Problem 5 - Chemistry The Central Science In Si Units

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 an \(\mathrm{Na}_{2} \mathrm{SO}_{4}(a q)\) solution is added and a white precipitate forms. What is the identity of the unknown white solid?

Step-by-Step Solution

Answer

The unknown white solid is barium chloride (BaCl₂).

Step 1: Evaluating Solubility in Water

The first observation is that the white solid dissolves to give a clear solution when added to water. Among the potential compounds, both barium chloride (BaCl₂) and zinc chloride (ZnCl₂) are soluble in water, whereas lead chloride (PbCl₂) is only sparingly soluble at room temperature. This suggests that the solid is more likely to be BaCl₂ or ZnCl₂.

Step 2: Reaction with Sodium Sulfate

When Na₂SO₄ solution is added to the clear solution, a white precipitate forms. This suggests that a sulfate-containing compound is forming with one of the cations. BaCl₂ reacts with sulfates to form a white precipitate of barium sulfate (BaSO₄), which is insoluble in water. In contrast, neither lead chloride in solution nor zinc chloride reacts with sulfate to form a noticeable white precipitate.

Step 3: Confirming the Reaction Product

The formation of a white precipitate upon adding Na₂SO₄ strongly indicates the formation of BaSO₄, confirming the presence of barium ions. ZnSO₄ is soluble in water and would not precipitate, further ruling out ZnCl₂ as the unknown solid.

Step 4: Conclusion

Given the solubility in water and the formation of a white precipitate with Na₂SO₄, it is most likely that the unknown white solid is barium chloride (BaCl₂), as the reactions and observations correspond to the behavior of barium compounds.

Key Concepts

Solubility

Solubility is a fascinating concept in chemistry, describing how much of a substance can dissolve in a solvent, such as water. In this exercise, solubility helps distinguish between different compounds. When a solid dissolves in water, it forms a homogeneous mixture known as a solution.
This property is crucial for identifying substances through experimentation because compounds exhibit varying levels of solubility.
For instance:

• Barium chloride (BaCl₂) is highly soluble in water, meaning it dissolves easily, producing a clear solution.
• Lead chloride (PbCl₂) is only sparingly soluble at room temperature, meaning it does not dissolve completely.
• Zinc chloride (ZnCl₂), similar to BaCl₂, is mostly soluble in water.

Understanding which compounds can dissolve helps narrow down possibilities when the composition of an unknown solid is unclear.

Barium Chloride

Barium chloride, with the chemical formula BaCl₂, is a white, crystalline salt. It is one of the barium compounds that is quite soluble in water. This property makes it an excellent candidate for various laboratory tests and reactions.
In its dissolved form, barium chloride splits into barium ions af(X = [-3..0,0,3], color="rgb(0,0,200)")(\(\text{Ba}^{2+}\)) and chloride ions (\(\text{Cl}^-\)).
Its high solubility allows for its use in laboratories to test reactions, especially those involving sulfate ions, as these will often produce a noticeable change such as a precipitate.
When mixed with sodium sulfate (\(\text{Na}_2\text{SO}_4\)), a white precipitate forms, which is key in qualitative analysis. This is due to the formation of barium sulfate (\(\text{BaSO}_4\))—an insoluble compound.

Precipitation Reaction

A precipitation reaction takes place when two soluble salts in aqueous solutions combine to form an insoluble salt, known as a precipitate. This type of reaction is vital in detecting and identifying ions in a solution.
In the context of this exercise, adding sodium sulfate (\(\text{Na}_2\text{SO}_4\)) to a clear solution of unknown chloride resulted in a white precipitate.
This occurrence points towards barium ions reacting with the sulfate ions.

• The reaction is chemical: aqua(X = [-2..0,0,7], color="rgb(0,200,50)")\[ \text{Ba}^{2+}(aq) + \text{SO}_4^{2-}(aq) \rightarrow \text{BaSO}_4(s) \]
• The product, barium sulfate (\(\text{BaSO}_4\)), is insoluble, leading to the formation of a solid precipitate.

This visible formation of a solid aids in confirming the presence of specific ions, and in this case, indicating the likelihood of barium chloride as the unknown compound.

Sodium Sulfate

Sodium sulfate (\(\text{Na}_2\text{SO}_4\)) is an important reagent in chemistry, often used in impacting solutions due to its notable reactivity with cations like barium. In its solid form, it appears as a white crystalline compound, highly soluble in water.

• Its aqueous form dissociates into sodium ions (\(\text{Na}^+\)) and sulfate ions (\(\text{SO}_4^{2-}\)).
• In qualitative analysis, its ability to interact and form precipitates with certain cations is exploited.

This property makes it particularly useful in experiments aimed at identifying unknown substances. When sodium sulfate is added to a solution containing barium ions, the resulting precipitation of \(\text{BaSO}_4\) provides clear evidence of the presence of barium, making it crucial for concluding the identity of unknown solids in chemical tests.