Particle Model of Dissolving
The particle model of dissolving explains that when a solute dissolves, its particles are separated from each other and spread evenly among the particles of the solvent, with no particles lost or destroyed. In brief: when sugar dissolves in water, sugar molecules leave the crystal and mix between the water molecules, so the sugar seems to vanish, but the mass of the solution equals the mass of sugar plus water.
The particle model gives clear answers to common questions about dissolving: where does the solute go, why does the level of water barely change and why does stirring help. This article describes the process step by step, explains why the volume change is small, compares dissolving ionic, molecular and non-dissolving substances at the particle level, works through mass and concentration calculations and gives some history.
How do particles behave during dissolving?
- Solvent particles move randomly and collide with the surface of the solute.
- Attractions between solvent particles and solute particles at the surface pull solute particles away from the solid.
- The freed solute particles are surrounded by solvent particles and move into the liquid.
- Random motion spreads the solute particles through the solution until the mixture is uniform.
The solute particles are far too small to see, so the liquid looks clear. The solute does not disappear, because it is present as separate particles. Stirring speeds up the process by moving the solution away from the surface of the solid, and warming the solvent makes its particles move faster, so they collide with the solid more often and with more energy.
Why is mass conserved?
No particles are created or destroyed when a solute dissolves. All the solute particles are still present in the solution, so the mass of the solution equals the mass of the solute plus the mass of the solvent. If 10 g of salt dissolves in 90 g of water, the solution has a mass of 100 g. This agrees with the law of conservation of mass.
Why does the volume barely change?
When a solute dissolves, the solute particles fit into the spaces between solvent particles. As a result, the total volume of the solution is often smaller than the sum of the separate volumes, or only a little larger. Adding a spoonful of salt to a full glass of water rarely makes it overflow immediately for this reason. Mixing 50 mL of ethanol with 50 mL of water gives about 96 to 97 mL, because the molecules pack more closely together than in the pure liquids.
Dissolving at the particle level
| Solute | Particles that enter solution | Why it happens |
|---|---|---|
| Sodium chloride | Na+ and Cl− ions | Water molecules attract and surround the ions |
| Sugar (sucrose) | Whole sucrose molecules | Hydrogen bonds with water |
| Ethanol | Whole ethanol molecules | Hydrogen bonds with water |
| Oxygen | O2 molecules (slightly) | Weak attractions to water |
| Cooking oil | None | Water particles attract each other more than oil particles |
| Sand | None | Strong bonds hold the solid together |
What is saturation in the particle model?
As more solute dissolves, some dissolved particles come back into contact with the solid and rejoin it. In a saturated solution, particles leave the solid at the same rate that other particles rejoin it. Dissolving and crystallizing continue, but there is no net change, which is called a dynamic equilibrium. Added solute then remains undissolved.
Worked example: mass and particles in a solution
A student dissolves 5.85 g of sodium chloride in 94.15 g of water.
- Mass of solution. 5.85 + 94.15 = 100.00 g.
- Moles of NaCl. The molar mass is 23.0 + 35.5 = 58.5 g/mol, so n = 5.85 ÷ 58.5 = 0.100 mol.
- Number of formula units. 0.100 × 6.02 × 1023 = 6.02 × 1022.
- Number of ions. Each formula unit gives 2 ions (Na+ and Cl−), so 1.20 × 1023 ions are in the solution.
- Mass percent. 5.85 ÷ 100.00 × 100% = 5.85%.
- Moles of water. 94.15 ÷ 18.0 = 5.23 mol, so there are about 5.23 ÷ 0.100 = 52 water molecules for each NaCl formula unit.
This shows how few solute particles there are compared with solvent particles, even in a solution that tastes quite salty.
How did scientists understand dissolving?
For a long time dissolving was explained as a kind of “melting” of the solid by the liquid. In the 19th century, the idea of particles led to explanations of dissolving as spreading. In 1887 Jacobus van ‘t Hoff showed that the osmotic pressure of dilute solutions follows laws similar to those of gases, which suggested that dissolved particles behave like a gas of freely moving particles. Svante Arrhenius added the idea of ions for salts in 1884, and van ‘t Hoff received the first Nobel Prize in Chemistry in 1901.
Which mistakes are common?
Mistake 1: Saying that the solute melts
Melting is a change of state caused by heat. Dissolving is the mixing of solute particles with solvent particles.
Mistake 2: Believing that the solute particles vanish
They are present as separate particles. Evaporating the water brings them back together as crystals.
Mistake 3: Thinking that the particles of the solute become smaller
The particles keep their size. Only the crystal is broken apart.
Mistake 4: Imagining that the solute fills the empty spaces completely
The solute particles fit partly between solvent particles, but they also push them apart a little, so the volume of the solution can increase slightly.