Question

Magnesium chloride is dissolved in a solution containing sodium bicarbonate. On heating, a white precipitate is formed. Explain what causes the precipitation.

Short answer

Heating causes \(\text{NaHCO}_3\) to decompose and form \(\text{Na}_2\text{CO}_3\), which reacts with \(\text{MgCl}_2\) to form the precipitate, \(\text{MgCO}_3\).

Step-by-step solution

Write the reaction equations

Magnesium chloride (\(\text{MgCl}_2\)) is added to a solution containing sodium bicarbonate (\(\text{NaHCO}_3\)). Upon heating, sodium bicarbonate decomposes according to the equation: \[ \text{2NaHCO}_3 \rightarrow \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2 \]Magnesium chloride reacts with sodium carbonate to produce magnesium carbonate, which is sparingly soluble. This is described by the following reaction: \[ \text{MgCl}_2 + \text{Na}_2\text{CO}_3 \rightarrow \text{MgCO}_3 \downarrow + 2\text{NaCl} \]

Identify precipitate formation

The reaction between \(\text{MgCl}_2\) and \(\text{Na}_2\text{CO}_3\) forms magnesium carbonate (\(\text{MgCO}_3\)), a white insoluble solid, as a precipitate. The presence of this compound is indicative of the precipitation process.

Summarize the effects of heating

Upon heating, \(\text{NaHCO}_3\) decomposes, generating \(\text{Na}_2\text{CO}_3\). The carbonate ions provided by \(\text{Na}_2\text{CO}_3\) react with magnesium ions from \(\text{MgCl}_2\), leading to the formation of the white precipitate, \(\text{MgCO}_3\). This explains the observed precipitation when the solution is heated.

Key concepts

Magnesium chloride

Magnesium chloride, abbreviated as \( \text{MgCl}_2 \), is an essential compound in the field of chemistry. It consists of magnesium and chlorine elements.
This compound is highly soluble in water, meaning that it dissolves easily when mixed with a liquid, resulting in magnesium ions (\(\text{Mg}^{2+}\)) and chloride ions (\(\text{Cl}^-\)) in solution.
Magnesium chloride plays a pivotal role in various chemical reactions, serving as a source of magnesium ions.

• It is commonly used in medicine, food preparation, and as an ice control substance on roads.
• Additionally, it is a key component in the preparation and synthesis of several types of magnesium metal and alloys.

When mixed with other compounds, magnesium ions can precipitate insoluble salts under certain conditions, which is evident during reactions like the one explained in this exercise, where magnesium carbonate is formed.

Sodium bicarbonate

Sodium bicarbonate, which is popularly known as baking soda, has the chemical formula \(\text{NaHCO}_3\). It is a white, crystalline solid that is slightly alkaline.
This compound decomposes when heated, resulting in the formation of sodium carbonate (\(\text{Na}_2\text{CO}_3\)), water, and carbon dioxide.

• The decomposition reaction is as follows: \[\text{2NaHCO}_3 \rightarrow \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2\]

Sodium bicarbonate is versatile and is widely used in household products and culinary applications for its leavening properties.
In the context of chemical reactions, upon heating, sodium bicarbonate can produce carbonate ions, which react with other substances to form new compounds, as seen in this exercise.

Magnesium carbonate

Magnesium carbonate, symbolized as \(\text{MgCO}_3\), is known for being a white solid that is sparingly soluble in water.
This compound precipitates out of solution when magnesium ions encounter carbonate ions.

• In the reaction described, \(\text{MgCO}_3\) is formed when the aforementioned ions react in the aqueous environment created by the solution.

Magnesium carbonate is commonly encountered in various industries, for instance:

• In health foods and pharmaceuticals, due to its properties as a dietary supplement and antacid.
• In sports, it is used in athletes' chalk to help grip.

The precipitation of \(\text{MgCO}_3\) as described in the exercise exemplifies the concepts of solubility and chemical reaction dynamics.

Sodium carbonate

Sodium carbonate, denoted by the chemical formula \(\text{Na}_2\text{CO}_3\), is sometimes referred to as soda ash or washing soda.
It is a sodium salt of carbonic acid and appears as a white, odorless, water-soluble compound.
In the exercise context, sodium carbonate is formed by the decomposition of sodium bicarbonate when heat is applied.
This reaction not only releases carbonate ions, but it also plays a significant role in triggering the precipitation reaction with magnesium chloride.

• Sodium carbonate's applications extend to glass making, water softening, as well as in the production of detergents and cleaning products.
• It serves as a key reactant that aids in understanding how different substances interact at various states of matter.

Through its formation and reaction pathways, sodium carbonate helps to demonstrate the underlying principles of chemical reactions.