An aqueous solution is a chemical solution in which the solvent is water. The solutes are dissolved molecules and ions that are surrounded by water molecules. An aqueous solution is shown by writing (aq) after a chemical formula. For example, an aqueous solution of salt (NaCl) in water is NaCl(aq) or Na+(aq) + Cl-(aq). In contrast, a solution in which the solvent is not water is called a non-aqueous solution.
Examples of Aqueous Solutions
Both ionic and covalent solutes dissolve in water and form aqueous solutions. Examples of aqueous solutions include:
- Saline solution
- Arrhenius acid and base solutions
- Sweet tea
Examples of non-aqueous solutions include any solutions in oil, hexane, benzene, toluene, or other solvents that are not water. When a substance combines with water and forms a mixture but does not dissolve, an aqueous solution is not formed. For example, mixing sand and water does not form an aqueous solution.
Example Aqueous Solution Chemistry Problems
Students encounter a few different types of chemistry problems concerning aqueous solutions. These mainly concern questions of solubility and colligative properties.
Example: Which solute forms an aqueous solution?
- sodium nitrate (NaNO3)
- calcium carbonate (CaCO3)
- silver hydroxide (AgOH)
- copper(I) sulfide (Cu2S)
Technically, this is not a great question because all ionic compounds form aqueous solutions, even if they are very poorly soluble. This is because ionic compounds, like water, are polar molecules. But, the point of a question like this is getting a student to understand solubility rules. Based on these rules, only sodium nitrate is highly soluble in water. Most carbonates, hydroxides, and sulfides are insoluble and these particular compounds are not exceptions to the rules.
Other common questions concern colligative properties. Colligative properties, like freezing point depression and boiling point elevation, depend on the number of particles dissolved in water. The more a compound dissociates into ions or the greater its concentration, the higher it raises boiling point or lowers freezing point.
Example: Which aqueous solution has the lowest freezing point?
- 0.1 molal urea (CH4N2O) solution
- 0.1 molal sucrose (C12H22O11) solution
- 0.1 molal sodium chloride (NaCl) solution
- 0.1 molal calcium chloride (CaCl2) solution
Note that the freezing points of the compounds does not matter. Since all the solutions have the same concentration, all you need to look at is how many particles each molecule breaks into in aqueous solution. Urea and sucrose are covalent compounds, so they dissolve in water but don’t dissociate into ions. You know this because the compounds are organic. This leaves sodium chloride and calcium chloride. Both of these compounds are ionic and soluble in water. They dissociate into their ions in aqueous solution. But, sodium chloride only breaks into two ions or particles (Na+, Cl–). Meanwhile, calcium chloride breaks into three ions (Ca2+, Cl–, Cl–). So, the 0.1 molal calcium chloride solution has the lowest freezing point.
Example: Which aqueous solution has the highest boiling point?
- 0.1 M NaCl
- 0.1 M sucrose (C12H22O11)
- 0.1 M CaCl2
- 0.1 M AlCl3
Work this problem exactly like the freezing point depression question. First, make certain the compounds are soluble in water. Next, check the concentration of the solutions. In this case, all four compounds are soluble and have the same concentration values. Finally, compare the number of particles released when each of the compounds dissolves in water. Sucrose dissolves but does not dissociate so it only forms one particle and has the least effect on boiling point. NaCl forms two particles, CaCl2 forms three particles, and AlCl3 forms four particles (Al3+, Cl–, Cl–, Cl–). The aluminum chloride solution has the highest boiling point.
- Castellan, Gilbert W. (1983). Physical Chemistry (3rd ed.). Addison-Wesley. ISBN 978-0201103861.
- IUPAC (1997). “Solution”. Compendium of Chemical Terminology (the “Gold Book”) (2nd ed.). Blackwell Scientific Publications. doi:10.1351/goldbook.S05746
- McQuarrie, Donald; et al. (2011). “Colligative properties of Solutions”. General Chemistry. Mill Valley: Library of Congress. ISBN 978-1-89138-960-3.
- Zumdahl, Steven S. (1997). Chemistry (4th ed.). Boston, MA: Houghton Mifflin Company. pp. 133–145. ISBN 9780669417944.