Solubility, Salts, and Solutions

Announcements

• Lab exam rescheduled to next Tuesday.

• Lecture exam is this Friday in the usual room/time, covering material from the last ~3 weeks.

• Scholarship recipients are encouraged to attend ceremonies and apply for future opportunities.

Solubility and Temperature

• The solubility of compounds varies; some metal sulfides are insoluble ($FeS$), while others ($CaS$) are soluble.

• Temperature is the most critical factor influencing solubility.

• Heating water generally increases salt solubility, allowing more to dissolve.

• Cooling water decreases solubility, which can lead to recrystallization of the solute.

• "Slightly soluble" compounds, like $MgCO_3$, are considered soluble, just at lower concentrations than others.

Definition of a Salt

• In chemistry, a salt is defined as a metal-nonmetal complex (e.g., $NaCl$, $CaCl_2$).

• An infinite number of salts can be theoretically created by varying chemical conditions.

Hydration and Hygroscopy

• Hygroscopic substances absorb water from the atmosphere (e.g., $MgCl<em>2 ext{•}7H</em>2O$, $NaCl$).

• Anhydrous compounds (without water) have different molecular masses and properties compared to their hydrated forms.

Dissociation of Salts in Water

• When salts dissolve, they ionize and dissociate into individual ions (e.g., $NaCl(s) <br>ightarrow Na^+(aq) + Cl^-(aq)$).

• Water is a polar molecule with a net negative charge on the oxygen and net positive charges on the hydrogen atoms.

• Hydration shells form around ions due to water's polarity:

  • $Na^+$ (positive) attracts the oxygen (negative) end of water.

  • $Cl^-$ (negative) attracts the hydrogen (positive) end of water.

• This process holds ions in solution and prevents them from re-associating.

• At the solubility peak (saturation point), no more solute dissolves, forming a precipitate unless conditions (temperature, pressure, pH) are altered.

Solution, Mixture, and Suspension

• Mixture: A general term for two or more substances, encompassing both solutions and suspensions.

• Solution: An operationally defined mixture where soluble components pass through a 0.45 micron filter.

• Suspension: An operationally defined mixture where particles do not pass through the specified filter.