Question

A solution contains both copper(II) nitrate and lead(II) nitrate. What substance could be added to the solution to precipitate the lead(II) ions, but leave the copper(II) ions in solution? After removal of the lead(II) compound, what could then be added to the solution to precipitate the copper(II) ions?

Short answer

To precipitate the lead(II) ions but leave the copper(II) ions in solution, a sulfate such as sodium sulfate could be added. After removal of the lead(II) compound, a hydroxide such as sodium hydroxide could then be added to the solution to precipitate the copper(II) ions.

Step-by-step solution

Analyze the Solubility Rules

Start by looking at the solubility rules for common ionic compounds. By utilizing the solubility rules, we can identify compounds that would help precipitate specific ions from a solution. Lead(II) ions (Pb2+) will precipitate with sulfate ions (SO4 2-) in the form of lead(II) sulfate (PbSO4), which is an insoluble compound. Copper(II) ions (Cu2+) will stay in the solution as copper sulfate (CuSO4) is soluble.

Identify the compound to precipitate Copper(II) ions

After removing the precipitate of lead(II) sulfate, we need to identify a compound that will precipitate copper(II) ions but not lead(II) ions (even if they had been present). The compound that will achieve this is sodium hydroxide. When sodium hydroxide (NaOH) is added to the solution, it will precipitate Copper(II) ions (Cu2+) out of the solution in the form of copper(II) hydroxide (Cu(OH)2), which is an insoluble compound.

Finalizing the substances

The substance that can be added to the solution to precipitate the lead(II) ions but leave the copper(II) ions in the solution is sodium sulfate or any other sulfate. Subsequently, the substance that can be added to precipitate the copper(II) ions is sodium hydroxide or any other hydroxide.

Key concepts

Solubility Rules

Solubility rules are essential guidelines that help us predict the behavior of ionic compounds in aqueous solutions.
These rules allow us to foresee whether a compound will dissolve in water or form a solid precipitate.
Understanding these rules is crucial when working with solutions containing multiple ions. Key solubility principles include:

• Most nitrate ( NO_3^- ) salts are soluble. This means that compounds like copper(II) nitrate and lead(II) nitrate will remain dissolved in water.
• Sulfates ( SO_4^{2-} ) are generally soluble, except those formed with lead ( Pb^{2+} ), barium ( Ba^{2+} ), and calcium ( Ca^{2+} ), among others.
• Most hydroxides ( OH^- ) are insoluble, except those of alkali metals and barium.

By applying these rules, we can strategically add specific compounds to make particular ions precipitate while keeping others dissolved.

Copper(II) Nitrate

Copper(II) nitrate, chemically represented as Cu(NO_3)_2 , is a blue crystalline solid that is highly soluble in water.
In solution, it dissociates into copper(II) ions, Cu^{2+} , and nitrate ions, NO_3^- .
The high solubility of copper(II) nitrate is primarily due to the presence of nitrate, a generally soluble anion. In chemical processes, the presence of copper(II) ions can be manipulated to selectively precipitate copper compounds. For example, adding sodium hydroxide to a copper(II) nitrate solution results in the formation of copper(II) hydroxide, Cu(OH)_2 , an insoluble solid that precipitates out of the solution. Applications involving copper(II) nitrate require knowledge of solubility to ensure desired reactions occur.

Lead(II) Nitrate

Lead(II) nitrate, Pb(NO_3)_2 , is another compound that dissolves readily in water, forming lead(II) ions, Pb^{2+} , and nitrate ions, NO_3^- .
Its solubility is typical of most nitrate-based salts, which dissolve to form clear solutions in water.
However, despite the solubility of its nitrate form, lead is notorious for forming insoluble compounds with certain anions. For instance, adding sulfate ions to a solution containing lead(II) ions results in the formation of lead(II) sulfate, PbSO_4 , an insoluble compound that will precipitate out of the mixture.
This behavior is exploited to selectively precipitate lead from solutions containing copper, as copper sulfates remain soluble. This precipitation property makes lead(II) nitrate crucial for separation techniques based on solubility differences.

Insoluble Compounds

Insoluble compounds are those that do not dissolve appreciably in water, often forming precipitates when specific ions in a solution combine.
These precipitates are valuable in separating and purifying elements from a mixture.
Understanding which compounds are insoluble is pivotal in chemical processes. Some common insoluble compounds include:

• Lead(II) sulfate ( PbSO_4 ): Formed from lead(II) and sulfate ions, a well-known insoluble compound.
• Copper(II) hydroxide ( Cu(OH)_2 ): Precipitates from copper(II) ions when hydroxide ions are present.
• Barium sulfate ( BaSO_4 ): Often used in medical imaging due to its insolubility.

These compounds typically emerge when solutions reach saturation or when specific reagents induce precipitation.
By leveraging the insolubility of these compounds, chemists can drive reactions and processes to efficiently separate desired ions.