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Solutionsand osmosis — why salt disappears in water

Salt dropped into water doesn't vanish — it breaks up into ions, which water molecules surround like a swarm. See it at the molecular scale, make saline, grow crystals, push water through a membrane against osmosis, salt an icy road and split black ink into its colors.

Solvent + solute = solution. How much dissolves depends on temperature, and water always “pulls” toward where there are more dissolved particles.

A crystal in water — the molecular view

20 °C · no stirring

What are we adding?

20 °C

0%

Dissolved
0%
Ions in water
0
Conduction
none
Time
0 s

Simplified model: a flat (2D) slice of water, with molecules a few hundred times slower than in reality. In real water an Na⁺ ion is usually surrounded by 5–6 water molecules, and Cl⁻ by about 6–7.

Hydration: water pulls ions away

A water molecule is a dipole: the oxygen has a partial negative charge (δ−), the hydrogens a partial positive one (δ+). Water turns its oxygen toward an Na⁺ cation and its hydrogens toward a Cl⁻ anion. Ions at the corners and edges of the crystal have the fewest neighbors, so water pulls them off first.

The lattice energy of NaCl (about 787 kJ/mol) is almost equal to the hydration energy of its ions — so dissolving salt barely changes the temperature (it absorbs about 3.9 kJ/mol).

Molecules and polarity →Ions and pH →

Like dissolves like

Water readily dissolves polar and ionic substances: salt, sugar (it has many –OH groups), alcohol. Oil is nonpolar — water has nothing to attract it with and prefers to stick to itself through hydrogen bonds. Oil will, however, dissolve in gasoline or hexane.

Sugar dissolves as whole molecules — there are no ions in the solution, so it doesn't conduct electricity. A salt solution does conduct, because the Na⁺ and Cl⁻ ions can move freely.

Electrochemistry →

Soap: a bridge between water and fat

A soap molecule has a charged “head” (it likes water) and a long hydrocarbon “tail” (it likes fat). The tails dig into a droplet of fat while the heads stay on the outside — forming a micelle. The charged micelles repel each other and don't merge, so the fat can be rinsed away with water.

Warm water and stirring speed up dissolving, but they don't change whether something will dissolve.

How does it work?

Solutions and hydration

A solution is a homogeneous mixture: a solvent (most often water) and a solute. Water is a dipole, so it wraps ions and polar molecules in a “coat” — this is hydration. Ionic compounds break up into ions (dissociation), which is why their solutions conduct electricity.

NaCl(s) → Na⁺(aq) + Cl⁻(aq)

Concentrations

C% = msmsoln · 100%    Cm = nV = msM · V    C1V1 = C2V2

ms — mass of solute, msoln — mass of the whole solution, M — molar mass (NaCl: 58.44 g/mol), V — volume of the solution in L. When diluting, the number of moles stays the same; only the volume changes.

Solubility and gases

Solubility is the largest mass of a substance that will dissolve in 100 g of water at a given temperature. When a saturated solution is cooled, the excess crystallizes: 100 g of water saturated with KNO₃ at 60 °C (106 g), cooled to 20 °C (31.6 g), drops about 74 g of crystals. A gas dissolves better the higher its pressure and the lower the temperature:

c = kH · p Henry's law

Osmosis

A semipermeable membrane lets water through but holds back larger particles. Water crosses in both directions, but more often toward the side with more solute. The flow stops when the pressure difference equals the osmotic pressure:

π = i · c · R · T    h = πρ · g i — number of particles from one formula unit (NaCl ideally 2, in practice about 1.8–1.9)

For 0.15 mol/L NaCl at 37 °C: π ≈ 1.9 · 150 mol/m³ · 8.314 · 310 K ≈ 7.3 bar. If we press on the solution harder than π, water flows the other way — this is reverse osmosis.

Freezing and boiling of solutions

ΔTf = i · Kf · m    ΔTb = i · Kb · m water: Kf = 1.86, Kb = 0.512 K·kg/mol

m is the molality (moles of solute per 1 kg of water). What counts is the number of particles, not their kind — these are colligative properties. 1 mol of NaCl per kg of water (2 mol of ions) ideally lowers the freezing point by about 3.72 °C. For concentrated solutions the formulas are no longer accurate, which is why the graph also shows measurements.

Mixtures and separation

In a solution the particles are smaller than about 1 nm; in a colloid they are about 1–1000 nm (they scatter light — the Tyndall effect); in a suspension they are bigger still and settle over time. We choose a separation method by the differences: grain size (filtration), volatility (evaporation, distillation), density (separatory funnel, centrifuge) and affinity for the paper (chromatography).

Simplifications: we treat solutions as ideal where reasonable, we add volumes when diluting, and the molecule simulations are two-dimensional and slowed down.

Challenges

  1. Make a 0.9% saline solutionConcentration: NaCl dissolved in water, concentration 0.88–0.92%.not completed
  2. Crystallize at least 20 g of KNO₃ by coolingSolubility: dissolve KNO₃ in hot water, then cool it.not completed
  3. Save the red blood cell — pick an isotonic solutionOsmosis: the cell is crinkled; choose a concentration that brings it back to normal (without hemolysis).not completed
  4. Reverse osmosis and get fresh waterOsmosis: push the piston harder than π and collect at least 100 mL of fresh water.not completed
  5. Lower the freezing point of water by 5 °CFreezing: choose the substance and the concentration.not completed
  6. Separate black ink into colorsMixtures: run a chromatogram of the black marker until the water has climbed 70% of the strip.not completed

Did you know…?

  • 0.9% saline has almost the same osmolarity as blood plasma (about 290 mOsm/L) — that's why red blood cells in it neither swell nor shrink.
  • The Dead Sea contains about 34% dissolved salts — almost 10 times more than the ocean. People float in it like corks, because the water's density reaches about 1.24 g/cm³.
  • Salting pasta water the usual way (about 10 g per liter) raises its boiling point by only about 0.2 °C.

Stay safe

  • Potassium permanganate (KMnO₄) is a strong oxidizer: it stains skin and clothes; don't combine it with other chemicals, and keep it away from children.
  • Don't drink seawater or “homemade” salt solutions — saline for a vein is given only by medical staff, from sterile packaging.
  • Hot solutions (syrup, sodium acetate) burn worse than water, because they are thick and sticky. Wear goggles.
  • Don't shake a warm bottle of fizzy drink before opening it — the CO₂ pressure is higher then.

Keys

Q W E R T Y
switch modes
Space
start / pause: molecules, cooling the beaker, osmosis, chromatography
N
reset the current mode
K
solubility: touch the sodium acetate with a crystal
L
mixtures: laser on / off
Esc
stop everything
Molecules →Acids and bases →Kitchen chemistry →Blood →The kidneys →The cell →States of matter →