The amount of water a substance chemically combined with is called water of

Analysis

The question asks for the specific term used to describe the water molecules that are chemically bound within the crystalline structure of a substance.

Concept Explanation

In chemistry, many salts exist in a crystalline form where water molecules are trapped within the crystal lattice in a definite ratio. This water is not "wetting" the substance; rather, it is an integral part of the crystal's chemical formula.

• Water of Crystallization: This refers to the water molecules that are present in a definite stoichiometric amount in a crystal. For example, copper(II) sulphate pentahydrate ($CuSO_4 \cdot 5H_2O$) contains five molecules of water of crystallization.
• Hydration: While "hydration" generally refers to the process of adding water to a substance, the specific term for the water chemically combined in a crystal is "water of crystallization."
• Dehydration: This is the process of removing water from a substance.
• Solution: This refers to a homogeneous mixture of a solute and a solvent.

Step-by-Step Solution

1. Identify the context: We are looking for the name given to the water molecules that are part of the chemical formula of a crystalline solid.
2. Evaluate the options: • A (Dehydration): Incorrect, as this is the removal of water. • B (Solution): Incorrect, as this describes a mixture, not the chemically combined water in a crystal. • C (Hydration): This is a closely related term, but in standard chemistry terminology, the specific water held in a crystal is defined as "water of crystallization." • D (Crystallization): This is the correct term used to describe the water molecules that form part of the crystal structure.
3. Conclusion: The standard chemical definition for water chemically combined in a crystal is "water of crystallization."

Exam Tip

When you see the phrase "chemically combined" in the context of crystals and water, always look for the word "crystallization." A common example to remember is $Na_2CO_3 \cdot 10H_2O$ (Sodium carbonate decahydrate), where the $10H_2O$ is the water of crystallization.