Molecular Structure and Intermolecular Forces
4. Solubility and "Like Dissolves Like"
Learning outcomes
- I can explain why some substances dissolve while others do not.
- I can relate polarity to solubility.
- I can apply the principle "like dissolves like."
- I can predict solubility based on molecular properties.
- I can explain common examples of solubility.
What Is Solubility?
Solubility describes how well a substance can dissolve in a particular solvent.
When a substance dissolves:
- The substance being dissolved is the solute.
- The substance doing the dissolving is the solvent.
- The resulting mixture is a solution.
For example, when sugar dissolves in water:
sugar = solute
water = solvent
sugar solution = solution
Not every substance dissolves equally well in every solvent. Whether a substance dissolves depends strongly on the molecular properties of both the solute and solvent.
What Happens When a Substance Dissolves?
Particles in a solute are attracted to one another.
Particles in a solvent are also attracted to one another.
For a substance to dissolve, solvent particles must interact strongly enough with the solute particles to separate them and surround them.
We can think about three types of interactions:
- Solute-solute attractions.
- Solvent-solvent attractions.
- Solute-solvent attractions.
Dissolving is favorable when the new solute-solvent interactions are sufficiently favorable compared with the interactions that must be disrupted.
Polarity and Solubility
One of the most useful ways to predict solubility is to consider polarity.
A polar molecule has an uneven distribution of electrical charge.
A nonpolar molecule has a relatively even distribution of charge and no permanent overall dipole.
Polar molecules tend to interact strongly with other polar molecules.
Nonpolar molecules tend to interact more favorably with other nonpolar molecules.
This leads to the useful guideline:
“Like dissolves like.”
What Does “Like Dissolves Like” Mean?
The phrase means:
polar solutes tend to dissolve in polar solvents
and:
nonpolar solutes tend to dissolve in nonpolar solvents
It does not mean that substances must be chemically identical.
Instead, their intermolecular properties should be reasonably compatible.
Water: A Polar Solvent
Water is one of the most important solvents in chemistry and biology.
Water molecules are:
polar
Oxygen attracts bonding electrons more strongly than hydrogen.
As a result:
- Oxygen has a partial negative charge.
- Hydrogen has a partial positive charge.
Water's bent molecular shape prevents these bond dipoles from cancelling.
Therefore:
H₂O is polar.
This allows water to interact strongly with many polar and ionic substances.
Why Sugar Dissolves in Water
Sugar molecules contain several polar regions, including many:
O–H groups
These groups can form strong intermolecular attractions with water molecules, including hydrogen bonds.
Water molecules interact with the sugar molecules and surround them.
Individual sugar molecules can then move throughout the water.
Therefore:
sugar dissolves readily in water.
Dissolving Does Not Mean Disappearing
When sugar dissolves, the sugar has not disappeared.
The sugar molecules are still present.
They have simply become:
- Separated from one another.
- Surrounded by water molecules.
- Distributed throughout the solution.
This is why evaporating the water can leave the sugar behind.
Dissolving is usually a physical process, not the destruction of the solute.
Why Salt Dissolves in Water
Sodium chloride is different from sugar.
NaCl is an ionic compound containing:
Na⁺ ions
and:
Cl⁻ ions
Water is polar.
The partially negative oxygen side of water is attracted to:
Na⁺
The partially positive hydrogen side is attracted to:
Cl⁻
Water molecules surround the separated ions.
This process is called hydration.
Hydration of Ions
Around Na⁺:
The oxygen ends of water molecules tend to point toward the positive ion.
Around Cl⁻:
The hydrogen ends tend to point toward the negative ion.
These ion-dipole attractions help stabilize the ions in solution.
We can represent dissolving sodium chloride as:
NaCl(s) → Na⁺(aq) + Cl⁻(aq)
The symbol:
(aq)
means that the ions are dissolved in water.
Why Oil Does Not Dissolve Well in Water
Most oils consist mainly of nonpolar molecules.
Water is strongly polar.
Water molecules interact strongly with one another through hydrogen bonding.
Oil molecules do not provide similarly favorable interactions with water.
As a result, water molecules tend to remain associated with other water molecules while oil molecules associate with other oil molecules.
The two substances separate.
This is why oil and water normally form separate layers.
Miscible and Immiscible Liquids
When two liquids mix completely, they are described as:
miscible
When they do not mix significantly and form separate layers, they are:
immiscible
For example:
Water and ethanol are miscible.
Water and many oils are largely immiscible.
The intermolecular attractions between the liquids help determine whether they mix.
Nonpolar Solvents
Nonpolar solvents interact well with many nonpolar substances.
Examples of largely nonpolar substances include:
- Oils.
- Waxes.
- Greases.
- Many hydrocarbons.
This explains why a nonpolar solvent may dissolve a greasy substance that water cannot remove effectively.
The principle is again:
like dissolves like
Intermolecular Forces and Dissolving
Different solutes and solvents can interact through different forces.
These include:
- London dispersion forces.
- Dipole-dipole attractions.
- Hydrogen bonding.
- Ion-dipole attractions.
The type and strength of these interactions influence solubility.
For example:
polar + polar
may involve dipole-dipole attractions or hydrogen bonding.
ion + polar
may involve ion-dipole attractions.
nonpolar + nonpolar
may involve London dispersion forces.
Comparing Common Examples
| Solute | Solute Type | Solvent | Solvent Type | Expected Solubility |
|---|---|---|---|---|
| Sugar | Polar | Water | Polar | High |
| NaCl | Ionic | Water | Polar | Often high |
| Oil | Nonpolar | Water | Polar | Low |
| Wax | Nonpolar | Water | Polar | Low |
| Oil | Nonpolar | Nonpolar solvent | Nonpolar | Often higher |
These are useful general predictions, although actual solubility depends on the specific substances involved.
Predicting Solubility
A useful process is:
Identify the solute
↓
Determine whether it is ionic, polar, or nonpolar
↓
Identify the solvent
↓
Determine whether the solvent is polar or nonpolar
↓
Compare their intermolecular interactions
↓
Predict whether significant dissolving is likely
Worked Example: Sugar and Water
Sugar:
polar
Water:
polar
Both can participate in strong intermolecular attractions.
Prediction:
Sugar should dissolve relatively well in water.
Observation:
Sugar does dissolve readily in water.
Worked Example: Oil and Water
Oil:
mostly nonpolar
Water:
polar
The intermolecular interactions are poorly matched.
Prediction:
Oil should have low solubility in water.
Observation:
Oil and water separate into layers.
Worked Example: Wax and Water
Wax consists mainly of long nonpolar hydrocarbon molecules.
Water is polar.
Therefore:
wax has very low solubility in water.
This is why wax coatings can help repel water.
Molecular Structure Matters
It is not always enough to label an entire molecule simply polar or nonpolar.
Some molecules contain:
- Polar regions.
- Nonpolar regions.
The relative sizes of these regions can strongly influence solubility.
A molecule containing a small polar group attached to a very large nonpolar hydrocarbon region may still have low water solubility.
Alcohols as an Example
Many alcohol molecules contain:
–OH
This group is polar and can form hydrogen bonds with water.
However, alcohols may also contain nonpolar hydrocarbon regions.
For small alcohol molecules, the polar –OH group can have a strong influence.
As the nonpolar hydrocarbon region becomes larger, water solubility generally decreases.
This demonstrates that solubility depends on the whole molecular structure.
Soap: A Molecule with Two Personalities
Soap molecules contain:
- A polar or ionic hydrophilic head.
- A nonpolar hydrophobic tail.
Hydrophilic means water-attracting.
Hydrophobic means water-avoiding or poorly interacting with water.
The nonpolar tails interact with grease.
The polar heads interact with water.
This allows soap to help disperse oily substances in water.
Micelles
In water, soap molecules can form structures called micelles.
The nonpolar tails point inward toward grease or other nonpolar substances.
The polar heads face outward toward the surrounding water.
This traps oily material inside structures that can remain dispersed in water.
The grease can then be washed away.
This is a practical application of polarity and solubility.
Solubility and Biological Membranes
Cell membranes contain phospholipids.
Phospholipids also contain:
- Polar heads.
- Nonpolar tails.
In water, they naturally organize into a bilayer.
The polar heads interact with water.
The nonpolar tails group together away from water.
The same principles of polarity and intermolecular attraction therefore help explain the basic structure of cell membranes.
Solubility and Medicines
The solubility of a medicine can influence:
- How it is formulated.
- How easily it dissolves.
- How it moves through watery body fluids.
- How it crosses biological membranes.
- How it can be delivered.
Some molecules contain both polar and nonpolar regions, allowing them to interact with different environments.
Solubility is therefore an important consideration in pharmaceutical chemistry.
Temperature and Solubility
Polarity is not the only factor affecting solubility.
Temperature can also have an important effect.
For many solid solutes dissolved in liquids:
higher temperature → greater solubility
However, this is not true for every substance.
The effect must be determined from experimental evidence or solubility data.
Solubility Curves
A solubility curve shows how the solubility of a substance changes with temperature.
A solubility curve can be used to:
- Compare substances.
- Determine how much solute can dissolve.
- Predict crystallization.
- Analyze the effect of temperature.
Gases and Temperature
Gases often behave differently from many solid solutes.
For many gases dissolved in liquids:
increasing temperature decreases gas solubility.
This is why a warm carbonated drink tends to lose dissolved carbon dioxide more readily than a cold one.
Temperature therefore affects solubility differently depending on the solute.
Pressure and Gas Solubility
Pressure can strongly affect the solubility of gases.
Increasing the pressure of a gas above a liquid generally increases the amount of that gas that can dissolve.
This principle is used in carbonated drinks.
Carbon dioxide is dissolved under elevated pressure.
When the container is opened, pressure decreases and carbon dioxide can escape from the solution as bubbles.
Saturated Solutions
A saturated solution contains the maximum amount of dissolved solute that can remain dissolved under particular conditions.
If additional solute is added, it may remain undissolved.
For example, repeatedly adding sugar to water eventually produces a point where additional sugar remains at the bottom.
The solution has reached its solubility limit under those conditions.
Unsaturated Solutions
An unsaturated solution contains less than the maximum amount of solute that could dissolve under the existing conditions.
Therefore, additional solute may still dissolve.
For example, if a small amount of sugar dissolves completely in a glass of water, the solution may still be capable of dissolving considerably more sugar.
Supersaturated Solutions
Under carefully controlled conditions, a solution can sometimes contain more dissolved solute than would normally remain dissolved at that temperature.
This is called a:
supersaturated solution
Supersaturated solutions are unstable.
Adding a small crystal or disturbing the solution can sometimes trigger rapid crystallization.
Soluble Does Not Mean Infinitely Soluble
A substance described as soluble does not necessarily dissolve in unlimited amounts.
For example, a certain mass of solute may dissolve in:
100 g of water
at a particular temperature.
Once the solubility limit is reached, additional solute remains undissolved.
Therefore, solubility describes an extent of dissolving under specified conditions.
Stirring and Dissolving
Stirring can make a substance dissolve faster because fresh solvent is continually brought into contact with the solute.
However, stirring does not necessarily increase the maximum solubility.
This distinction is important:
rate of dissolving ≠ solubility
One describes how quickly dissolving occurs.
The other describes how much can dissolve.
Crushing a Solute
Crushing a solid into smaller pieces increases its surface area.
This can make it dissolve faster.
However, crushing usually does not change the maximum amount that can eventually dissolve at equilibrium.
Again:
faster dissolving does not necessarily mean greater solubility.
Real-World Example: Making a Drink
When sugar is added to a drink:
- Stirring makes the sugar dissolve faster.
- Warmer liquid may allow more sugar to dissolve, depending on the substance.
- The polarity of water allows strong interactions with the polar regions of sugar molecules.
Several solution concepts therefore operate at the same time.
Real-World Example: Removing Grease
Grease is largely nonpolar.
Water is polar.
Therefore, water alone does not interact strongly with grease.
Soap contains both:
- Nonpolar regions that interact with grease.
- Polar regions that interact with water.
This allows grease to become dispersed and washed away.
Real-World Example: Nail Products and Paints
Some coatings, paints, adhesives, and cosmetic products contain substances that do not dissolve well in water.
Appropriate solvents are chosen based partly on the molecular properties of the materials being dissolved.
This is another practical application of:
like dissolves like
Real-World Example: Vitamins
Some vitamins are relatively water-soluble because they contain polar regions that interact favorably with water.
Others are more soluble in fats because they contain larger nonpolar regions.
This helps explain why vitamins are sometimes described broadly as:
- Water-soluble.
- Fat-soluble.
Their molecular structures influence which environments they interact with most readily.
Predicting Solubility from Structure
Consider a molecule containing:
- Several O–H groups.
- Several polar bonds.
- A relatively small nonpolar region.
It is likely to interact strongly with water.
Therefore, we might predict:
relatively high water solubility
Now consider a molecule consisting almost entirely of:
C–C and C–H bonds
with a long hydrocarbon chain.
It is largely nonpolar.
We would predict:
low water solubility
and greater compatibility with nonpolar substances.
Interpreting a Molecular Model
When predicting solubility from a molecular model, ask:
Does the molecule contain polar bonds?
What is its overall molecular polarity?
Can it form hydrogen bonds?
Does it contain ionic groups?
How large is its nonpolar region?
What type of solvent is being used?
Then compare the possible solute-solvent interactions.
Comparing Solubility and Concentration
These terms are related but different.
Solubility describes how much of a substance can dissolve under particular conditions.
Concentration describes how much solute is actually present in a particular amount of solution.
For example, a solution may be:
- Dilute and unsaturated.
- Concentrated and unsaturated.
- Saturated.
Therefore:
concentration and solubility are not the same thing.
Common Mistakes
Thinking “Like Dissolves Like” Means Identical Substances
It refers mainly to similarities in polarity and intermolecular interactions.
Assuming Every Polar Substance Dissolves Completely in Water
Polarity is important, but molecular size, structure, temperature and other interactions also matter.
Assuming Ionic Compounds Always Dissolve in Water
Many ionic compounds are water-soluble, but some have low solubility because their ionic attractions are too difficult for hydration interactions to overcome sufficiently.
Saying Oil Is Insoluble Because It Is More Dense or Less Dense
Density determines whether one liquid tends to sit above another.
It does not determine whether they dissolve in one another.
Oil and water separate mainly because of their molecular interactions.
Confusing Solubility With Dissolving Rate
A substance can dissolve slowly but still have high solubility.
Assuming Stirring Increases Solubility
Stirring usually increases the rate of dissolving rather than the maximum amount that can dissolve.
Thinking Dissolved Particles Disappear
They remain present but become distributed throughout the solvent.
Check Your Understanding
1. Define solubility.
2. Identify the solute and solvent in a sugar-water solution.
3. Explain what happens to particles when a substance dissolves.
4. What does “like dissolves like” mean?
5. Why do many polar substances dissolve well in water?
6. Explain why sugar dissolves in water.
7. Explain why NaCl can dissolve in water.
8. What is an ion-dipole attraction?
9. Explain why oil does not mix well with water.
10. Distinguish between miscible and immiscible liquids.
11. Predict whether a largely nonpolar wax would dissolve well in water. Explain.
12. Explain why nonpolar substances may dissolve better in nonpolar solvents.
13. How does soap help oil interact with water?
14. What is a micelle?
15. Explain how phospholipids demonstrate interactions between polar and nonpolar regions.
16. Distinguish between a saturated and unsaturated solution.
17. What is a supersaturated solution?
18. Explain why stirring can increase the rate of dissolving without increasing solubility.
19. Why does crushing a solid often make it dissolve faster?
20. Explain the difference between concentration and solubility.
21. Predict whether a molecule containing several O–H groups is likely to interact well with water.
22. Predict whether a long hydrocarbon molecule would have high or low water solubility.
23. Explain why carbonated drinks lose gas more easily after being opened.
24. Explain why “like dissolves like” is useful as a guideline but should not be treated as an absolute rule.
Key Terms
- Solubility – extent to which a solute can dissolve in a particular solvent under specified conditions.
- Solute – substance being dissolved.
- Solvent – substance doing the dissolving.
- Solution – homogeneous mixture containing dissolved solute.
- Polar – having an uneven distribution of electrical charge.
- Nonpolar – having no permanent overall separation of charge.
- Like dissolves like – guideline stating that substances with similar polarity and intermolecular interactions tend to dissolve in one another.
- Miscible – liquids capable of mixing to form a homogeneous solution.
- Immiscible – liquids that do not mix significantly and form separate phases.
- Hydration – surrounding and stabilizing dissolved particles with water molecules.
- Ion-dipole attraction – attraction between an ion and a polar molecule.
- Hydrophilic – interacting favorably with water.
- Hydrophobic – interacting poorly with water.
- Micelle – structure formed by molecules such as soaps with nonpolar regions grouped inward and polar regions interacting with water.
- Saturated solution – solution containing the maximum equilibrium amount of dissolved solute under specified conditions.
- Unsaturated solution – solution containing less than the maximum amount of solute that can dissolve.
- Supersaturated solution – unstable solution containing more dissolved solute than normally remains dissolved at that temperature.
Key Takeaways
- Solubility describes how well a substance dissolves in a particular solvent.
- Dissolving involves interactions between solute and solvent particles.
- Polarity is an important factor controlling solubility.
- “Like dissolves like” is a useful guideline for predicting solubility.
- Polar substances often dissolve well in polar solvents.
- Nonpolar substances often dissolve better in nonpolar solvents.
- Water is a polar solvent.
- Sugar contains polar regions that interact strongly with water.
- Ionic substances can dissolve when polar solvent molecules stabilize their separated ions.
- Water forms ion-dipole attractions with dissolved ions.
- Oil is largely nonpolar and therefore interacts poorly with polar water.
- Miscible liquids mix completely, while immiscible liquids form separate phases.
- Soap molecules contain both polar and nonpolar regions.
- This allows soap to interact with both water and grease.
- Molecular structure, not simply molecular formula, is important when predicting solubility.
- Temperature can affect solubility.
- Pressure is especially important for the solubility of gases.
- Stirring and crushing can increase the rate of dissolving without necessarily changing maximum solubility.
- Solubility and concentration are different concepts.
- Density does not determine whether substances dissolve in one another.
- To predict solubility, compare the polarity and intermolecular interactions of the solute and solvent while also considering molecular structure and conditions.