1. Unit 1 – Day 12
Solubility Rules

2. Solubility
When discussing ionic and covalent compounds we learned that ionic compounds are soluble in water.
This is not always true however. Many ionic compounds are insoluble.
This depends on both the positive (cation) and negative (anion) in the compound.

3. Why care?
Creating insoluble compounds can create many problems at home or at work.
One example is hard water.
Hard water is not toxic or harmful, but it can be inconvenient.
The water contains dissolved ions of calcium, iron and magnesium.
These ions react with soap to form soap scum, an insoluble precipitate.

5. Scaling
Hard water in a kettle or coffee maker can cause calcium carbonate to build up as scale deposits.
It can impede the flow of water in hot water tanks.

6. Solubility Rules
We will be using a table of rules to determine if a compound is soluble or not.
For example, lead (IV) chloride is soluble, while lead (II) chloride is not.
Negative Ions (Anions) +
Positive Ions (Cations) =
Solubility of compounds in water
Example
Chloride (Cl-), Bromide (Br-), Iodide (I-)
Thiocyanate (SCN-)
silver (Ag+), lead (Pb2+), mercury (Hg2+), copper (Cu+), thallium (Tl+)
low solubility (insoluble)
silver chloride, AgCl, forms a white precipitate (a white solid)
any other cation
soluble
potassium bromide, KBr, is soluble

7. Some rules that are always true
All NITRATES are always soluble.
All ACETATES are always soluble.
All Alkali metal compounds (Li+, Na+, K+ etc) are always soluble.
All ammonium (NH4+) compounds are always soluble.
For these, and all other compounds, you can always look on the chart.

8. Examples
Try and determine if the following compounds are soluble:
Iron (III) carbonate
Calcium hydroxide
Sodium phosphate
Aluminum nitrate
Barium sulphate No
Yes

9. When two solutions mix…
When we mix two solutions, there is a double displacement reaction.
The compounds can switch ions.
If one of these combinations is insoluble, it will form as a solid precipitate.

10. Examples
Copper (II) chloride and Potassium Carbonate are mixed. Will a precipitate form?
CuCl2 + K2CO3  KCl + CuCO3
Check the solubility of both products.
CuCO3 is insoluble, and will form a precipitate.
Try it again for copper (II) nitrate and lead (II) acetate. Will a precipitate form if they mix?