Question

explain what a precipitate is and how it is formed based on solubility principle and how can be separated.

Answer 1

Precipitation

Precipitation reactions occur when cations and anions in aqueous solution combine to form an insoluble ionic solid called a precipitate. Whether or not such a reaction occurs can be determined by using the solubility rules for common ionic solids. Because not all aqueous reactions form precipitates, one must consult the solubility rules before determining the state of the products and writing a net ionic equation. The ability to predict these reactions allows scientists to determine which ions are present in a solution, and allows industries to form chemicals by extracting components from these reactions.

Properties of Precipitates

Precipitates are insoluble ionic solid products of a reaction, formed when certain cations and anions combine in an aqueous solution. The determining factors of the formation of a precipitate can vary. Some reactions depend on temperature, such as solutions used for buffers, whereas others are dependent only on solution concentration. The solids produced in precipitate reactions are crystalline solids, and can be suspended throughout the liquid or fall to the bottom of the solution. The remaining fluid is called supernatant liquid. The two components of the mixture (precipitate and supernate) can be separated by various methods, such as filtration, centrifuging, or decanting.

SOLUBILITY RULES

Whether or not a reaction forms a precipitate is dictated by the solubility rules. These rules provide guidelines that tell which ions form solids and which remain in their ionic form in aqueous solution. The rules are to be followed from the top down, meaning that if something is insoluble (or soluble) due to rule 1, it has precedence over a higher-numbered rule.

1. Salts formed with group 1 cations and NH+4NH4+ cations are soluble. There are some exceptions for certain Li+Li+ salts.
2. Acetates (C2H3O−2C2H3O2−), nitrates (NO−3NO3−), and perchlorates (ClO−4ClO4−) are soluble.
3. Bromides, chlorides, and iodides are soluble.
4. Sulfates (SO2−4SO42−) are soluble with the exception of sulfates formed with Ca2+Ca2+, Sr2+Sr2+, and Ba2+Ba2+.
5. Salts containing silver, lead, and mercury (I) are insoluble.
6. Carbonates (CO2−3CO32−), phosphates (PO3−4PO43−), sulfides, oxides, and hydroxides (OH−OH−) are insoluble. Sulfides formed with group 2 cations and hydroxides formed with calcium, strontium, and barium are exceptions. there are three possible conditions for an aqueous solution of an ionic solid:Q < Ksp. The solution is unsaturated, and more of the ionic solid, if available, will dissolve.

there are three possible conditions for an aqueous solution of an ionic solid:

Q < Ksp. The solution is unsaturated, and more of the ionic solid, if available, will dissolve.
Q = Ksp. The solution is saturated and at equilibrium.
Q > Ksp. The solution is supersaturated, and ionic solid will precipitate.

(The ion product (Q) of a salt is the product of the concentrations of the ions in solution raised to the same powers as in the solubility product expression. It is analogous to the reaction quotient (Q) discussed for gaseous equilibria. Whereas Ksp describes equilibrium concentrations, the ion product describes concentrations that are not necessarily equilibrium concentrations.

An ion product can in principle have any positive value, depending on the concentrations of the ions involved. Only in the special case when its value is identical with Ks does it become the solubility product. A solution in which this is the case is said to be saturated)