The common ion effect happens when a solution already contains an ion that is part of a salt. This reduces the solubility of that salt.
Here’s how it works:
- When a salt dissolves in water, it breaks into its ions.
- If the solution already has one of those ions from another source, the solution is less willing to dissolve more salt.
- This is because the solution tries to maintain equilibrium, so the salt dissolves less to balance the extra ions.
Example in words:
If you try to dissolve table salt in water that already has a lot of sodium ions, the salt won’t dissolve as much as in pure water.
In short: The presence of a common ion decreases solubility because the solution is “already crowded” with that ion.