Solubility is one of most interesting parts of chemistry. Watching a colorful precipitate form, or redissolve, can be very exciting. In this article, we look at the common solubility rules of chemistry, which state which anions and cations are usually soluble, and which aren’t. We will also display a solubility chart that states the solubility of many common ionic compounds.
These solubility rules can help you predict if a precipitate will form, when two ionic compounds are mixed in solution. If all combinations of ions are soluble, then no precipitate will be formed, and you often not be able to isolate a single compound.
- Salts of the alkali metals, plus NH4+, are usually soluble. This includes Li+, Na+, K+, Rb+, Cs+
- Nitrates, with the NO3– ion, are always soluble. So are acetates, chlorates and perchlorates.
- Chlorides, bromides and iodides are soluble, except for Ag+, Pb+2, and Hg2+2
- Silver compounds are insoluble, except for silver nitrate and silver acetate
- Sulfates are soluble, except for Ca+2, Sr+2, Ba+2, Pb+2, and Ag+2
- Hydroxides are insoluble, except for the alkali metals which are soluble, and the alkaline earth metals Mg+2, Ca+2, Sr+2, Ba+2 which are slightly soluble
- Sulfides are highly insoluble, except for the alkali metals and alkaline earth metals
- Carbonates are insoluble, except for the alkali metals and NH₄⁺
- Chromates are insoluble, except for the alkali metals and NH₄⁺
- Phosphates are insoluble, except for the alkali metals and NH₄⁺
- Fluorides are insoluble, except for the alkali metals and NH₄⁺
What is Solubility?
Let’s discuss solubility, and some terms associated with it.
- Insoluble – less than 1 gram dissolves in a liter
- Slightly soluble 1-10 grams dissolves in a liter
- Sparingly soluble 10-30 grams dissolves in a liter
- Soluble – more than 30 grams dissolve in a liter
- Precipitate – what comes out of solution when a compound that is formed is not soluble
- Coffee filter – a great way to separate your precipitate from the solution!
- Saturated solution – a solution that has the maximum amount of the compound dissolved into solution
- Supersaturated solution – one that has more than the maximum dissolved. This is done usually by forming a saturated solution in hot water, and then letting the solution very slowly cool without any seed crystals present
- Solvent – the liquid that dissolves your compound
- Supernate – the solution that is left after a precipitate is removed or filtered out
The solubility rules in this article are in water at room temperature. Some compounds can have very different solubilities in hot or cold water. For example, potassium bromate is quite soluble in hot water, but only slightly soluble in very cold water.
Even the most insoluble ionic compounds will dissolve into ions to a very small degree. The solubility product constant, known as the Ksp value, allows you to calculate how much will actually dissolve.
Some compounds can take a while to dissolve. For example, crystals of copper (II) sulfate pentahydrate seem to take forever to dissolve, and a magnetic stirrer must often be run to help speed things up.
Interesting Solubility Facts & Exceptions
- Potassium chlorate, bromate and perchlorate are only slightly soluble in cold water. This fact can be used to separate potassium from sodium in solution.
- Sodium acetate is so soluble in water, that it can be difficult to form a super-saturated solution
- Rubidium formate, thallium formate, and silver perchlorate are 3 of the most highly soluble compounds, with over 5,000 grams of each dissolving in a liter of water at room temperature!
- A solution of equal parts thallium formate and thallium malonate, both extremely soluble, is known as Clerici solution. It has possibly the highest density of any aqueous solution. It allows minerals to be separated by density.
- Bromates and formates are generally soluble
- Copper (I) halides are insoluble, although this exception is usually not listed in standard solubility rules
- Lead, mercury (II) and silver sulfate are slightly soluble
- Sulfides are one of the most insoluble ions. For example, when all the lead needs to be removed from solution, sodium sulfide is often added to precipitate out as much lead as possible. One of the most insoluble ionic compounds is mercury (II) sulfide.
- Oxalates are highly insoluble, except for Na+, K+ and NH4+. Even rubidium and cesium oxalate are insoluble.