Which of the following compounds is the least soluble in water?

The solubility of a compound in water is determined by its ability to dissociate into ions in the water. Compounds that dissociate completely into ions are more soluble in water than those that do not dissociate as much. Looking at the given compounds, CaCl₂ dissociates into three ions (Ca²⁺ and two Cl^-) in water and NaCl dissociates into two ions (Na⁺ and Cl^-). CaSO₄ dissociates into two ions (Ca²⁺ and SO₄^2-) but it is less soluble than CaCl₂ because the SO₄^2- ion is larger and more polar than the Cl^- ion, so it has a stronger attraction to the Ca²⁺ ion, making it more difficult to dissociate. Na₂SO₄ dissociates into three ions (two Na⁺ and SO₄^2-) but it is also less soluble than NaCl because of the larger, more polar SO₄^2- ion. Therefore, CaSO₄ is the least soluble compound in water because it has a weaker ability to dissociate into ions in water compared to the other given compounds.