Problem 5 - Chemistry The Central Science

Question

You are presented with a white solid and told that due to careless labeling it is not clear if the substance is barium chloride, lead chloride, or zinc chloride. When you transfer the solid to a beaker and add water, the solid dissolves to give a clear solution. Next a \(\mathrm{Na}_{2} \mathrm{SO}_{4}(a q)\) solution is added and a white precipitate forms. What is the identity of the unknown white solid? \([\) Section 4.2\(]\)

Step-by-Step Solution

Answer

The unknown white solid can be identified as either barium chloride (BaCl₂) or lead chloride (PbCl₂). This is based on the fact that the addition of sodium sulfate (\(Na_2SO_4\)) to the clear solution formed a white precipitate, indicating the presence of an insoluble sulfate salt, which is consistent with either barium sulfate (BaSO₄) or lead sulfate (PbSO₄). Zinc chloride (ZnCl₂) can be ruled out as it would not form a precipitate with sodium sulfate because zinc sulfate (ZnSO₄) is soluble.

Step 1: Recall the solubility rules

Generally, salts containing Group 1 elements (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) and ammonium (NH₄⁺) are soluble. Additionally, nitrates (NO₃⁻), acetates (CH₃COO⁻), and most perchlorates (ClO₄⁻) are soluble. Sulfates (SO₄²⁻) are usually soluble, with barium, strontium, and lead sulfates being exceptions. A common saying to remember this is "all sulfates are soluble, except those of barium, strontium, and lead".

Step 2: Analyze the initial solubility observations

When the unknown white solid is transferred to a beaker and mixed with water, it dissolves to give a clear solution. This observation indicates that the solid is an ionic compound that dissociates in water, forming ions. All of the given chloride salts (barium chloride, lead chloride, and zinc chloride) are water-soluble, so this information does not help us distinguish the identity of the unknown solid.

Step 3: Analyze the reaction with sodium sulfate

When a sodium sulfate (Na₂SO₄) solution is added to the clear solution obtained in Step 2, a white precipitate forms. This precipitate suggests that one of the cations in the unknown chloride salt forms an insoluble compound with the sulfate anion (SO₄²⁻).

Step 4: Identify the insoluble sulfate salt

Based on the solubility rules mentioned in Step 1, we know that barium sulfate (BaSO₄) and lead sulfate (PbSO₄) are insoluble, while zinc sulfate (ZnSO₄) is soluble. Given that the addition of sodium sulfate led to the formation of a white precipitate, the unknown solid should be either barium chloride (BaCl₂) or lead chloride (PbCl₂). Zinc chloride (ZnCl₂) can be ruled out since it would not form a precipitate with sodium sulfate.

Step 5: Conclusion

Based on the reactions observed with water and sodium sulfate, the unknown white solid is either barium chloride or lead chloride. The fact that a white precipitate formed after the addition of sodium sulfate indicates the presence of an insoluble sulfate salt, consistent with either barium sulfate or lead sulfate. Zinc chloride could be ruled out since its sulfate salt is soluble.

Key Concepts

Insoluble Sulfate Salts

In chemistry, understanding which compounds are soluble or insoluble in water is crucial. Solubility rules, specifically for sulfate salts, help us predict the outcomes of chemical reactions. Most sulfate salts dissolve in water. However, there are notable exceptions, including barium sulfate (\(\text{BaSO}_4\)), lead sulfate (\(\text{PbSO}_4\)), and strontium sulfate (\(\text{SrSO}_4\)). These salts remain insoluble, meaning that they do not dissolve and will form a solid precipitate when sulfate ions are present.

In many laboratory scenarios, such as in our exercise, determining the solubility of a salt is key to identifying unknown substances. Here, when a sodium sulfate (\(\text{Na}_2 ext{SO}_4\)) solution was added, an insoluble sulfate compound precipitated, indicating the presence of either barium or lead in the initial solution. By remembering which sulfates remain insoluble, we can deduce the identities of unknown compounds effectively.

Ionic Compounds

Ionic compounds are formed when metals transfer electrons to nonmetals, resulting in positive and negative ions that attract each other to form a compound. They are typically solid, crystalline substances with high melting points. For example, barium chloride (\(\text{BaCl}_2\)), lead chloride (\(\text{PbCl}_2\)), and zinc chloride (\(\text{ZnCl}_2\)) are ionic compounds, each composed of metal cations and chloride anions.

One of the key properties of ionic compounds is their ability to dissolve in water and form electrolytic solutions. Upon dissolving, ionic compounds dissociate into their constituent ions, allowing the solution to conduct electricity. This characteristic explains why the white solid in the exercise dissolves in water, forming a clear solution, despite the identity of its metal ion being initially uncertain. Since all the given chloride salts dissolve uniformly, this step didn't narrow down the options, further analysis with other reagents was necessary.

Chemical Precipitation

Chemical precipitation occurs when a solid forms from a solution during a chemical reaction. It is an important process for isolating and identifying substances. In the exercise, adding sodium sulfate (\(\text{Na}_2 ext{SO}_4\)) to a solution of the unknown white solid resulted in the formation of a white precipitate. This event utilizes the solubility rules to create an insoluble compound out of dissolved ions.

When identifying metals such as barium and lead in chloride salts, the reaction with a sulfate solution is effective. If the resulting product is a solid, this indicates the presence of an insoluble sulfate. This practice confirms the presence of either barium or lead, as both their sulfates precipitate from solution due to their low solubility. Thus, chemical precipitation provides a powerful tool for differentiating species in a mixture, based on the solubility properties of their possible reaction products.