice of sodium ions and chloride ions held together by the attraction between their opposite charges. Inside the crystal, every ion has six neighbours of the other kind around it, above, below and on each side, so it is held from every direction. That is why salt is hard and why it only melts at about 801 °C.

Water molecules are polar. The oxygen atom pulls the shared electrons towards itself, so the oxygen end of the molecule is slightly negative (written δ−) and the two hydrogen ends are slightly positive (δ+).

When salt goes into water, the water molecules crowd round the ions on the surface. Their oxygen ends turn towards the positive sodium ions. Their hydrogen ends turn towards the negative chloride ions. An ion on a corner or an edge is held by fewer neighbours than one in the middle, so enough water molecules pulling together can drag it away. Once it is out, a shell of water stays wrapped around it, which stops it rejoining the crystal.

The equation under the simulation shows this, and the state symbols tell the story: the solid, (s), becomes ions dissolved in water, (aq).

Hot water and stirring

Turn up the temperature in the simulation and the water moves faster. Molecules hit the crystal harder and more often, so ions come away sooner. Stirring brings fresh water to the surface. Both make salt dissolve faster.

They make very little difference to how much salt can dissolve. About 36 g of salt dissolves in 100 g of water at room temperature, and only about 39 g near boiling. Sugar is different: far more of it dissolves in hot water than in cold. How fast something dissolves and how much of it dissolves are separate questions, and exam questions often test whether you can tell them apart.

Why it matters

  • Salt water conducts electricity. In the solid, the ions are locked in place. Dissolved, they are free to move, so they can carry charge through the liquid. Pure water barely conducts at all.
  • You can get the salt back. Dissolving is a physical change. Evaporate the water and the ions build a crystal again, which is how sea salt is made.
  • Not everything ionic dissolves. Calcium carbonate (chalk) and silver chloride are ionic but hardly dissolve, because their ions are held together too strongly for water to win. The Deeper page explains how to weigh that up.
Safety

Dissolving table salt is harmless, but this page isn’t a recipe. Many ionic compounds are toxic or corrosive. Do practical work only in a school or college lab, with a teacher.