Which type of substance is always uniformly distributed?

A pure substance is defined as a material that has a consistent and uniform composition throughout its entire structure. This means that every sample of a pure substance possesses the same properties and characteristics, regardless of where it is obtained. For example, elements like gold (Au) or compounds like water (H₂O) are pure substances because their composition does not vary — they have a fixed elemental makeup and fixed chemical properties.

In contrast, mixtures, solutions, and colloids can have varying compositions. Mixtures consist of two or more substances that are physically combined but not chemically bonded, which may lead to regions that do not have uniformity. Solutions, even though they are often homogeneous and can appear uniform to the naked eye, can still have variations at a molecular level under certain conditions. Colloids contain particles that are distributed throughout another substance but may not be uniformly distributed on a microscopic level due to the size of their dispersed particles.

Therefore, a pure substance stands out as the option that is always uniformly distributed, as it maintains the same composition and properties throughout its entirety.