Here's why:

* Mixtures are formed by physically combining different substances, and the substances retain their individual properties.

* Compounds are formed by chemically combining elements in a fixed ratio. The resulting compound has entirely new properties compared to the original elements.

Potassium dichromate is formed by the chemical combination of potassium (K), chromium (Cr), and oxygen (O) in a specific ratio. It has unique properties distinct from its component elements.

Therefore, potassium dichromate is a compound, not a mixture.