A calcium sulfate molecule would be considered a dissolved solid.

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

Answer: (B)

When a solid enters water and dissolves, it no longer exists as a visible solid. Instead, it breaks into small species that become part of the liquid, typically as ions or very small molecules. Calcium sulfate, being an ionic compound, dissociates in water into calcium ions and sulfate ions. You wouldn’t find a discrete calcium sulfate molecule floating around in the solution; you’d have dissolved ions. Those dissolved species are what water-treatment classifications refer to as dissolved solids. If the solid didn’t dissolve and remained as tiny particles, it would be a colloidal or suspended solid, not dissolved. So the dissolved form is the correct description.