A calcium sulfate molecule would be considered a dissolved solid.

• A. colloidal solid
• B. dissolved solid
• C. suspended solid
• D. molecular solid

Answer: (B)

The concept here is how solids behave once they enter water. Dissolved solids are substances that go into solution and form ions or very small molecules that mix uniformly with the water, rather than staying as visible particles. Calcium sulfate dissolves in water and dissociates into calcium ions and sulfate ions; once dissolved, these species are in the aqueous phase rather than as solid particles. That dissolved form contributes to the total dissolved solids (TDS) in the water. If the substance remained as discrete particles that are too large to dissolve, it would be a suspended or colloidal solid, not a dissolved solid. A molecular solid would imply intact molecules in a solid state, which isn’t the case here because the compound dissociates into ions in solution.