Question

Select the incorrect statement(s): (a) Milk of magnesia is an aqueous solution of \(\mathrm{Mg}(\mathrm{OH})_{2}\) (b) \(\mathrm{KOH}\) is a weaker base than \(\mathrm{NaOH}\) (c) \(\mathrm{CaO}_{2}\) is less stable than \(\mathrm{MgO}_{2}\) (d) \(\mathrm{Mg}^{2+}\) ions are precipitated with the addition of \(\mathrm{NH}_{4} \mathrm{OH}\) in the presence of \(\mathrm{NH}_{4} \mathrm{Cl}\)

Short answer

The incorrect statements are (b) and (c).

Step-by-step solution

Understanding Statement (a)

Statement (a) claims that milk of magnesia is an aqueous solution of \(\mathrm{Mg}(forms{OH})_{2}\). This statement is correct because milk of magnesia consists mainly of a suspension of magnesium hydroxide in water.

Analyzing Statement (b)

Statement (b) says \(\mathrm{KOH}\) is a weaker base than \(\mathrm{NaOH}\). This is incorrect because both \(\mathrm{KOH}\) and \(\mathrm{NaOH}\) are strong bases, and their dissociation in water is almost complete. While \(\mathrm{KOH}\) and \(\mathrm{NaOH}\) both fully dissociate, \(\mathrm{NaOH}\) is generally considered a slightly stronger base due to the more electronegative nature of sodium compared to potassium.

Evaluating Statement (c)

Statement (c) asserts that \(\mathrm{CaO}_{2}\) is less stable than \(\mathrm{MgO}_{2}\). This statement is incorrect. In reality, \(\mathrm{MgO}_{2}\) doesn't exist as a stable compound under normal circumstances. \(\mathrm{CaO}_{2}\), referring to calcium peroxide, is less stable due to the peroxide bond, making the comparison with a non-existent compound invalid.

Checking Statement (d)

Statement (d) mentions \(\mathrm{Mg}^{2+}\) ions are precipitated with the addition of \(\mathrm{NH}_{4} \mathrm{OH}\) in the presence of \(\mathrm{NH}_{4} \mathrm{Cl}\). This statement is correct. \(\mathrm{Mg}^{2+}\) ions do form a precipitate as magnesium hydroxide when \(\mathrm{NH}_{4} \mathrm{OH}\) is added, although the presence of \(\mathrm{NH}_{4}\) ions from \(\mathrm{NH}_{4} \mathrm{Cl}\) can reduce the precipitation slightly.

Key concepts

Milk of Magnesia

Milk of Magnesia is a well-known household item often used for its soothing laxative properties and its ability to neutralize stomach acid. It is composed mainly of tiny, undissolved particles of magnesium hydroxide \( \mathrm{Mg}(\mathrm{OH})_2 \) dispersed in water. This "milk" is not a true solution but a suspension, where the magnesium hydroxide remains mostly undissolved, suspended throughout the liquid. This "milky" appearance gives it the name Milk of Magnesia.

Magnesium hydroxide is only slightly soluble in water, which is why it forms a suspension rather than a solution. When consumed, Milk of Magnesia reacts with stomach acid, \( \mathrm{HCl} \), to form water and magnesium chloride, which helps soothe indigestion and relieve constipation. Its ability to engage in such chemical reactions highlights its usefulness in various home remedies.

Strong Bases

In chemistry, bases are substances that can accept hydrogen ions \( \mathrm{H}^+ \) or donate hydroxide ions \( \mathrm{OH}^- \) when dissolved in water. Strong bases, such as sodium hydroxide (NaOH) and potassium hydroxide (KOH), completely dissociate in water to release ions.

Both KOH and NaOH are considered strong bases because they fully break apart into their respective ions, \( \mathrm{K}^+ \) and \( \mathrm{OH}^- \), or \( \mathrm{Na}^+ \) and \( \mathrm{OH}^- \). Their dissociation is nearly 100% in water, making them highly effective in neutralizing acids and conducting electricity.

• NaOH is often preferred in industrial settings due to its slightly stronger basicity, which stems from the more electronegative sodium atom providing a stronger ionic attraction for \( \mathrm{OH}^- \)
• KOH, while equally effective in principle, is often utilized in applications requiring less thermal resistance and cost.

Magnesium Hydroxide

Magnesium hydroxide, represented chemically as \( \mathrm{Mg}(\mathrm{OH})_2 \), is an inorganic compound with many practical uses. It is known primarily for its role in antacids and laxatives, but also finds applications in water treatment and as a refractory material.

One of its prominent features is its low solubility in water, making it ideal for applications where a solid form of magnesium is desirable. This low solubility, however, does not impede its reactivity with acids, as demonstrated by its utility in neutralizing stomach acid.

Applications and Benefits

• Antacid Use: Neutralizes stomach acid to alleviate symptoms of indigestion and heartburn
• Laxative Use: Draws water into the intestines to stimulate bowel movements
• Water Treatment: Precipitates heavy metals from wastewater as hydroxides
• Flame Retardant: Releases water vapor when heated, inhibiting fire spread

Calcium Peroxide

Calcium peroxide (\( \mathrm{CaO}_2 \)) is an intriguing compound due to its structural properties and usefulness in several industries. Unlike magnesium peroxide, which does not form a stable compound, calcium peroxide forms due to the presence of a peroxide bond (\( \mathrm{O}_2^{2-} \)).

This bond introduces additional stability concerns since peroxides are often reactive and can decompose over time.

Uses of Calcium Peroxide

• Agriculture: Enhances oxygen levels in soil, promoting seed germination and root growth.
• Environmental: Used in soil remediation and wastewater treatment as it releases oxygen when moist, supporting aerobic biodegradation.
• Food Industry: Acts as a flour bleaching agent and dough conditioner.

Precipitation Reaction

A precipitation reaction is a type of chemical reaction in which two soluble salts react in aqueous solution to form one or more insoluble products, known as precipitates. A common example involves the reaction between ionic species in a solution \( \mathrm{Mg}^{2+} \) ions.

When you add \( \mathrm{NH}_4\mathrm{OH} \) in the presence of \( \mathrm{NH}_4\mathrm{Cl} \), magnesium ions form an insoluble product of magnesium hydroxide (\( \mathrm{Mg}(\mathrm{OH})_2 \)), manifesting as a solid precipitate.

Understanding the Process

• Formation of Precipitates: The reaction relies on the solubility product principle, where the solution becomes saturated, and excess ions begin forming a solid.
• Influence of Other Ions: The presence of ammonium chloride (\( \mathrm{NH}_4\mathrm{Cl} \)) can affect the degree of precipitation, as it provides a source of common ions that shift the equilibrium.
• Applications: This type of reaction is frequently employed in analytical chemistry to separate ions from mixtures and in various purification processes.