Solubility of ionic solids in water is determined by two forces governing the interaction between polar water molecules and ions, which forms crystals. The two forces are (1) Force of attraction between water and solid ions and (2) force of attraction between the oppositely charged ions. It is however not practical to know the exact values of these two forces for all combination. Hence, the use of solubility rules to predict if an ionic solid would be soluble in water. Part 2 of Berry Berry Easy notes on Heterogeneous Ionic Equilibrium for STPM Chemistry is about solubility rules.
[Tips: MEMORISE! Memorise the solubility rules whether you like it or not, as it'll come in handy. Use any method you can to memorise first, eventually you'll see the set of generalisation rules that will make it second nature to you. It is also important to know that a compound is highly soluble in water when Force (1) is the dominant factor, due to the force tendency to bring ions into solution. Water solubility is low is Force (2) is dominant as it tends to keep the ions in solid state.]
STPM Chemistry Form 6 Notes – Heterogeneous Ionic Equilibrium (Part 2)
General Solubility Rules (It is a good estimation when no solubility data is provided)
- Most salts containing alkali metal cations (Li+, Na+, K+, Cs+, Rb+) and ammonium (NH4+) are water-soluble.
- Most nitrate (NO3- ) salts are water-soluble.
- Most salts containing halide anions (SO42- ) are water-soluble (exceptions such as Ba2+, Pb2+, Hg2+ and Ca2+)
- Most hydroxide anion (OH- ) salts are only slightly water-soluble. KOH and NaOH are substantially soluble, while Ca(OH)2, Sr(OH)2 and Ba(OH)2 are fairly soluble in water.
- Most carbonate anion (CO32- ) and sulfide anions (S2-) salts are only slightly water-soluble.
- Most salts composed of a +1 cation (excluding transition metals) and -1 anions are soluble in water at room temperature.
- Nitrate (NO3- ) is a large anion that forms weak lattice interactions and forms strong hydrogen bonds with water, so more nitrate salts are water-soluble.
- Most salts containing sulphate anions (SO42- ) with +1 cations (excluding transition metals) are water-soluble
- Most salts with -2 or -3 anions are insoluble in water, excluding the sulphate salts.
- Most oxide (O2- ) and hydroxide anion (OH- ) salts are only slightly water-soluble. KOH and NaOH are exceptions that are substantially soluble.
This concludes Part 2 notes on the topic of Heterogeneous Ionic Equilibrium for STPM Form 6 Chemistry students from Berry Berry Easy. In Part 3, Berry Readers will be introduced to the concept of solubility products (an equilibrium constant) and also molar solubility. So keep logging on to our site for more quality and concised notes.