Solutions are homogeneous mixtures of two or more substances. In civil engineering, solutions are everywhere: from the hydration of cement to the chlorination of water supplies and the prevention of ice on roadways.

Aqueous solutions, where water is the solvent, possess unique properties based on whether the dissolved solute forms ions.

To effectively utilize a solution in an engineering context, we must know its concentration—the exact ratio of solute (the substance being dissolved) to solvent (the dissolving medium, usually water). Different applications require different units of concentration depending on whether temperature changes are a factor.

Chemical analysis of solutions often relies on stoichiometry. When working with solutions, molarity ($M$) and volume ($V$) are used to calculate the moles of solute involved in a reaction ($n = M \times V$).

Solubility represents the maximum limit of solute that can dissolve in a specific volume of solvent at a given temperature to form a stable, saturated solution. In civil engineering, knowing the solubility rules is crucial, particularly in water and wastewater treatment, to predict when heavy metals or minerals will precipitate out as solids.

Knowing what dissolves is crucial for predicting precipitation reactions.

1. Always Soluble:

   • Alkali metal ions ($Li^+$, $Na^+$, $K^+$, ...) and Ammonium ($NH_4^+$).
   • Nitrates ($NO_3^-$), Acetates ($CH_3COO^-$), Chlorates ($ClO_3^-$), Perchlorates ($ClO_4^-$).

2. Usually Soluble:

   • Chlorides ($Cl^-$), Bromides ($Br^-$), Iodides ($I^-$) — Except with $Ag^+$, $Pb^{2+}$, $Hg_2^{2+}$.
   • Sulfates ($SO_4^{2-}$) — Except with $Ca^{2+}$, $Sr^{2+}$, $Ba^{2+}$, $Pb^{2+}$.

3. Usually Insoluble:

   • Carbonates ($CO_3^{2-}$), Phosphates ($PO_4^{3-}$), Sulfides ($S^{2-}$) — Except with Alkali metals or $NH_4^+$.
   • Hydroxides ($OH^-$) — Except with Alkali metals, $Ca^{2+}$, $Sr^{2+}$, $Ba^{2+}$.

Colligative properties are unique because they depend purely on the number (concentration) of solute particles in a solution, rather than the chemical identity or mass of those particles. This principle governs critical engineering applications, such as adjusting the freezing point of water on road surfaces.

Henry's law relates the solubility of a gas in a liquid to the partial pressure of that gas above the liquid. In environmental engineering, it is used to model the dissolution of oxygen in rivers and wastewater treatment aeration tanks.