Which one of the following is a heterogeneous mixture?

A mixture is classified as homogeneous if its components are uniformly distributed and heterogeneous if they are non-uniform [1]. Hydrochloric acid is an aqueous solution where HCl gas dissolves in water to form a single homogeneous phase. Vinegar is a homogeneous mixture of acetic acid in water [1]. Soda water (aerated water) is a solution of carbon dioxide gas in water, which is also uniform [1]. In contrast, milk is a colloid or emulsion where fat globules are dispersed in water [2]. While colloids may appear uniform to the naked eye, they are microscopically heterogeneous because they consist of at least two distinct phases that are not uniformly dispersed on a microscopic scale [2]. Therefore, milk is the only heterogeneous mixture among the options, as it is a two-phase system where particles scatter light and can be distinguished under a microscope [2].

Sources

1. [1] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures > 8.1.2 Types of mixtures > p. 120
2. [2] https://www.ncbi.nlm.nih.gov/books/NBK559084/