2.6.1: Like Dissolves Like
- Page ID
- 493469
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- Explain why certain substances dissolve in other substances.
Like Dissolves Like
A simple way to predict which compounds will dissolve in other compounds is the phrase "like dissolves like". What this means is that polar compounds dissolve polar compounds, nonpolar compounds dissolve nonpolar compounds, but polar and nonpolar do not dissolve in each other.
Even some nonpolar substances dissolve in water but only to a limited degree. Have you ever wondered why fish are able to breathe? Oxygen gas, an nonpolar molecules, does dissolve in water and it is this oxygen that the fish take in through their gills. Or, one more example of a nonpolar compound that dissolves in water is the reason we can enjoy carbonated sodas. Pepsi-cola and all the other sodas have carbon dioxide gas, \(\ce{CO_2}\), a nonpolar compound, dissolved in a sugar-water solution. In this case, to keep as much gas in solution as possible, the sodas are kept under pressure.
This general trend of "like dissolves like" is summarized in the following table:
| Solute | Solvent | Is Solution Formed? |
|---|---|---|
| Polar Covalent | Polar | yes |
| Non-polar Covalent | Non-polar | yes |
| Polar Covalent | Non-polar | no |
| Non-polar Covalent | Polar | no |
| Ionic | Polar | yes |
| Ionic | Non-polar | no |
Note that every time charged particles (ionic compounds or polar substances) are mixed, a solution is formed. When particles with no charges (nonpolar compounds) are mixed, they will form a solution. However, if substances with charges are mixed with other substances without charges a solution does not form.
When an ionic compound is considered "insoluble", it doesn't necessarily mean the compound is completely untouched by water. All ionic compounds dissolve to some extent. An insoluble compound just doesn't dissolve in any noticeable or appreciable amount.
What is it that makes a solute soluble in some solvents but not others?
The answer is intermolecular interactions. The intermolecular interactions include London dispersion forces, dipole-dipole interactions, and hydrogen bonding.
From experimental studies, it has been determined that if molecules of a solute experience the same intermolecular forces that the solvent does, the solute will likely dissolve in that solvent. So, NaCl—a very polar substance because it is composed of ions—dissolves in water, which is very polar, but not in oil, which is generally nonpolar. Nonpolar wax dissolves in nonpolar hexane, but not in polar water. Liquids that dissolve in one another in all proportions are said to be miscible. Liquids that do not dissolve in one another are called immiscible.
Figure \(\PageIndex{2}\) Water (clear liquid) and oil (yellow) do not form liquid solutions. (CC BY-SA 1.0 Generic; Victor Blacus)
Summary
- “Like dissolves like” is a useful rule for deciding if a solute will be soluble in a solvent.
- Liquids that dissolve in one another in all proportions are said to be miscible.
- Liquids that do not dissolve in one another are called immiscible.
Contributors and Attributions
Henry Agnew (UC Davis)
- Libretext: Chemistry for Allied Health (Soult)
- Libretext: The Basics of GOB Chemistry (Ball et al.)