Here's why:

* Composition: A nickel is made primarily of nickel (about 75%), but it also contains other metals like copper (about 25%) and trace amounts of other elements.

* Mixture: This combination of different elements means a nickel is a mixture, specifically an alloy. Alloys are created by combining metals to achieve specific properties like strength, durability, or corrosion resistance.

So, while a nickel is primarily composed of the element nickel, it's not a pure substance because it's mixed with other elements.