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.
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.