Answer:
The salts that are insoluble in water are CaS (calcium sulfide) and PbF2 (lead(II) fluoride).
Step-by-step explanation:
The solubility of a salt in water depends on the balance between the forces holding the ions together in the solid state (ionic lattice) and the forces between the ions and water molecules.
In the case of calcium sulfide (CaS), it is considered insoluble in water. This is because the forces of attraction between the calcium cations (Ca2+) and sulfide anions (S2-) in the solid lattice are relatively stronger than the forces of attraction between these ions and water molecules. As a result, the solid CaS does not readily dissociate into its ions in water, leading to low solubility.
Similarly, lead(II) fluoride (PbF2) is also considered insoluble in water. The forces between the lead cations (Pb2+) and fluoride anions (F-) in the solid lattice are strong enough to prevent easy dissociation in water.
On the other hand, magnesium sulfate (MgSO4) and copper(II) chloride (CuCl2) are both soluble in water. The forces between the ions in these salts are weaker than the forces between the ions and water molecules, allowing them to dissociate into their respective ions and form a solution in water.
Here are some general rules to follow when predicting solubility.
1. Most nitrate (NO3-) salts are soluble in water.
2. Most salts of alkali metals (Group 1 elements) and ammonium (NH4+) are soluble.
3. Most chloride (Cl-), bromide (Br-), and iodide (I-) salts are soluble, except for those of silver (Ag+), lead (Pb2+), and mercury (Hg2+).
4. Most sulfate (SO4 2-) salts are soluble, except for those of barium (Ba2+), strontium (Sr2+), lead (Pb2+), and calcium (Ca2+).
5. Most hydroxide (OH-) salts are insoluble, except for those of alkali metals (Group 1 elements) and barium (Ba2+), strontium (Sr2+), and calcium (Ca2+).
6. Most carbonate (CO3 2-) and phosphate (PO4 3-) salts are insoluble, except for those of alkali metals (Group 1 elements) and ammonium (NH4+).