Solutions and Mixtures

Difference between a mixture and a solution

For your AP chemistry exam, you should know the following definitions regarding solutions and mixtures.

A solution is composed of a solute and a solvent. A solute is a substance that gets dissolved in a solvent. A solvent is a medium in which the solute gets dissolved. In solutions, the macroscopic properties do not vary throughout the sample.

To form a solution, the intermolecular forces present in both the solute and the solvent must be broken, and then new intermolecular forces need to form between them.

A mixture, on the other hand, consists of particles that cannot mix evenly and therefore are considered heterogeneous. In mixtures, the macroscopic properties vary depending on the location in the mixture.

Before diving into the different types of mixtures and solutions, we need to remember the basics of solubility.

• In solids, the solubility in water increases with an increase in temperature.
• In gases, the solubility in water decreases with an increase in temperature.
• Most ionic compounds that have Li⁺, Na⁺, K⁺, NH₄⁺, NO₃^- or CH₃CO₂^- are considered soluble in water.

Types of solutions and mixtures

Solutions can be formed from any combination of solid, liquid, or gas. In the table below, you can find some examples of solutions!

Examples of solutions

Solutions can be categorized as:

• Dilute solutions

• Concentrated solutions

• Saturated solutions

• Supersaturated solutions

• Unsaturated solutions

Dilute vs concentration solutions

When you add a cup of concentrated orange juice to a jar containing three cups of water to make orange juice, you are actually making a dilution solution! Dilute solutions are solutions that have a low amount of solute in the solution.

Dilutions are usually performed by chemists to reduce the concentration of solutions. Concentration is a measurement of how much solute is dissolved in the solvent.

Concentrated solutions are the opposite of dilute solutions and they have a high amount of solute in the solution. Concentrated solutions can be further divided into unsaturated, saturated, and supersaturated solutions.

Unsaturated Solutions

Unsaturated solutions are solutions that have less than the maximum amount of solute that can be dissolved in the solvent. So, if you decided to add more solute to an unsaturated solution, the solute would dissolve without a problem, leaving no traces of the solute!

For example, if you added salt to a cup of water and the salt completely dissolves, then you have an unsaturated solution.

Saturated solutions

Saturated solutions are solutions that have the maximum amount of solute dissolved. In other words, if you added more solute to it, the solute would not dissolve. Instead, it would sink to the bottom of the solution.

When a solution becomes saturated, it means that the rate at which the solute dissolves in the solvent is equal to the rate at which the saturated solution is formed. This is called crystallization.

Fig.1-Crystallization

Think about a time when you added sugar to your coffee or tea, and it got to a point where the sugar stopped dissolving. This is an example of a saturated solution!

Supersaturated solutions

Supersaturated solutions are solutions that contain more than the maximum amount of solute that can be dissolved in the solvent. Supersaturated solutions are formed when a saturated solution gets heated to a high temperature and then more solute is added to it. When the solution cools down, no precipitate is formed.

Fig.2-Formation of a supersaturated solution

Now, let's look at the different types of mixtures! Mixtures can be homogeneous and heterogeneous.

However, when dealing with AP exams, mixtures are the term used to refer to heterogeneous mixtures only! To make things simpler, let's focus on what heterogeneous mixtures are.

Heterogeneous Mixtures

When a mixture contains substances that are not uniform in composition, we give it the name heterogeneous mixture. This type of mixture can be separated by physical means. Your favorite pizza is a type of heterogeneous mixture!

Suspensions are a type of heterogeneous mixture. In order to mix the substances found in a suspension, an outside force is needed. But, after a while, the substances will separate again. A common example of a suspension is salad dressing, made up of oil and vinegar.

Now that we learned about what mixtures and solutions are, and the types that exist, let's focus on the properties of mixtures and solutions!

Properties of Mixtures and Solutions

Solutions are a type of homogenous mixture consisting of particles with very small diameters that completely dissolve in the solution and cannot be seen with the naked eye. They are not capable of scattering beams of light, and they cannot be separated by filtration. Solutes are also stable at a given temperature.

Mixtures, on the other hand, are heterogeneous mixtures consisting of particles that can be separated. Mixtures do not have a uniform composition and the different parts may be seen with the naked eye. Mixtures are able to scatter light.

Molarity (Molar Concentration)

We can express the composition of a solution by using molarity. Molarity is the concentration of the solute.

The equation for molarity is as follows:

$\mathrm{Molarity}\left(M\right)=\frac{n_{\mathrm{solute}}}{L_{\mathrm{solution}}}$

Let's look at an example!

Dilution Calculation involving Molarity

We stated before that when more solvent is added to a sample, it becomes less concentrated (diluted). The dilution equation is:

${\mathrm{M}}_1{\mathrm{V}}_1={\mathrm{M}}_2{\mathrm{V}}_2$

Where,

• M₁ is the molarity before dilution
• M₂ is the molarity after dilution
• V₁ is the volume of solution before dilution (in L)
• V₂is the volume of solution after dilution (in L)

Pure substances mixture and solution

Pure water is made up of hydrogen and oxygen molecules, and it is considered a pure substance. Some examples of pure substances include Iron, NaCl (table salt), sugar (sucrose), and ethanol.

If a solution has a constant composition, then it can also be considered a type of pure substance. For example, a solution containing salt dissolved in water is a pure substance because the composition of the solution stays the same throughout.

Mixtures (heterogeneous mixtures) are not considered pure substances due to the differences in composition.

After reading this, I hope that you feel more confident about the difference between solutions and mixtures, and ready to tackle any problem that comes your way!