Why some solutes are soluble in water?
SOLUBILITY is understood as a maximum amount of solute that dissolves in a solvent at so called equilibrium. In chemistry an equilibrium is a state where reactants and products reach a balance - no more solute can be dissolved in the solvent in the set conditions (temerature, pressure). Such a solution is called a saturated solution.
To put it in simple words:
If you take one liter of water and you start dissolving table salt in it (chemical formula of salt is NaCl) and:
- temperature of water is 25oC
- pressure is 1 ATM (Atmosphere - standard pressure in the open air on Earth)
you should be able to dissolve exactly 357.00 grams and not a gram more. The rest of the salt will stay on the bottom as residue and will not dissolve. Solubility of salt in water is therefore 357.00g/L. When this amount of salt is dissolved the solution reaches its equilibrium. Every chemical substance which dissolves in water has a fixed solubility. If it does not dissolve - its solubility is zero. Many of these solublities have been measured and special charts are produced displaying solubility of many substances at once.
To complete our introduction to solubility, we will discribe two groups of substances in case of which solubility measure cannot be applied. These are miscible and immiscible substances.
On a thermodynamic prespective, for a solid solute and taking water as solvent, when heat of hydration outweighs the lattice energy of the solid, dissolution takes place. In general terms, when a molecule is placed in water, it faces interaction with the molecules of water. Dissolution takes place when the water-solute forces of attraction outweighs solute-solute interactions. Water is a polar protic solvent, capable of favourably interacting with highly polar or ionic molecules with either end of its dipole. Non polar molecules on the other hand, does not show strong affinity towards water, rendering them insoluble in it.