In this explainer, we will learn how to describe polar and nonpolar solvents.
Solubility can be defined as the tendency of one chemical substance to dissolve in a solvent and form a solution. The solute can be a solid, a liquid, or even a gas and the solvent can be made up of almost any type of molecule.
Some substances will be soluble in polar solvents and insoluble in nonpolar solvents. Other substances will be soluble in nonpolar solvents and insoluble in polar solvents. The different solubilities can be understood if we consider how electrons are distributed in each molecule type and the strength of intermolecular interactions between the solute and solvent molecules. Once we understand all of these concepts, we will know why oil does not mix with water.
Solubility is the maximum amount of solute that can dissolve in a given amount of solvent at a given temperature.
It is important to first properly define concepts like electronegativity and molecular polarity before we discuss intermolecular interaction strengths and solute and solvent solubilities. Electronegativity values quantify the ability of an atom to withdraw bonding pairs of electrons from covalent bonds. Atoms are effective at withdrawing a lot of electron density from covalent bonds when they have high electronegativity values and less effective at withdrawing bonding pairs of electrons when they have low electronegativity values.
Covalent bonds are described as being polar when they are made up of atoms that have significantly different electronegativity values. The more electronegative atom withdraws most of the electron density from the covalent bond and it ends up carrying a partial negative electric charge . The other atom ends up carrying a partial positive electric charge . Hydrogen chloride () molecule is a good example of a molecule that contains a polar covalent bond. The highly electronegative chlorine atom withdraws a significant amount of electron density from the hydrogen–chloride covalent bond and it ends up having a partial negative electric charge, while the hydrogen has a partial positive charge.
Molecules usually end up being polar if they contain polar covalent bonds, but this is not always the case. Molecules can have certain shapes and geometries, which means that their exposed surface areas all have essentially the same electric charge value even if the molecule contains multiple polar bonds.
The following figure shows how some molecules are nonpolar despite the fact that they contain polar bonds. It shows that carbon dioxide () molecules are nonpolar despite having two polar carbon–oxygen bonds. Carbon dioxide molecules are nonpolar because they are highly symmetrical. They have two electric dipole moments, but these dipole moments effectively cancel each other out. The figure also shows that water molecules are polar. They have two electric dipole moments that reinforce each other. Water molecules generate a molecular dipole moment and carbon dioxide molecules do not generate a molecular dipole moment. The following image will conclude this introductory scientific background section and we can now start discussing why some substances are soluble in other substances.
Solute and solvent substances will usually only mix together and form a single homogeneous solution if the solute–solvent interactions are similar to or the same as the solvent–solvent or solute–solute interactions. Polar solvent molecules have an asymmetrical distribution of electron density, and some areas of a polar solvent molecule carry a partial negative charge while other areas carry a partial positive charge. Polar solvent molecules have an electric dipole moment and they experience strong electrostatic interactions with other polar solvent molecules. Polar solvent molecules will only break apart and mix with solute molecules if this dissolution process makes solvent–solute interactions that are similar to or the same as the electrostatic-based solvent–solvent interactions.
The following figure can be used to better understand the information in the previous paragraph. The figure shows that there are strong bridging interactions between polar solvent molecules. These bridging interactions can be called solvent–solvent interactions. Some of these bridging interactions have to be disrupted if the solvent is to mix with some solute. Some of the bridging interactions have to be disrupted so that a cavity can be formed in between the solvent molecules. The cavity can then be filled with a solute molecule. This process will only tend to happen if the solute can effectively “fill the place” of the displaced solvent. The process will only tend to happen if the solute can make new solute–solvent bridging interactions that are similar to or the same as the old solvent–solvent bridging interactions.
Water () is one of the simplest polar solvents and it contains just one electronegative oxygen atom that is covalently bonded to two hydrogen atoms. The oxygen atom attracts most of the electron density from the two covalent bonds. The oxygen atom ends up being relatively enriched with electron density , while the two hydrogen atoms end up being depleted of electron density . The asymmetrical electron density distribution creates an electric dipole moment, and the water molecules can have strong electrostatic interactions with other polar molecules or charged atoms.
Example 1: Remembering Whether Water Molecules Are Classed as Polar or Nonpolar Solvents
Is a water () molecule a polar or nonpolar solvent?
- Polar solvent
- Nonpolar solvent
Water molecules have areas where the electron density is relatively low and other areas where the electron density is relatively high . Water molecules are highly polar molecules and they have strong electrostatic interactions with other charged and polar molecules. The previous statements can be used to determine that water molecules are polar solvents. Option A has to be the correct answer for this question.
Water–water intermolecular interactions are strong because water molecules have an electric dipole moment. The electron-dense areas of one water molecule can form strong hydrogen bonds with the electron-depleted areas of other water molecules. Water molecules will only break away from each other and break down strong water–water hydrogen bonds if they can create similarly strong electrostatic interactions with other solute molecules or atoms.
Water molecules readily break away from each other and break up water–water hydrogen bonds if they can form new and strong electrostatic interactions with charged ions or polar molecules. This happens when ionic salts such as sodium chloride () are dissolved in water. The water molecules break away from each other and they form strong electrostatic interactions with the positively and negatively charged ions that make up the sodium chloride lattice. The positively charged sodium ions () interact with the partially negatively charged ends of some water molecules. The negatively charged chloride () ions interact with the partially positively charged ends of other water molecules. The sodium and chloride ions can effectively “fill the place” of displaced solvent water molecules.
Water molecules also tend to have strong electrostatic interactions with short-chain alcohol and carboxylic acid molecules. The hydrogen atom of one water molecule can form hydrogen bonds with the electronegative oxygen atom of adjacent alcohol or carboxylic acid molecules. Water molecules will readily break away from each other if they can form strong hydrogen bonds with polar alcohol or carboxylic acid molecules. This tendency of water molecules to break apart and then form hydrogen bonds with polar solutes explains why short-chain alcohols and short-chain carboxylic acids are soluble in water.
The first three members of the straight-chain alcohol and carboxylic acid homologous series are all soluble in water. Methanol, ethanol, and propanol can all form a single homogeneous solution with water at room temperature and atmospheric pressure. Chemists define methanol, ethanol, and propanol as being miscible with water because they can form a single homogeneous solution with water under standard conditions.
Miscible substances can fully dissolve in each other at any concentration and form a single homogeneous solution.
Example 2: Identifying the Term That Describes Substances That Mix Together in Any Proportion
Fill in the blank: Ethanol and ethanoic acid both mix in any proportion with water. Substances that mix in this way are said to be .
The words blends, mingled, and concocted are quite ambiguous terms that are usually not used by chemists to describe how two or more substances can combine together. Miscible and immiscible are much less ambiguous terms that are regularly used by chemists to describe how two or more substances can combine together. Miscible substances can fully dissolve in each other at any concentration and form a single homogenous solution. Immiscible substances cannot fully dissolve in each other and they do not form a single homogeneous solution. These statements can be used to determine that option D is the correct answer for this question.
The larger members of the alcohol and carboxylic acid homologous series are insoluble in water at room temperature and atmospheric pressure. Large alcohol and carboxylic acid molecules are insoluble in water because they have relatively large nonpolar hydrocarbon chain domains and relatively small polar hydroxyl () or carboxyl () groups. The small hydroxyl or carboxyl groups can form strong hydrogen bonds with water molecules, but the larger nonpolar carbon chains cannot form hydrogen bonds with water molecules. It is energetically favorable for the small hydroxyl and carboxyl groups to dissolve in water, but it is more energetically favorable for the large carbon chains to not dissolve in water. The polar side of the large alcohol or carboxylic acid molecule can effectively “fill the place” of displaced polar water molecules, but the nonpolar side cannot.
The following table shows that the solubility of an alcohol depends on the length of its nonpolar hydrogen carbon chain domain. It shows that alcohol molecules are highly soluble in water if they have a small nonpolar hydrocarbon chain domain and that alcohol molecules are much less soluble if they have a relatively large hydrocarbon chain domain.
|Alcohol Name||Chemical Formula||Water Solubility |
(g/100 g Water)
|Water solubility are determined at|
Example 3: Understanding the Association between Carboxylic Acid Molecule Length and Solubility
Why is pentanoic acid insoluble in water, whereas ethanoic acid is soluble?
- Pentanoic acid does not react with water.
- The hydration energy is too high.
- The shape of the molecule prevents hydrogen bonding.
- Pentanoic acid has a high boiling point.
- Pentanoic acid has a long hydrocarbon chain.
The solubility of a carboxylic acid molecule depends on its balance of nonpolar and polar parts of the molecule. Large carboxylic acids are insoluble in water because they have relatively large nonpolar hydrocarbon chain and relatively small polar carboxyl () groups.
Smaller carboxylic acids are usually more soluble in water because they tend to have smaller nonpolar hydrocarbon chain. Pentanoic acid is insoluble in water because it has a five-membered carbon chain and only one carboxyl group. Ethanoic is much more soluble in water because it has a similar carboxyl group but only a two-membered carbon chain, which is smaller.
These statements can be used to determine that answer E is the correct answer for this question.
Water does not tend to mix with nonpolar substances because the water–water interactions tend to be much stronger than the water–nonpolar molecule interactions. Water will not mix with nonpolar substances like oil or grease. It is not energetically favorable to break strong water–water interactions and replace them with weaker water–oil or water–grease interactions. Water molecules tend to stay together if they are surrounded by nonpolar molecules like oil or grease molecules.
The intermolecular interactions between nonpolar molecules are generally very weak, and nonpolar molecules do not tend to be capable of “filling the place” of displaced polar solvent molecules. Nonpolar substances tend to have relatively weak intermolecular interactions, and they are usually not miscible with polar substances like water. Polar substances do not mix with nonpolar substances because the interactions between polar molecules are stronger than the interactions between nonpolar substances. It is energetically favorable to have polar and nonpolar substances separated and less energetically favorable to have them mixed together. There is a high total number of strong intermolecular interactions if the nonpolar and polar substances are separated. There is a lower number of strong intermolecular interactions if the nonpolar and polar substances are mixed together.
Nonpolar substances do tend to be miscible with other nonpolar substances though. There will be approximately the same number of weak intermolecular interactions if nonpolar solute and solvent substances are separated or mixed together. There are number of weak intermolecular interactions if the nonpolar solvent and solute substances are separated and approximately number of weak intermolecular interactions if the nonpolar solvent and solute substances are mixed to form a single homogenous solution. This line of reasoning can be used to understand why most nonpolar substances are miscible with each other. Let us consider some examples in the next paragraph.
The following figure shows the intermolecular interactions between two types of nonpolar molecules. One image shows that there are weak intermolecular interactions between nonpolar hexane () molecules and the other image shows that there are similar weak intermolecular interactions between nonpolar bromine () molecules. Hexane–hexane and bromine–bromine intermolecular interactions are both weak because hexane and bromine molecules do not contain polar bonds. They have nonpolar bonds because they are made up of atoms that have similar electronegativity values. Any one bromine atom cannot withdraw most of the electron density from a bromine–bromine bond in a molecule and neither carbon nor hydrogen atoms can withdraw a significant amount of electron density from the carbon–hydrogen bonds in a molecule.
The bromine and hexane substances are miscible with each other because the hexane–hexane interactions are similar to the bromine–bromine interactions. There are roughly number of weak intermolecular interactions in a solution of hexane and bromine regardless of how the hexane and bromine molecules are arranged around each other.
We can also consider the nonpolar benzene molecule that has the chemical formula . The molecule is made up of atoms that have similar electronegativity numbers, and it is nonpolar. Benzene molecules tend to be miscible with most other nonpolar substances. There are roughly number of weak intermolecular interactions in a solution of benzene and another nonpolar substance regardless of how the two types of molecules are arranged around each other. Benzene molecules can dissolve in nonpolar substances like hexane because benzene–hexane interactions are very similar to hexane–hexane interactions. The benzene molecules can effectively “fill the place” of displaced hexane solvent. Solutions of benzene and hexane have roughly number of weak intermolecular interactions regardless of how benzene and hexane molecules are arranged around each other.
Example 4: Identifying the Correct Description for Bromine Dissolving in Hexane
Which of the following statements describes bromine dissolving in hexane?
- A nonpolar substance dissolving in a polar solvent
- A polar substance dissolving in a nonpolar solvent
- A polar substance dissolving in a polar solvent
- A nonpolar substance dissolving in a nonpolar solvent
Diatomic molecules are described as being nonpolar if they contain two atoms of the same chemical element. The bromine element has a relatively high electronegativity value but diatomic bromine molecules are nonetheless still nonpolar molecules, because diatomic bromine molecules contain two atoms of the same chemical element. This line of reasoning can be used to infer that bromine should be described as a nonpolar substance.
Simple alkane hydrocarbons are described as being nonpolar substances because they are made up of two types of atoms that have very similar electronegativity values. Hydrocarbons are made up of carbon and hydrogen atoms. The hydrogen and carbon atoms have similar electronegativity values and neither atom can withdraw most of the electron density from the carbon–hydrogen () covalent bonds. This paragraph can be used with the first paragraph to determine that option D is the correct answer for this question. It is scientifically accurate to describe the process of dissolving bromine in hexane as a process of dissolving a nonpolar substance in a nonpolar solvent.
Our reasoning has helped us to understand why most polar substances can dissolve in most polar solvents and also why most nonpolar solutes can dissolve in most nonpolar solvents. The miscibility values are ultimately determined by the strengths of the intermolecular interactions that keep the solute and solvent substances together.
Nonpolar solute and nonpolar solvents generally have similar intermolecular interaction strengths and they can usually mix together and form a single homogeneous solution. Polar solute and polar solvents generally have similar intermolecular interaction strengths and they also tend to mix together and form a single homogeneous solution. Nonpolar and polar substances have very different intermolecular interaction strengths, and this explains why polar and nonpolar substances are usually not miscible with each other. We have been able to satisfactorily explain why nonpolar substances like oil do not usually mix with polar substances like water. Let us now consider some common nonpolar and polar solvents.
The following table shows some of the most common polar and nonpolar solvents. The first column shows common nonpolar solvents and the second column shows common polar solvents. The substances in the left column are almost all made-up atoms that have similar electronegativity values and the substances in the right column have atoms with significantly different electronegativity values.
|Common Nonpolar Solvents||Common Polar Solvents|
Example 5: Determining Whether Some Common Simple Molecular Compounds Should Be Classed as Polar or Nonpolar Solvents
Which of the following molecules is a nonpolar solvent?
- Acetic acid
Nonpolar solvents contain atoms that have similar electronegativity values, like carbon and hydrogen. Compounds A–D have some atoms with relatively low electronegativity values such as hydrogen and others with relatively high electronegativity values such as oxygen and nitrogen. This suggests that compounds A–D are polar molecules that can form polar solvents when they group together.
We have determined that option E is the correct answer through the process of elimination, but we can also apply some logic to validate this inference. Benzene () contains carbon and hydrogen atoms that have similar electronegativity values, and this means that the carbon–hydrogen bonds in benzene should not be polar. The benzene molecules are not polar and they must form a nonpolar solvent when they group together. This statement can be used to deduce that option E is the correct answer for this question.
Chemists tend to further subdivide polar solvents into the polar protic and polar aprotic classes. Polar protic solvent molecules can form hydrogen bonds, because they usually contain at least one hydrogen atom that is covalently bonded to a nitrogen, oxygen, or fluorine atom. The hydrogen atom carries a partial positive electrostatic charge and it can make hydrogen bonds with another electronegative atom bearing a lone pair of electrons.
Definition: Polar Protic Solvent
Polar protic solvent molecules have at least one hydrogen atom that is covalently bonded to an oxygen, a nitrogen, or a fluorine atom.
Polar aprotic solvents are polar solvents that do not contain any hydrogen atoms that can make hydrogen bonds with electron lone pairs. There is no hydrogen bonding between adjacent polar aprotic solvent molecules because one polar aprotic molecule cannot make a hydrogen bond with another polar aprotic molecule.
The following figure compares the structure of one polar protic solvent molecule with one polar aprotic solvent molecule. The acetic acid molecule has one hydrogen atom that is covalently bonded to a highly electronegative oxygen atom. The dichloromethane molecule does not have any hydrogen atoms that are similarly bonded to a highly electronegative atom. Acetic acid can form hydrogen bonds and dichloromethane molecules cannot form hydrogen bonds.
- Polar solvents contain some atoms with relatively high electronegativity values and others with relatively low electronegativity values.
- The polarity of a molecule depends on the electronegativity difference between the bonded atoms and the shape and geometry of the molecule.
- Nonpolar solvents contain atoms with similar electronegativity values.
- Polar substances are generally miscible with other polar substances, and nonpolar substances are generally miscible with other nonpolar substances.
- Polar and nonpolar substances are generally not miscible with each other.
- Polar protic solvent molecules tend to form strong intermolecular hydrogen bonds.
- Polar aprotic solvents molecules do not generally form intermolecular hydrogen bonds.