Unit 8: Comprehensive Summary

Solubility is the maximum amount of a solute that can dissolve in a solvent at a specific temperature. A solution containing this maximum amount is saturated and exists in a state of dynamic equilibrium.

Solubility curves are graphs that plot solubility against temperature. They allow us to determine the solubility at any temperature and calculate the mass of solute that will crystallize when a saturated solution is cooled.

Crystallisation is a technique used to separate and purify soluble solids. It relies on the principle that the solubility of most solids decreases as temperature decreases.

A precipitation reaction occurs when two soluble aqueous solutions are mixed to form an insoluble solid product, called a precipitate.

We can predict whether a precipitate will form by using a set of solubility rules. Key rules include:

• All Group 1, ammonium, and nitrate compounds are soluble.
• Most chlorides are soluble (except AgCl, PbCl₂).
• Most sulfates are soluble (except BaSO₄, CaSO₄, PbSO₄).
• Most carbonates and hydroxides are insoluble (except for Group 1 and ammonium).

These reactions are best represented by net ionic equations, which show only the ions that form the precipitate.

Precipitation reactions are essential for identifying unknown ions in solution.

• Cations: Adding NaOH(aq) and NH₃(aq) produces hydroxide precipitates with characteristic colors. Whether the precipitate redissolves in excess reagent helps identify the cation. Amphoteric hydroxides ($Al^{3+}$, $Zn^{2+}$, $Pb^{2+}$) dissolve in excess NaOH. Complex-forming ions ($Cu^{2+}$, $Zn^{2+}$) dissolve in excess NH₃.
• Anions: Halides ($Cl^-, Br^-, I^-$) are identified by the colored precipitates they form with acidified silver nitrate. The sulfate ion ($SO_4^{2-}$) is identified by the white precipitate it forms with acidified barium chloride.

Hard water contains a high concentration of dissolved $Ca^{2+}$ and $Mg^{2+}$ ions.

• Disadvantages: Forms limescale ($CaCO_3$) when heated, which reduces the efficiency of appliances. It also reacts with soap to form insoluble scum, preventing lathering.
• Hardness Types: Temporary hardness is caused by calcium hydrogencarbonate and can be removed by boiling. Permanent hardness is caused by calcium sulfate and cannot be removed by boiling.
• Water Softening: Methods to remove $Ca^{2+}$/$Mg^{2+}$ ions include boiling (for temporary hardness only), adding washing soda ($Na_2CO_3$) to precipitate the ions, distillation, and ion exchange resins.