Dissolving is the process in which particles of a solute spread evenly through a solvent to form a solution. It matters because many reactions in chemistry, biology, medicine, and environmental science happen in solution. When salt dissolves in water or sugar dissolves in tea, the change depends on attractions between particles, not just on stirring.
A solution looks uniform because its particles are mixed at the molecular or ionic level.
At the particle level, solvent molecules pull solute particles away from one another and surround them. Water dissolves many ionic compounds because its polar molecules attract positive and negative ions. The overall dissolving process depends on the balance between energy needed to separate solute particles, energy needed to separate solvent particles, and energy released when solute and solvent particles attract.
This is why some substances dissolve easily, some dissolve only slightly, and others do not dissolve in a given solvent.
Understanding Chemistry: The Dissolving Process
A solid does not vanish when it dissolves. Its particles remain present, but they become too small and too widely separated to see. In a crystal such as table salt, ions are held in a repeating structure.
Water molecules move around the crystal surface and orient themselves toward the charged ions. The oxygen end of a water molecule is slightly negative, while the hydrogen ends are slightly positive. This arrangement helps water carry sodium and chloride ions away from the crystal.
Each ion becomes surrounded by water molecules. This surrounding is called hydration. Sugar behaves differently because it stays as whole molecules rather than splitting into ions.
Dissolving has a limit at a particular temperature. When no more solute can remain dissolved, the solution is saturated. Adding more solid after this point leaves some undissolved material at the bottom.
This does not mean particle motion has stopped. In a saturated solution, particles leave the solid and enter the liquid while dissolved particles return to the solid. These changes occur at equal rates.
This is a dynamic equilibrium. A supersaturated solution contains more dissolved solute than is normally stable.
It can form when a hot saturated solution cools carefully. A small crystal or even a scratch on the container can then trigger rapid crystal growth.
It is important to separate the rate of dissolving from solubility. Crushing a solid creates more exposed surface, so solvent particles can reach it more easily. Stirring replaces liquid near the solid with fresh solvent.
These actions usually make dissolving faster, but they do not necessarily increase the final amount that can dissolve. Temperature often increases the solubility of solid substances because warmer particles move more energetically. Gas solutes show a different pattern.
Cold water generally holds more dissolved gas than warm water. Heating a fizzy drink releases carbon dioxide, which is why it loses its bubbles. Higher pressure above a liquid can force more gas into solution, as in unopened soft drink bottles.
Concentration affects properties that students can measure. A concentrated salt solution contains more dissolved ions in the same volume than a dilute one. Since ions can move through the liquid, salt solution conducts electric current.
Sugar solution does not conduct well because its dissolved molecules have no overall charge. Concentration matters in blood, medicines, sports drinks, pools, and rivers. Cells can gain or lose water when the concentration outside them differs from the concentration inside.
When studying dissolving, pay attention to the type of particles present, the temperature, and whether the observation concerns speed, maximum solubility, or concentration. These ideas explain many results that can otherwise seem contradictory.
Key Facts
- A solution contains a solute dissolved uniformly in a solvent.
- For ionic solids in water, dissociation can be written as NaCl(s) -> Na+(aq) + Cl-(aq).
- Like dissolves like: polar solvents dissolve polar or ionic solutes best, while nonpolar solvents dissolve nonpolar solutes best.
- Enthalpy of solution: ΔHsoln = ΔHsolute separation + ΔHsolvent separation + ΔHmixing.
- Molarity measures concentration: M = moles of solute / liters of solution.
- Dissolving rate increases with stirring, smaller particle size, and higher temperature for many solid solutes.
Vocabulary
- Solute
- The substance that is dissolved in a solvent to form a solution.
- Solvent
- The substance that does the dissolving and is usually present in the greater amount.
- Solvation
- The process in which solvent molecules surround and stabilize dissolved solute particles.
- Hydration
- Solvation by water molecules, especially when water surrounds ions or polar molecules.
- Saturated solution
- A solution that contains the maximum amount of dissolved solute possible at a given temperature.
Common Mistakes to Avoid
- Thinking dissolving always means melting, which is wrong because dissolving separates particles into a solvent while melting changes a solid into a liquid by heating.
- Ignoring solvent polarity, which is wrong because solubility depends strongly on whether solute and solvent particles can attract each other.
- Assuming stirring increases solubility, which is wrong because stirring usually increases the rate of dissolving but does not change the maximum amount that can dissolve at a fixed temperature.
- Writing ions as neutral atoms after dissolving, which is wrong because ionic compounds separate into charged ions such as Na+ and Cl- in water.
Practice Questions
- 1 A student dissolves 5.85 g of NaCl in enough water to make 0.500 L of solution. If the molar mass of NaCl is 58.44 g/mol, what is the molarity?
- 2 A solution contains 0.250 mol of glucose in 2.00 L of solution. What is the molarity of the glucose solution?
- 3 Water dissolves sodium chloride well but does not dissolve cooking oil well. Explain this using polarity, solute-solvent attractions, and the idea of like dissolves like.