9.1: Solutions

Skills to Develop

• To understand what causes solutions to form.

• Distinguish among suspensions. colloids, and solutions.

Solutions

A solution is another name for a homogeneous mixture. A mixture as a material composed of two or more substances. In a solution, the combination is so intimate that the different substances cannot be differentiated by sight, even with a microscope. Compare, for example, a mixture of salt and pepper and another mixture consisting of salt and water. In the first mixture, we can readily see individual grains of salt and the flecks of pepper. A mixture of salt and pepper is not a solution. However, in the second mixture, no matter how carefully we look, we cannot see two different substances. Salt dissolved in water is a solution.

The major component of a solution, called the solvent, is typically the same phase as the solution itself. Each minor component of a solution (and there may be more than one) is called the solute. In most of the solutions we will describe in this textbook, there will be no ambiguity about whether a component is the solvent or the solute.) For example, in a solution of salt in water, the solute is salt, and solvent is water.

Solutions come in all phases, and the solvent and the solute do not have to be in the same phase to form a solution (such as salt and water). For example, air is a gaseous solution of about 80% nitrogen and about 20% oxygen, with some other gases present in much smaller amounts. An alloy is a solid solution consisting of a metal (like iron) with some other metals or nonmetals dissolved in it. Steel, an alloy of iron and carbon and small amounts of other metals, is an example of a solid solution. Table \(\PageIndex{1}\) lists some common types of solutions, with examples of each.

Table \(\PageIndex{1}\): Types of Solutions

Solvent Phase	Solute Phase	Example
gas	gas	air
liquid	gas	carbonated beverages
liquid	liquid	ethanol (C2H5OH) in H2O (alcoholic beverages)
liquid	solid	saltwater
solid	gas	H2 gas absorbed by Pd metal
solid	liquid	Hg(ℓ) in dental fillings
solid	solid	steel alloys

What causes a solution to form? The simple answer is that the solvent and the solute must have similar intermolecular interactions. When this is the case, the individual particles of solvent and solute can easily mix so intimately that each particle of solute is surrounded by particles of solute, forming a solution. However, if two substances have very different intermolecular interactions, large amounts of energy are required to force their individual particles to mix intimately, so a solution does not form.

This process leads to a simple rule of thumb: like dissolves like. Solvents that are very polar will dissolve solutes that are very polar or even ionic. Solvents that are nonpolar will dissolve nonpolar solutes. Thus water, being polar, is a good solvent for ionic compounds and polar solutes like ethanol (C₂H₅OH). However, water does not dissolve nonpolar solutes, such as many oils and greases (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): A beaker holds water with blue food dye (upper liquid layer) and a much more dense perfluoroheptane (a fluorocarbon) lower liquid layer. The two fluids cannot mix and the dye cannot dissolve in fluorocarbon. A goldfish and a crab have been introduced into the water. The goldfish cannot penetrate the dense fluorocarbon. The crab floats at the liquid boundary with only parts of his legs penetrating the fluorocarbon fluid, unable to sink to the bottom of the beaker. Quarter coins rest on the bottom of the beaker. Animals were rescued from their predicament after the photo was taken. Figure used with permission from Wikipedia (Sbharris (Steven B. Harris)).

We use the word soluble to describe a solute that dissolves in a particular solvent, and the word insoluble for a solute that does not dissolve in a solvent. Thus, we say that sodium chloride is soluble in water but insoluble in hexane (C₆H₁₄). If the solute and the solvent are both liquids and soluble in any proportion, we use the word miscible, and the word immiscible if they are not.

Example \(\PageIndex{1}\)

Water is considered a polar solvent. Which substances should dissolve in water?

1. methanol (CH₃OH)
2. sodium sulfate (Na₂SO₄)
3. octane (C₈H₁₈)

SOLUTION

Because water is polar, substances that are polar or ionic will dissolve in it.

1. Because of the OH group in methanol, we expect its molecules to be polar. Thus, we expect it to be soluble in water. As both water and methanol are liquids, the word miscible can be used in place of soluble.
2. Sodium sulfate is an ionic compound, so we expect it to be soluble in water.
3. Like other hydrocarbons that only have non-polar C-C and C-H bonds, octane is nonpolar, so we expect that it would not be soluble in water.

Suspensions

Take a glass of water and throw in a handful of sand or dirt. Stir it and stir it and stir it. Have you made a solution? Sand and dirt do not dissolve in water, and, though it may look homogenous for a few moments, the sand or dirt gradually sinks to the bottom of the glass (see figure below). Some medications are delivered as suspensions and must be mixed well before the doses measured to make sure the patient is receiving the correct amount of medication.

Figure \(\PageIndex{2}\) : A mixture of sand and water forms a suspension.

A suspension is a heterogeneous mixture in which some of the particles settle out of the mixture upon standing. The particles in a suspension are far larger than those of a solution, so gravity is able to pull them down out of the dispersion medium (water). The diameter for the dispersed particles in a suspension, such as the sand in the suspension described above, is typically at least 1000 times greater than those in a solution. Solutions are typically transparent, whereas the particles in a suspension will block some or all light from passing through. Unlike a solution, the dispersed particles can be separated from the dispersion medium by filtering. Suspensions are considered heterogeneous because the different substances in the mixture will not remain uniformly distributed if they are not actively being mixed.

Colloids

A colloid is a heterogeneous mixture in which the dispersed particles are intermediate in size between those of a solution and a suspension. The particles are spread evenly throughout the dispersion medium, which can be a solid, liquid, or gas. Because the dispersed particles of a colloid are not as large as those of a suspension, they do not settle out upon standing. The table below summarizes the properties and distinctions between solutions, colloids, and suspensions.

Table \(\PageIndex{2}\): Properties of Solutions, Colloids, and Suspensions

Solution	Colloids	Suspensions
Homogeneous	Heterogeneous	Heterogeneous
Particle size: \(0.01\)-\(1 \: \text{nm}\); atoms, ions or molecules	Particle size: \(1\)-\(1000 \: \text{nm}\), dispersed; large molecules or aggregates	Particle size: over \(1000 \: \text{nm}\), suspended: large particles or aggregates
Do not separate on standing	Do not separate on standing	Particles settle out
Cannot be separated by filtration	Cannot be separated by filtration	Can be separated by filtration
Do not scatter light (transparent)	Usually scatter light, but may block (translucent or opaque)	Usually block light, but may scatter (opaque or translucent)

Colloids are unlike solutions because their dispersed particles are much larger than those of a solution. The dispersed particles of a colloid cannot be separated by filtration, but they scatter light, a phenomenon called the Tyndall effect.

Tyndall Effect

Colloids are often confused with true homogenous solutions because the individual dispersed particles of a colloid cannot be seen. When light is passed through a true solution, the dissolved particles are too small to deflect the light. However, the dispersed particles of a colloid, being larger, do deflect light (see figure below). The Tyndall effect is the scattering of visible light by colloidal particles. You have undoubtedly "seen" a light beam as it passes through fog, smoke, or a scattering of dust particles suspended in air. All three are examples of colloids. Suspensions may scatter light, but if the number of suspended particles is sufficiently large, the suspension may simply be opaque, and the light scattering will not occur.

Figure \(\PageIndex{3}\): Light passes through a colorless solution and is not scattered. When it passes through a diluted milk solution, the light is scattered by colloidal particles, an observation of the Tyndall effect. The Tyndall effect allows sunlight to be seen as it passes through a fine mist.

Examples of Colloids

Listed in the table below are examples of colloidal systems, most of which are very familiar. Some of these are shown below (see figure below). The dispersed phase describes the particles, while the dispersion medium is the material in which the particles are distributed.

Table \(\PageIndex{3}\): Classes of Colloids

Class of Colloid	Dispersed Phase	Dispersion Medium	Examples
Solid and gel	solid	liquid	paint, jellies, blood, gelatin, mud
Solid aerosol	solid	gas	smoke, dust in air
Solid emulsion	liquid	solid	cheese, butter
Liquid emulsion	liquid	liquid	milk, mayonnaise
Liquid aerosol	liquid	gas	fog, mist, clouds, aerosol spray
Foam	gas	solid	marshmallow
Foam	gas	liquid	whipped cream, shaving cream

Figure \(\PageIndex{4}\): Some common colloids (A) gelatin dessert, (B) smoke (solid aerosol), (C) butter (solid emulsion), (D) mayonnaise (liquid emulsion), (E) fog (liquid aerosol), (F) marshmallows (foam), (G) whipped cream (foam)

Emulsions

Butter and mayonnaise are examples of a class of colloids called emulsions. An emulsion is a colloidal dispersion of a liquid in either a liquid or a solid. A stable emulsion requires an emulsifying agent to be present. Mayonnaise is made in part of oil and vinegar. Since oil is nonpolar, and vinegar is a polar aqueous solution, the two do not mix and would quickly separate into layers. However, the addition of egg yolk causes the mixture to become stable and not separate. Egg yolk is capable of interacting with both the polar vinegar and the nonpolar oil. The egg yolk is called the emulsifying agent. Soap acts as an emulsifying agent because one end of a soap molecule is polar, and the other end is nonpolar. This allows the grease to be removed from your hands or your clothing by washing with soapy water.

Concept Review Exercises

1. What causes a solution to form?
2. How does the phrase like dissolves like relate to solutions?
3. How can you distinguish between a solution, a suspension, and a colloid?

Answers

1. Solutions form because a solute and a solvent have similar intermolecular interactions.
2. It means that substances with similar intermolecular interactions will dissolve in each other.
3. A suspension can be separated from the solvent by filtration while a solution and colloid cannot. Suspensions and colloids block or scatter light, whereas typically do not.

Key Takeaway

• Solutions form because a solute and a solvent experience similar intermolecular interactions.

Exercises

1. Define solution.

2. What is the difference between a solvent and a solute?

3. Which substances will probably be soluble in water, a very polar solvent?

   1. sodium nitrate (NaNO₃)
   2. hexane (C₆H₁₄)
   3. isopropyl alcohol [(CH₃)₂CHOH]
   4. benzene (C₆H₆)

4.  The solubility of alcohols in water varies with the length of carbon chain. For example, ethanol (CH₃CH₂OH) is soluble in water in any ratio, while only 0.0008 mL of heptanol (CH₃CH₂CH₂CH₂CH₂CH₂CH₂OH) will dissolve in 100 mL of water. Propose an explanation for this behavior.

5.  Identify each of the following descriptions or examples as being representative of a solution, suspension, or colloid. More than one answer may apply.

a. dispersed particles can be filtered out
b. heterogeneous
c. particles are not visible to the unaided eye
d. paint
e. lemonade with no pulp
f. particle size larger than 1 nm
g. milk
h. particles do not settle upon standing
i. fog

Answers

1. a homogeneous mixture
2. A solvent is the majority component of a solution; a solute is the minority component of a solution.
3. a. probably soluble;  b. probably not soluble;  c. probably soluble;  d. probably not soluble
4. Small alcohol molecules have strong polar intermolecular interactions (polar C-O bond and very polar O-H bond), so they dissolve in water. In large alcohol molecules, the nonpolar hydrocarbon portion overwhelms the polar end, so they do not dissolve very well in water.
5.  

a. suspension

b. colloids and suspensions

c. solution

d. colloid

e. solution

f. colloids and suspensions

g. colloid

h. solutions and colloids

i. colloids