Problem 107 - Chemistry The Molecular Science

Question

When all the water is evaporated from a sodium hydroxide solution, solid sodium hydroxide is obtained. However, if you evaporate the water in an ammonium hydroxide solution, you will not produce solid ammonium hydroxide. Explain why. What will remain after the water is evaporated?

Step-by-Step Solution

Answer

Ammonium hydroxide evaporates as ammonia and water without any residue.

Step 1: Understanding the Composition

Ammonium hydroxide (\(NH_4OH\)) is an aqueous solution of ammonia (\(NH_3\)) in water. When it dissolves, it forms a dynamic equilibrium between the ammonium ions (\(NH_4^+\)), hydroxide ions (\(OH^-\)), and dissolved ammonia.

Step 2: Evaporation of Water from Sodium Hydroxide Solution

Sodium hydroxide (\(NaOH\)) is a strong base and fully dissociates into sodium ions (\(Na^+\)) and hydroxide ions (\(OH^-\)) in water. When the water evaporates, these ions recombine to form solid sodium hydroxide.

Step 3: Evaporation of Water from Ammonium Hydroxide Solution

In ammonium hydroxide solution, the equilibrium involves dissolved ammonia gas. When water evaporates, the equilibrium shifts to release ammonia gas, which escapes, leaving no solid residue behind.

Step 4: Reason for No Solid Formation

Ammonia is a gas and separate from the water, evaporates easily when heated. Thus, upon complete evaporation of water, there will be no ammonium hydroxide remaining, unlike sodium hydroxide, which forms a solid.

Step 5: What Remains After Evaporation?

After evaporation of water from an ammonium hydroxide solution, only the gases (\(NH_3\)) and the remaining air will be left since ammonium hydroxide itself does not exist as a solid.

Key Concepts

Ammonium Hydroxide

Ammonium hydroxide is often confused with being a solid substance, but in reality, it is an aqueous solution of ammonia gas in water. When ammonia (\(NH_3\)) dissolves in water, it creates a balance of ammonium ions (\(NH_4^+\)) and hydroxide ions (\(OH^-\)). This chemical concoction does not exist in a truly solid form under ordinary conditions.
When the water in the solution starts to evaporate, the delicate balance of the ion equilibrium tips. As water diminishes, ammonia gas is released into the air.
The result? There's no solid ammonium hydroxide left behind because ammonia has left the building! The solution's state significantly depends on the presence of water to keep ammonia dissolved. Without water, it's just ammonia gas.

Sodium Hydroxide

Sodium hydroxide, or \(NaOH\), behaves differently from ammonium hydroxide. This compound is known as a strong base, meaning it dissociates fully into sodium (\(Na^+\)) and hydroxide ions (\(OH^-\)) in aqueous solution. Even when the water evaporates from a sodium hydroxide solution, the ions recombine to form solid sodium hydroxide.
This means you will be left with a solid substance after the water is gone.

• Sodium hydroxide has a strong affinity for water.
• It easily transitions from liquid to solid as the solvent disappears.
• This property makes it useful in various industrial processes.

Even though it starts similarly in a dissolved state, its readiness to become solid contrasts sharply with ammonium hydroxide.

Water Evaporation

Evaporating water from a solution significantly impacts its components. Evaporation involves the transition of water from liquid to vapor, changing the states of substances in the solution.
In the case of ammonium hydroxide, evaporating water allows ammonia to escape as gas.

• The lack of solid formation after evaporation indicates that ammonium ions and hydroxide ions depend on water.
• As water leaves, ammonia does too, leaving no solid behind.

Conversely, when evaporating water from sodium hydroxide solution, evaporation facilitates recrystallization of sodium hydroxide, forming a solid.
This demonstrates how evaporation determines whether a compound will settle into a solid or disperse into vapor, governed by each substance's intrinsic nature and interaction with water.