**Question:** Gypsum product having least expansion

**Core Concept:** Gypsum is a mineral compound that is commonly used in medical applications, particularly in making casts and splints. When mixed with water, gypsum forms an expandable gel that hardens to a solid, creating a mold or cast. The expansion of the gel is a result of the hydration process and the release of water molecules.

**Why the Correct Answer is Right:** The correct answer is **Option C: Calcium sulfate hemihydrate (Gypsum Type III)**. Calcium sulfate hemihydrate is the least hydrated form of gypsum, meaning it contains the smallest amount of water molecules within its structure. When mixed with water, calcium sulfate hemihydrate undergoes a faster and more complete hydration process, resulting in the least expansion compared to other types of gypsum.

**Why Each Wrong Option is Incorrect:**

A. **Calcium sulfate dihydrate (Gypsum Type I)** is another form of gypsum that contains two water molecules in its structure. When mixed with water, it undergoes a slower hydration process, leading to more significant expansion.

B. **Calcium sulfate anhydrite (Gypsum Type II)** is an even more hydrated form of gypsum, with no water molecules present. When mixed with water, it undergoes an extremely rapid hydration process causing a significant expansion and is not suitable for making casts or splints.

C. **Calcium sulfate hemihydrate (Gypsum Type III)** is the correct answer, as mentioned earlier. It has the smallest amount of water molecules in its structure, resulting in the least expansion when mixed with water.

D. **Calcium sulfate decahydrate (Gypsum Type IV)** is another hydrated form of gypsum, containing ten water molecules. When mixed with water, it undergoes a rapid hydration process, leading to significant expansion and is not suitable for making casts or splints.

**Clinical Pearls:** Understanding the hydration process of gypsum forms is crucial for selecting the appropriate type of gypsum product for making casts or splints. Gypsum Type I and II are not suitable due to their rapid or excessive expansion, while Type IV gypsum is also not suitable due to its rapid expansion. Type III gypsum (Calcium sulfate hemihydrate) is the optimal choice for making casts or splints as it undergoes a slower hydration process, resulting in minimal expansion.