Final answer:
Pure substances and homogeneous mixtures have a uniform composition, meaning that every part of the substance or mixture has the same amount and type of components.
Step-by-step explanation:
When we talk about the uniform composition of pure substances (elements and compounds) and homogeneous mixtures, we mean that these substances have the same composition throughout the entire sample. In other words, every part of the substance or mixture has the same amount and type of components.
For example, let's consider a bottle of pure water. No matter where you take a sample from within the bottle, the composition of water will be the same. It will always consist of hydrogen and oxygen atoms in a ratio of 2:1.
Similarly, if you have a homogeneous mixture like sugar dissolved in water, the composition will be the same throughout the entire solution. Whether you take a sample from the top or the bottom of the container, it will always contain the same amount of sugar molecules dissolved in water.