Problem 175 - Chemistry Matter and Change

Question

What is a hydrated compound? Use an example to illustrate your answer.

Step-by-Step Solution

Answer

A hydrated compound contains water molecules within its crystal structure, such as Copper(II) sulfate pentahydrate (CuSO₄ · 5H₂O), where water is part of the crystal lattice.

Step 1: Understanding Hydrated Compounds

Hydrated compounds are chemical compounds that include water molecules within their crystal structure. These water molecules are not just physically trapped; they are part of the structure at a molecular level. The water is known as "water of crystallization" or "water of hydration."

Step 2: Identifying an Example

A common example of a hydrated compound is Copper(II) sulfate pentahydrate, with the chemical formula \( ext{CuSO}_4 \cdot 5 ext{H}_2 ext{O}\). This means that for each unit of copper sulfate in the crystal, there are five water molecules associated with it.

Step 3: Examining the Structure

In Copper(II) sulfate pentahydrate, the five water molecules are integrated within the crystal lattice, causing its blue color. If the water is removed, the compound turns into anhydrous copper sulfate, which is white.

Key Concepts

Water of Crystallization

In chemistry, water of crystallization refers to water molecules that are an integral part of a crystal structure in hydrated compounds. This water is not merely absorbed but is chemically bound within the crystal itself, often lending specific properties to the solid. Consider the analogy of bricks (the compound) held together by mortar (the water of crystallization).
These water molecules impact several aspects of the compound such as:

• Physical properties like color and shape
• Chemical properties that determine how the compound behaves in reactions

When a compound with water of crystallization is heated, the water can be removed, usually resulting in a change of color or structure. This can transform the hydrated compound into an anhydrous form, which lacks the water of crystallization and typically behaves differently.

Copper(II) Sulfate Pentahydrate

Copper(II) sulfate pentahydrate is a classic example of a hydrated compound. Its chemical formula is \( \text{CuSO}_4 \cdot 5\text{H}_2\text{O} \), indicating that each molecule of copper sulfate is associated with five water molecules. This bonding to water is why the compound typically appears as bright blue crystals.
The presence of water molecules influences the following:

• The appearance—generally, the more water molecules, the richer the color
• The stability—water of crystallization helps maintain the crystal's structure at lower temperatures
• The chemical reactivity—the water can participate in reactions or affect how the compound dissolves

On heating, these blue crystals lose their water molecules and change to a white powder, known as anhydrous copper sulfate. This demonstrates the shift from a hydrated to an anhydrous state.

Crystal Structure

The crystal structure of hydrated compounds is a complex, ordered arrangement where water molecules fill specific positions within the lattice. This precise positioning often impacts the compound significantly.

• Color: Hydrated compounds often exhibit distinct colors due to how water molecules alter the spacing and electron arrangement in the crystal. For example, the brilliant blue of copper(II) sulfate pentahydrate.
• Solubility: The presence of water can change how the compound dissolves, often making it easier for the solid to go into a solution.
• Thermal properties: The water within the crystal can absorb heat, impacting the melting point or boiling conditions. This makes hydrated compounds useful in various applications.

Understanding the crystal structure is essential for predicting how a hydrated compound might react or change under different conditions, such as temperature or chemical environment.