Question

A sodium salt on treatment with \(\mathrm{MgCl}_{2}\) gives white precipitate only on heating. The anion of the sodium salt is: (a) \(\mathrm{HCO}_{3}^{-}\) (b) \(\mathrm{CO}_{3}^{2-}\) (c) \(\mathrm{NO}_{3}^{-}\) (d) \(\mathrm{SO}_{4}{ }^{2-}\)

Short answer

The anion of the sodium salt is 0{HCO_{3}^{-}}0.

Step-by-step solution

Identify Precipitate Formation

Consider the given information that a white precipitate forms only on heating. This suggests that the process involves the formation of a less soluble compound upon the action of heat.

Determine Possible Anion Interaction with MgCl2

We need to consider which anions can form precipitates with the magnesium ion (0{Mg^{2+}}0), especially upon heating. 0{MgCO_{3}}0 tends to precipitate, but it is soluble in cold water and precipitates upon heating.

Eliminate Options Based on Solubility

Evaluate each option: - Option (a) 0{HCO_{3}^{-}}0: 0{Mg(HCO_{3})_{2}}0 is soluble in water but can convert to 0{MgCO_{3}}0 upon heating. - Option (b) 0{CO_{3}^{2-}}0: 0{MgCO_{3}}0 is less soluble, precipitating upon heating. - Option (c) 0{NO_{3}^{-}}0: 0{Mg(NO_{3})_{2}}0 is soluble in water. - Option (d) 0{SO_{4}{ }^{2-}}0: Normally forms a white precipitate immediately, not requiring heating.

Conclude Based on Solubility and Reaction Conditions

The options can further be concluded by considering the conditions of heating in the question; the anionic species that best fits the description based on magnesium precipitate formation and the heating condition is 0{HCO_{3}^{-}}0, which forms 0{MgCO_{3}}0 when heated.

Key concepts

Sodium Salt Reactions

Sodium salts are versatile in chemical reactions. They usually contain an anion paired with a sodium ion, which can participate in a variety of reactions depending on the other substances present. In the context of wet chemistry, sodium often serves as a counterion to keep the main reacting anion dissolved in water. Sodium salts, like sodium bicarbonate, can undergo reactions where the anion itself interacts with other chemical species.

Common interactions include those with acids, bases, and other salts, where the anion may form new compounds or participate in ionic exchanges. When these salts are mixed with solutions of other ions, such as magnesium chloride (

• Anion exchange: The anion in sodium salt can exchange with another anion in solution, forming a new compound.
• Complex formation: Some anions can bind to metal ions to create complex ions.
• Precipitation: A common outcome where an insoluble compound forms, often seen as a precipitate.

Sodium salts offer an incredible array of reactions due to the diversity of anions they can contain. Understanding the nature of the anion is key to predicting the outcome of such reactions.

Precipitate Formation

Precipitate formation occurs when two soluble substances in solution react to form an insoluble solid. This process is evaluated in qualitative analysis as it helps identify ions, like magnesium ions, which form precipitates under certain conditions. Understanding the conditions under which these ions precipitate is crucial for predicting chemical reactions.

When considering precipitate formation:

• Supersaturation: It may lead to the formation of a solid from a solution, especially when the solution is concentrated or temperature plays a role.
• Temperature Influence: Some substances only precipitate when the solution is heated, as seen with magnesium carbonate precipitating from bicarbonate solutions.
• Solubility Product: It dictates whether a precipitate will form based on the product of the ion concentrations exceeding a certain value.

Precipitation is not just a physical change; it provides insight into the solubility and reactivity of ions. Heat can drive reactions toward precipitate formation, especially when endothermic processes are involved.

Magnesium Chloride Interaction

Magnesium chloride (\(\mathrm{MgCl}_{2}\)) is highly soluble in water and is a useful reagent for detecting the presence of certain anions through precipitation reactions. It is particularly reactive with carbonate or bicarbonate anions, which are part of many sodium salts.

Here's how it works:

• Simple Ion Exchange: The magnesium ion (\(\mathrm{Mg^{2+}}\)) can pair with an anion once displacement occurs, sometimes resulting in a precipitate.
• Heat-Mediated Reaction: For the bicarbonate ion (\(\mathrm{HCO}_{3}^{-}\)), magnesium chloride doesn't immediately cause a precipitate. However, when heated, bicarbonate ions convert to carbonate ions (\(\mathrm{CO}_{3}^{2-}\)), eventually forming insoluble \(\mathrm{MgCO}_{3}\).
• Reaction Conditions: Heating is crucial; it drives the conversion and affects reaction kinetics, causing magnesium ions to precipitate as white magnesium carbonate.

In laboratory settings, magnesium chloride is an effective reagent to visualize and identify specific anions based on their tendency to form distinct precipitates. The role of heat in these reactions is explicitly pronounced during such experiments, offering a clear pathway to precipitate formation.