What do maple syrup, saltwater, and a bowl containing cereal and milk have in common? There are different types of solutions and mixtures! These two are very similar expressions, but it can be important to understand the subtle differences between them. Let's take a closer look at Solutions and Mixtures!
- First, we will talk about the difference between a mixture and a solution.
- Then, we will look at the different types of mixtures and solutions.
- Next, we will learn about their properties.
- Lastly, we will talk about the meaning of pure substances.
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 a mixture in which all the particles are evenly mixed. Solutions are considered homogeneous mixtures, and they can involve solids, liquids, and gases.
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.
In summary, a solution is referred to as a homogeneous mixture. Solutions have a uniform composition.
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.
Water is considered a universal solvent because of its capability of dissolving many substances! Water is able to dissolve ionic compounds, and also polar covalent compounds. When water dissociates ionic compounds, electrolyte solutions are formed. These solutions are capable of conducting electricity due to the presence of ions in the solution!
When water is used as a solvent, the solution is called an aqueous solution.
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.
A mixture is referred to as a heterogeneous 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.
The solubility of a solute is referred to as the maximum amount of solute that is able to dissolve in 100 grams of solvent at a given temperature.
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
| Primary solute | Solvent | Solution |
Acetic acid (liquid) | Water (liquid) | Vinegar (liquid-liquid) |
Zinc (solid) | Copper (solid) | Brass (solid-solid) |
Oxygen (gas) | | Air (gas-gas) |
Sodium chloride (solid) | Water (liquid) | Saltwater (solid-liquid) |
| Water (liquid) | Soda water (gas-liquid) |
Solutions can be categorized as:
Dilute solutions
Concentrated solutions
Saturated solutions
Supersaturated solutions
Unsaturated solutions
These days a super intensely researched area of chemistry is how to store hydrogen gas efficiently. One of the main problems with green energy production is the need to store this energy. Producing hydrogen from the energy (for example solar) is a very nice approach. However, what do you do with hydrogen? One idea is to dissolve it in metals like Palladium. Yes, that would be gas in a "solid solution". Many other elements are capable of dissolving hydrogen gas inside them these are called interstitial hydrides by the way. This is a very good solution for hydrogen transport but sadly very expensive.
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.
Dilution is the process of adding more solvent to a fixed amount of solute, increasing volume, and decreasing the concentration of the solution.
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.
Did you know that dilute solutions of phenol (carbolic acid) were used in hospitals before as antiseptics to kill infectious microorganisms? Joseph Lister was actually the first person to ever sterilize surgical instruments with phenol and also use phenol to disinfect wounds!
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!
If you mix two substances and they do not dissolve in one another (mixing oil and water or mixing salt and pepper), a saturated solution cannot be formed.
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
Supersaturated solutions don't always have to be heated in order to be formed. Honey is a supersaturated solution made from more than 70% sugar added to very low water content. Supersaturated solutions are unstable and, as seen in honey, will crystallize over time to form a stable saturated 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.
Try mixing oil and vinegar at home and watch how the two substances separate: oil on top and vinegar on the bottom!
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.
Molarity, which is also known as molar concentration, indicates the number of moles of a solute in 1 L of solution.
The equation for molarity is as follows:
Let's look at an example!
How many moles of MgSO4 is found in 0.15 L of a 5.00 M solution?
The questions give us molarity and liters of solution. So, all we have to do is rearrange the equation and solve for moles of MgSO4.
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:
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)
Find the molarity of 0.07 L of a 4.00 M KCl solution when diluted to a volume of 0.3 L.
Notice that the question gives us M1, V1, and V2. So, we need to solve for M2 using the dilution equation above.
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.
A pure substance is referred to an element or compound that has a definite composition and distinct chemical properties.
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.
Some substances are considered a gray area in terms of whether they are pure substances or not. Substances in this category as usually those that do not have a chemical formula, like milk, air, honey, and even coffee!
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!
Solutions and Mixtures - Key takeaways
- A solution is referred to as a homogeneous mixture composed of solute and solvent.
- A mixture is referred to as a heterogeneous mixture, also composed of solute and solvent.
- Solutions can be categorized as dilute, concentrated, unsaturated, saturated, and supersaturated.
- A pure substance is referred to an element or compound that has a definite composition and distinct chemical properties. Solutions can be pure substances, mixtures cannot.
References
- Brown, T. L. (2009). Chemistry: The Central Science. Pearson Education.
- The Princeton Review. (2019). Cracking the AP Chemistry Exam 2020. Princeton Review.
- AP Chemistry course and exam description ... - AP central. (n.d.). Retrieved April 29, 2022, from https://apcentral.collegeboard.org/pdf/ap-chemistry-course-and-exam-description.pdf?course=ap-chemistry
- Swanson, J. W. (2020). Everything you need to Ace Chemistry in one big fat notebook. Workman Pub.
- Timberlake, K. C., & Orgill, M. (2020). General, organic, and Biological Chemistry: Structures Of Life. Upper Saddle River: Pearson.
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