Question

When solutions of \(\mathrm{BaCl}_{2}\) and \(\mathrm{Na}_{2} \mathrm{SO}_{4}\) are mixed, the mixture becomes cloudy. After a few days, a white solid is observed on the bottom of the beaker with a clear liquid above it. (a) Write a balanced equation for the reaction that occurs. (b) Why is the solution cloudy at first? (c) What happens during the few days of waiting?

Short answer

(a) \( \mathrm{BaCl}_{2}(aq) + \mathrm{Na}_{2} \mathrm{SO}_{4}(aq) \rightarrow \mathrm{BaSO}_{4}(s) + 2\mathrm{NaCl}(aq) \). (b) The solution becomes cloudy due to the formation of insoluble \( \mathrm{BaSO}_{4} \). (c) The \( \mathrm{BaSO}_{4} \) particles settle, forming a precipitate.

Step-by-step solution

Identify the Reactants

The reactants in this chemical process are Barium Chloride, \( \mathrm{BaCl}_{2} \), and Sodium Sulfate, \( \mathrm{Na}_{2} \mathrm{SO}_{4} \). When mixed in an aqueous solution, they undergo a double displacement reaction.

Write the Chemical Equation

For a double displacement reaction between \( \mathrm{BaCl}_{2} \) and \( \mathrm{Na}_{2} \mathrm{SO}_{4} \), Barium Sulfate \( \mathrm{BaSO}_{4} \) and Sodium Chloride \( \mathrm{NaCl} \) are formed: \[ \mathrm{BaCl}_{2}(aq) + \mathrm{Na}_{2} \mathrm{SO}_{4}(aq) \rightarrow \mathrm{BaSO}_{4}(s) + 2\mathrm{NaCl}(aq) \]. This equation represents the balanced reaction.

Explain Cloudiness in the Solution

The solution becomes cloudy initially because \( \mathrm{BaSO}_{4} \), which is an insoluble precipitate, forms and disperses throughout the solution. This suspension of solid particles causes cloudiness.

Describe Changes Over Time

Over time, the particles of \( \mathrm{BaSO}_{4} \) settle to the bottom due to gravity, forming a white solid. This results in a clear liquid phase above the solid precipitate as the suspension is resolved into its separate components.

Key concepts

Barium Sulfate

Barium sulfate, represented by the chemical formula \( \mathrm{BaSO}_{4} \), is a compound that is known for being highly insoluble in water. This feature makes it a prominent example of an insoluble salt in chemical reactions. When barium sulfate is formed in a reaction, it frequently precipitates out of the solution as a solid.
This property is distinctly observable in reactions such as the one between barium chloride \( \mathrm{BaCl}_{2} \) and sodium sulfate \( \mathrm{Na}_{2} \mathrm{SO}_{4} \). The formation of a white solid precipitate in such reactions pinpoints the creation of barium sulfate.

• Uses of barium sulfate include its role in medical imaging as a contrast agent because it is non-toxic despite its heavy metal status.
• It's important in the formation of sediments in chemical processes.
• In industry, it is used in paints and plastics for its high density and opaque nature.

Double Displacement Reaction

A double displacement reaction, also known as a metathesis reaction, occurs when parts of two ionic compounds are exchanged to form two new compounds.
The general form for a double displacement reaction is \( AB + CD \rightarrow AD + CB \). In this context, the double displacement reaction between barium chloride \( \mathrm{BaCl}_{2} \) and sodium sulfate \( \mathrm{Na}_{2} \mathrm{SO}_{4} \) leads to the formation of barium sulfate \( \mathrm{BaSO}_{4} \) and sodium chloride \( \mathrm{NaCl} \).
This type of reaction is characterized by:

• Swapping of cations and anions between reactants.
• The potential formation of a precipitate, which is a solid product, in aqueous solutions.
• The exchange leads to the generation of new compounds, which may have different solubility properties than the original substances.

Double displacement reactions are significant in teaching about chemical reactivity and solubility rules.

Chemical Equation Balancing

Balancing a chemical equation is essential to uphold the Law of Conservation of Mass, which states that mass is neither created nor destroyed in a chemical reaction. Each side of the equation must have the same amount of each element.
For the reaction between barium chloride and sodium sulfate, the balanced equation is:
\[ \mathrm{BaCl}_{2}(aq) + \mathrm{Na}_{2} \mathrm{SO}_{4}(aq) \rightarrow \mathrm{BaSO}_{4}(s) + 2\mathrm{NaCl}(aq) \] This balanced equation shows that one molecule of \( \mathrm{BaCl}_{2} \) reacts with one molecule of \( \mathrm{Na}_{2} \mathrm{SO}_{4} \) to form one molecule of \( \mathrm{BaSO}_{4} \) and two molecules of \( \mathrm{NaCl} \).

• Balancing equations ensures that all atoms present in the reactants are accounted for in the products.
• It illustrates the stoichiometric ratios of reactants and products.
• This process requires adjusting coefficients in front of compounds to achieve balance, without changing the compounds themselves.

Insoluble Salts

Insoluble salts are substances that do not dissolve significantly in water. Instead, they form a precipitate, which is a solid that falls out of the solution.
Barium sulfate is an excellent example of an insoluble salt. When barium ions \( \mathrm{Ba}^{2+} \) and sulfate ions \( \mathrm{SO}_{4}^{2-} \) meet in solution, they combine to form barium sulfate, which remains as a solid because it does not dissolve.
Insoluble salts are critical to understanding reactions that result in precipitates.

• They are often used to drive reactions to completion, as the formation of a solid removes ions from the solution.
• Understanding solubility principles helps predict whether a double displacement reaction will result in a precipitate.
• Despite being called insoluble, many salts considered insoluble will actually have slight solubility that could be noticeable in highly sensitive situations.

The concept of insolubility is foundational in chemical reactions dealing with solutions.