Explain why a solution is considered a homogeneous mixture, while a suspension is a heterogeneous mixture. What are the key differences in their particle sizes and visibility?

A solution is homogeneous because its components are uniformly distributed throughout, and individual particles are not visible even under a microscope. The particle size in a solution is less than 1 nm. For example, sugar dissolved in water. 

A suspension is heterogeneous because its components are not uniformly distributed, and the particles are large enough to be seen with the naked eye and settle down upon standing. The particle size in a suspension is greater than 100 nm. For example, sand in water.