Which of the following is an example of a homogeneous mixture?

A homogeneous mixture is one in which the components are uniformly distributed throughout the mixture, and the individual substances cannot be easily distinguished from one another.

Boiler flue gas serves as a prime example of a homogeneous mixture. It is composed of various gases produced during combustion, including carbon dioxide, water vapor, and nitrogen, among others. These gases mix uniformly and, despite being made up of different substances, they cannot be separated easily by physical means. The uniform characteristics of flue gas illustrate the defining trait of a homogeneous mixture.

On the other hand, the other options represent different states or compositions. Acid can be homogeneous, but it depends on its concentration; it’s not specifically indicative of a mixture. Pure water is a compound rather than a mixture, as it consists solely of H2O molecules. Sand and water form a heterogeneous mixture because sand particles do not dissolve or blend well within the water; they remain distinguishable and can settle to the bottom. Thus, these choices do not fulfill the criteria of a homogeneous mixture in the same way that boiler flue gas does.