Is AlPO4 Soluble or Insoluble in Water?

AlPO4 (Aluminium phosphate) is Insoluble in water because water dipole strength is too weak to pull away the anions and cations of AlSO4 as both Al 3+ and PO4 3- ions are strongly stabilized, as a result, AlPO4 has a higher lattice energy.

Why is AlPO4 insoluble in water?

for any compound to be soluble or insoluble in water, lattice enthalpy and hydration enthalpy are two major aspects. So let's have some ideas about it first,

Lattice Enthalpy

Lattice enthalpy (lattice energy) is a measure of the total energy required to split a molecule into its constituent cations and ions. In simple words, it is a cohesive force that is required to bind or split ions of the compound. For example: consider a compound NaCl which has two ions i.e. Na+ cation and Cl- cation. Now, the lattice energy of the NaCl molecule is the total energy required to stick Na+ and Cl- together to stabilize NaCl.

Hydration Enthalpy

Hydration energy is the total amount of energy released when an ion is combined with a water molecule. This energy is released due to the water tension produced on any molecule which comes in contact with water.

Explanation:

As we know, “higher the lattice energy, the molecule will be less soluble and reverse of it, higher the hydration energy, the molecule will be more soluble”. This is a simple concept to check the solubility of any compound.

In the case of AlPO4, it has higher lattice energy because both of its Al 3+ and PO4 3- ions are big and bigger ions stabilize more strongly. So that when AlSO4 is dissolved in water, its hydration enthalpy decreases more rapidly than hydration enthalpy.

But any compound to be soluble in water, its hydration energy must decrease quickly than its lattice energy. But when AlSO4 dissolved in water, its hydration enthalpy decreases rapidly.

Moreover quickly, you can also find out, AlPO4 soluble or not according to the solubility rule i.e. in the rule, it is stated most of phosphates like Ca3(PO4)3, Ag3PO4, and AlPO4 are frequently insoluble.

AlSO4 soluble or insoluble (Determining factors)

For any compound to be soluble or insoluble in water, there are definitely many factors that are directly or indirectly responsible. Same here, some of the characteristics determining factors for the solubility of AlSO4:

Solubility and Solubility Product

Higher the solubility and solubility product, the more often molecules are soluble in water. Both solubility and solubility products of AlSO4 are very low (near to zero) when compared with a highly soluble compound like NaCl.

Solubility Principle

According to the solubility principle “likes dissolve likes” which means a group of compounds (solute) is more likely to be soluble in a similar group of compounds (solvent). More likely, a polar molecule is mostly soluble in polar solvents and nonpolar molecules are soluble in nonpolar solvents.

Temperature

As the temperature has a direct relation with the solubility of molecules, a molecule's solubility started increasing when its temperature gradually started increasing. When you fluctuate the temperature of the solution, you will also see fluctuation in the solubility and solubility product.

pH value

pH is reciprocal to the solubility of the molecule i.e. if pH value increases, solubility decreases, and vice-versa.

Note: For your kind of information, most of the compounds which are considered to be insoluble, are not totally insoluble. They are sparingly soluble in water as outer ions are more often pull by water molecule tension and the small amounts of separated ions are quickly saturated with the largely occupied undissolved precipitation i.e. undissolved compound.