Here's why:
* Elements: Hydrogen is an element, meaning it cannot be broken down into simpler substances by chemical means. All elements are pure substances by definition.
* Fixed Composition: A pure substance has a fixed and definite composition. In the case of hydrogen, it's always composed of only hydrogen atoms, regardless of its source or form (e.g., hydrogen gas, liquid hydrogen).
* Uniform Properties: Pure substances have uniform properties throughout. All samples of pure hydrogen will have the same chemical and physical properties.
In contrast, mixtures consist of two or more different substances combined physically, not chemically. They don't have a fixed composition and can vary in their properties.
So, because hydrogen is a single, distinct element with a fixed composition and uniform properties, it is classified as a pure substance.
