. A student observes what happens when two different aqueous solutions are mixed in a test tube. Which of the following observations would be most indicative of a precipitation reaction?
Gas bubbles are produced.
There is a color change.
The test tube warms.
A solid settles out.

a solid settles out

Explanation:

a precipitate is when a solid forms

a  solution  is always transparent, light passes through with no scattering from solute particles which are molecule in size. the  solution  is homogeneous and does not settle out. a colloid is intermediate between asolution  and a  suspension. while asuspension  will separate out a colloid will not.