Question

Two solutions are labeled A and B. Solution A contains \(\mathrm{Na}_{2} \mathrm{CO}_{3},\) and solution \(\mathrm{B}\) contains \(\mathrm{NaHCO}_{3} .\) Describe how you would distinguish between the two solutions if you were provided with an \(\mathrm{MgCl}_{2}\) solution. (Hint: You need to know the solubilities of \(\mathrm{MgCO}_{3}\) and \(\mathrm{MgHCO}_{3} .\) )

Short answer

Add \(\mathrm{MgCl}_{2}\); Solution A gives a precipitate; Solution B stays clear.

Step-by-step solution

Understanding the Chemical Compounds

Identify the key chemical compounds involved in each solution. Solution A contains sodium carbonate \((\text{Na}_2\text{CO}_3)\), and Solution B contains sodium bicarbonate \((\text{NaHCO}_3)\). We will use magnesium chloride \((\text{MgCl}_2)\) to test these two solutions.

Reaction with MgCl₂

Consider the reaction of each anion with magnesium chloride. Both \(\text{Na}_2\text{CO}_3\) and \(\text{NaHCO}_3\) can react to form magnesium carbonate \((\text{MgCO}_3)\), but their behaviors differ due to solubility. \[\text{Na}_2\text{CO}_3 + \text{MgCl}_2 \rightarrow \text{MgCO}_3 \text{(s)} + 2\text{NaCl} \] \[2\text{NaHCO}_3 + \text{MgCl}_2 \rightarrow \text{Mg(HCO}_3)_2 \text{(aq)} + 2\text{NaCl} \]

Observing the Precipitation

Focus on the potential for precipitation. \(\text{MgCO}_3\) is sparingly soluble and will precipitate out of the solution, while \(\text{Mg}\text{(HCO}_3)_2 \) remains soluble in water. In Solution A, a white precipitate of \(\text{MgCO}_3\) forms, while in Solution B, no precipitate is observed.

Conclusion

Mix \(\text{MgCl}_2\) solution with Solution A and Solution B separately. The formation of a white precipitate indicates Solution A (\(\text{Na}_2\text{CO}_3\)), while a clear solution indicates Solution B (\(\text{NaHCO}_3\)).

Key concepts

Sodium Carbonate

Sodium carbonate, also known by its chemical formula \( \text{Na}_2\text{CO}_3 \), is a common substance found in various household and industrial products. It's often known as washing soda and is used in laundry detergents, glass manufacturing, and for a host of other applications. This compound is composed of sodium, carbon, and oxygen. It is a white powder at room temperature and is highly soluble in water.

When sodium carbonate comes into contact with magnesium chloride \( \text{MgCl}_2 \), a chemical reaction occurs to form magnesium carbonate \( \text{MgCO}_3 \) and sodium chloride \( \text{NaCl} \). Here is the reaction formula once again for reference:

• \( \text{Na}_2\text{CO}_3 + \text{MgCl}_2 \rightarrow \text{MgCO}_3 \text{(s)} + 2\text{NaCl} \)

Magnesium carbonate is not very soluble in water, which leads to the formation of a white precipitate, a solid that settles out of the solution. This characteristic makes sodium carbonate quite useful for distinguishing between solutions containing different bicarbonates or carbonates based on precipitation reactions.

Sodium Bicarbonate

Sodium bicarbonate, or \( \text{NaHCO}_3 \), is a chemical compound commonly referred to as baking soda. This compound is a staple in kitchens as a leavening agent, but it also finds uses in cleaning and deodorizing due to its slightly alkaline nature. It's made of sodium, hydrogen, carbon, and oxygen and appears as a white crystalline powder.

One of the key interactions of sodium bicarbonate is its reaction with magnesium chloride \( \text{MgCl}_2 \). This reaction results in the formation of magnesium bicarbonate \( \text{Mg(HCO}_3)_2 \), which is soluble in water:

• \( 2\text{NaHCO}_3 + \text{MgCl}_2 \rightarrow \text{Mg(HCO}_3)_2 \text{(aq)} + 2\text{NaCl} \)

Unlike magnesium carbonate, which precipitates out, magnesium bicarbonate remains in solution. This means there is no formation of a visible precipitate, which highlights an important difference between solutions of sodium carbonate and sodium bicarbonate when tested with magnesium chloride. This reaction is particularly useful in experiments aimed at differentiating between the two compounds based on solubility and reactivity.

Magnesium Chloride Reactivity

Magnesium chloride \( \text{MgCl}_2 \) plays a pivotal role in detecting and differentiating between sodium carbonate and sodium bicarbonate through observable chemical reactions. When it reacts with sodium carbonate, a white precipitate of magnesium carbonate \( \text{MgCO}_3 \) forms. This result signifies a lack of solubility, which is critical for visually distinguishing the compound from those that do not precipitate.

Conversely, when magnesium chloride interacts with sodium bicarbonate \( \text{NaHCO}_3 \), magnesium bicarbonate \( \text{Mg(HCO}_3)_2 \) is produced and remains in the aqueous phase. This solubility difference allows one to deduce the nature of the solution being tested without requiring complex analytical equipment or methods.

Magnesium chloride's dual reactivity with both sodium carbonate and sodium bicarbonate effectively highlights the fundamental principles of solubility and precipitation in chemical reactions, making it a valuable tool in educational settings for studying the properties and behaviors of various compounds and their interactions.