precipitation

Examples of precipitation in the following topics:

• Precipitation Reactions

  • Precipitation reactions transform ions into an insoluble salt in aqueous solution.
  • Precipitation refers to a chemical reaction that occurs in aqueous solution when two ions bond together to form an insoluble salt, which is known as the precipitate.
  • The following is a common laboratory example of a precipitation reaction.
  • A final way to represent a precipitation reaction is known as the net ionic equation.
  • Distinguish ways to write precipitation reactions (complete ionic equation and net ionic equation) and use a solubility table to determine whether a precipitation reaction will occur

• Ion Separation by Fractional Precipitation

  • Silver chloride (AgCl) is not soluble and forms a white precipitate.
  • So how does one tell if the precipitate is a carbonate or a chloride?
  • If the precipitate remains unchanged, then the salt is a chloride.
  • Bromides and iodides also form precipitates when they are reacted with silver nitrate, but both precipitates are pale yellow.
  • Which ion will precipitate first?

• Predicting Precipitation Reactions

  • Sometimes ions in solution react with each other to form a new substance that precipitates; this reaction is called a precipitation reaction.
  • A precipitate, which looks like light blue specks of dust, forms.
  • No precipitate forms.
  • Therefore Na2CO3 will remain in solution, but CuCO3 will precipitate out.
  • This is why no precipitate forms in this second reaction.

• Chemical Analysis

  • Two common techniques are precipitation analysis and colorimetric analysis.
  • The color of the precipitate will indicate the metal.
  • Barium, calcium, and strontium will precipitate at this point, but not before.
  • However, precipitation methods similar to those mentioned above are often used.
  • Difference in the visual appearance of an aggregate and a precipitate.

• Solubility Product Principle and Qualitative Analysis

  • Solubility-product constants can be used to devise methods for separating ions in a solution by selective precipitation.
  • Selective precipitation is used to form a solid with one of the ions in solution without disturbing the other ions.
  • Can either Ba2+ or Sr2+ be precipitated selectively with concentrated sodium sulfate (Na2SO4) solution?
  • Which ion will precipitate first?
  • Precipitation is used in qualitative chemical analysis.

• Molecular, Ionic, and Complete Ionic Equations

  • Precipitation reactions can be written as molecular, ionic, or complete ionic equations.
  • There are various ways to write out precipitation reactions.
  • On the right hand side of the equation, the precipitant (AgCl) is written in its full formula and designated as a solid, since this is the precipitate that is formed in the reaction.
  • In this representation, the ions that are uninvolved in the precipitation reaction ($Ca^{2+}$ and $NO_{3}^{-}$) are simply excluded from the reaction, because they are essentially redundant on both the left and right side.
  • Silver chloride is a precipitant of silver and chloride ions reacting in solution.

• Effect of a Common Ion on Solubility

  • Adding a common ion decreases the solubility of a solute, causing it to precipitate.
  • When equilibrium is shifted toward the reactants, the solute precipitates.
  • In the water treatment process, sodium carbonate salt is added to precipitate the calcium carbonate.
  • The very pure and finely divided precipitate of calcium carbonate that is generated is used in the manufacture of toothpaste.
  • Lithium hydroxide forms less-soluble lithium carbonate, which precipitates because of the common ion effect.

• Copper

  • Adding aqueous sodium hydroxide causes the precipitation of light blue solid copper (II) hydroxide.
  • Aqueous ammonia results in the same precipitate.
  • Upon adding excess ammonia, the precipitate dissolves, forming tetraamminecopper (II):
  • Aqueous ammonia results in the same precipitate.
  • Upon adding excess ammonia, the precipitate dissolves, forming tetraamminecopper(II):

• Measuring Reaction Rates

  • In a reaction in which a precipitate is formed, the amount of precipitate formed in a period of time can be used as a measure of the reaction rate.
  • For example, when sodium thiosulphate reacts with an acid, a yellow precipitate of sulfur is formed.
  • However, as the reaction progresses and more precipitate is formed, the cross will gradually become less clear and will eventually disappear altogether.
  • By using a stopwatch to time how long it takes for the cross to disappear, and then massing the amount of precipitate formed during this time, an average reaction rate can be calculated.

• Oxidation

  • This deep blue reagent is reduced to cuprous oxide, which precipitates as a red to yellow solid.
  • To avoid precipitation of the insoluble metal hydroxides, the cations must be stabilized as complexed ions.