The question asks to identify the statement that is NOT true about Plaster of Paris (PoP).

Plaster of Paris Properties and Uses

Let's analyze the given options:

• Option 1: Plaster of Paris is used for making ornamental designs. This statement is true. PoP is widely used in crafting decorative items and ornamental designs due to its moldability.
• Option 2: Plaster of Paris doesn't change on wetting with water. This statement is false. When Plaster of Paris (calcium sulfate hemihydrate, $ \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} $) is mixed with water, it undergoes a chemical reaction and sets into a hard solid mass of gypsum (calcium sulfate dihydrate, $ \text{CaSO}_4 \cdot 2\text{H}_2\text{O} $). This process is fundamental to its use as a casting material.
• Option 3: Plaster of Paris is white. This statement is generally true. Pure Plaster of Paris is a white powder.
• Option 4: Plaster of Paris is used to repair the broken bones. This statement is true. It is famously used by doctors to create casts for immobilizing fractured bones, allowing them to heal properly.

Identifying the Incorrect Statement

Based on the analysis, the statement that is NOT true about Plaster of Paris is that it doesn't change on wetting with water. In reality, it reacts with water and hardens.

Therefore, the correct option is B.