In this explainer, we will learn how to identify and describe different types of mixtures and their physical properties.
Chemistry is the study of matter, any material that has both mass and volume. Matter can further be classified as either a pure substance or a mixture. Elements such as aluminum (), carbon (), and iron () are pure substances. When elements chemically bond together, compounds such as water (), carbon dioxide (), and sodium chloride () are formed. Compounds are also classified as pure substances.
When two or more substances, elements or compounds, are physically combined rather than chemically, a mixture is formed. In order to be classified as a mixture, the substances must retain their distinct identities and should be able to be separated from one another by physical means. Examples of mixtures include air, milk, salt water, and cement.
A mixture is the combination of two or more substances by physical means in which the substances retain their distinct identities.
The figure below shows various atoms, compounds, and mixtures on a molecular level.
Example 1: Identifying the Definition of a Mixture
Which of the following statements best describes a mixture?
- A chemical combination of two or more substances
- A molecule that is composed of different types of atoms
- A nonchemical combination of two or more substances
- A substance composed of only one type of atom or molecule
- The combination of two substances in different physical states
A mixture is a combination of two or more substances. Answer choice D cannot describe a mixture as only one type of atom or molecule is present. Furthermore, to be a mixture, the substances must be physically blended not chemically combined. We can eliminate answer choices A and B as these describe chemical combinations. While the physical states of the substances combined to form a mixture may be different as in the mixture formed when salt dissolves in water, the physical states may be the same as in the mixture formed when mixing milk with coffee. As such, we can eliminate answer choice E. The statement that best describes a mixture is answer choice C.
When two or more substances combine to form a mixture that has a uniform composition, the mixture can be classified as homogeneous. The prefix “homo-” means the same and indicates that the composition and properties of the mixture are the same throughout. A homogeneous mixture is also called a solution.
Definition: Homogeneous Mixture
A homogeneous mixture is one with a uniform composition throughout. This is also often called a solution.
Salt water is an example of a homogeneous solution. The mixture consists of sodium chloride () and water (). Once combined, the resulting mixture is uniform throughout and the salt cannot be distinguished from the water.
Another example of a homogenous mixture is air. Air is composed of nitrogen, oxygen, argon, carbon dioxide, water vapor, and other gases. The composition of air is uniform, and the various gases cannot be distinguished from one another.
A solution consists of a solvent, the major component, and solutes, the minor components. The solute may also be described as the substance that dissolves while the solvent is described as the substance that does the dissolving.
A solvent is the major component of a solution into which other substances dissolve.
A solute is the minor component of a solution, the substance that is described as dissolving.
In the example of salt water, the salt is the solute and water is the solvent. In air, nitrogen is the solvent as it makes up of the total composition of the mixture. Therefore, when considering air as a mixture, oxygen, argon, carbon dioxide, and water vapor are all considered to be solutes.
We often think of solutions as liquids consisting of a solid dissolved in water, but the solute and solvent could be solid, liquid, or gas. Examples of various solutions are shown in the table below.
|Types of Solutions|
|State of Solute||State of Solvent||State of Solution||Example|
|Liquid||Liquid||Liquid||Ethanol in water|
|Gas||Solid||Solid||() in palladium or platinum|
|Liquid||Solid||Solid||Dental amalgam ( dissolved in silver),|
|Solid||Solid||Solid||Alloys such as brass ( dissolved in )|
Example 2: Identifying the Physical State of the Solute and Solvent in Atmospheric Air
Fill in the blanks: Atmospheric air is a type of solution in which the physical state of the solute is and the physical state of the solvent is .
A solution is a homogeneous mixture that consists of one or more solutes dissolved in a solvent. The solvent is the major component of the solution, and the solute is the minor component of the solution. The solution in question is atmospheric air. Atmospheric air primarily consists of nitrogen, oxygen, and argon with an approximate percent composition of , , and respectively. This means that nitrogen is the solvent while oxygen and argon are solutes. Both the solvent and the solutes are in the gaseous state. Therefore, we should fill in both blanks in the question with the word gas.
Only a certain amount of solute can be dissolved into a solvent at a given temperature. Solutions that contain the maximum amount of solute that can be dissolved in the solvent at a particular temperature are called saturated solutions.
Definition: Saturated Solution
A saturated solution is one that contains the maximum amount of solute dissolved in a solvent at a given temperature.
If less than the maximum amount of solute is dissolved in the solvent at a particular temperature, the solution is considered to be unsaturated.
Definition: Unsaturated Solution
An unsaturated solution is one that contains less than the maximum amount of solute dissolved in a solvent at a given temperature.
It is possible to exceed the maximum amount of solute that a solvent should be able to contain. If a saturated solution is heated, the solvent may be able to dissolve more solute. Once the additional solute has been dissolved, the solution is slowly cooled back to the original temperature. The resulting cooled solution contains more than the maximum amount of solute and is called a supersaturated solution.
Definition: Supersaturated Solution
A supersaturated solution is one that contains more than the maximum amount of solute than should be able to be dissolved in a solvent at a given temperature.
Supersaturated solutions are unstable. Slight vibrations or dust particles may cause the excess solute to crystallize and precipitate out of the solution. For example, a reusable hand warmer contains a supersaturated solution of sodium acetate. When the disk inside the pouch is agitated, the excess sodium acetate precipitates out of the solution as white crystals and heat is generated.
Example 3: Describing the Type of Solution Produced by Dissolving Sodium Acetate in Water
In an experiment, a student added sodium acetate to a fixed volume of water. Eventually, the student reached a point at which no more sodium acetate dissolved. The student then warmed the solution until no crystals of sodium acetate were visible. The solution was then allowed to cool slightly. The diagram below shows the student’s observations.
- Which of the following terms describes solution B?
- Which of the following terms describes solution D?
A solution is a homogeneous mixture that consists of one or more solutes dissolved in a solvent. The solvent is the major component of the solution and the solute is the minor component of the solution. In this example, sodium acetate is the solute and water is the solvent.
When making solutions, the solvent can only dissolve a certain amount of solute at a given temperature. When the maximum amount of solute has been dissolved in the solvent at the desired temperature, the solution is considered saturated. When less than the maximum amount of solute has been dissolved in the solvent, the solution is considered unsaturated.
At the beginning of the experiment, solution A, sodium acetate is added to a fixed volume of water. Initially, more sodium acetate was able to be dissolved. This means that the solution was unsaturated. Eventually, a point was reached where no more sodium acetate dissolved, solution B. The maximum amount of sodium acetate had been dissolved in the water. Solution B can be described as saturated, answer choice B.
When more than the maximum amount of solute is dissolved into the solvent at a given temperature, the solution is considered supersaturated. Increasing the temperature of a solvent often allows more solute to be dissolved. If the resulting solution is cooled slowly, the excess solute can remain in the solvent producing a supersaturated solution.
In the experiment, solution B was heated until the excess sodium acetate crystals had dissolved, solution C. Then, the solution was allowed to cool, solution D. Notice that there are no sodium acetate crystals present in solution D. This means that the excess sodium acetate is still dissolved in the solvent. Solution B can be described as supersaturated, answer choice E.
Some solutes, when dissolved in water, ionize or dissociate into ions. These solutes are called electrolytes because the resulting solution conducts electricity.
An electrolyte is a substance with freely moving ions that can conduct electricity when molten or dissolved in water.
Electrolytes may be strong or weak. Strong electrolytes are solutes that completely ionize or dissociate when dissolved in water. Examples include ionic sodium chloride () and polar covalent hydrochloric acid ():
The () ions do not remain free in solution and are gained by the water molecules, forming hydronium ions (). The chemical equation for the ionization of () above is often used to abbreviate the reaction, though the chemical equation below is more accurate:
Weak electrolytes are solutes that only partially dissociate when dissolved in water. An example would be acetic acid (). Notice that, in the equation below, an equilibrium arrow is used in place of the forward arrow to indicate that only some of the acetic acid molecules ionize:
Solutes that do not produce ions when dissolved in water are called nonelectrolytes.
A nonelectrolyte is a substance that does not have freely moving ions that can conduct electricity when molten or dissolved in water.
Sugar is an example of a nonelectrolyte. Sugar does dissolve in water to form a solution, but the sugar molecules do not ionize. Therefore, the resulting solution does not conduct electricity.
A conductivity tester is a simple device consisting of a light bulb, electrodes, and a power source. When the electrodes are placed in a solution containing an electrolyte, the electric current can flow through the solution to complete the circuit and the light bulb glows. The light bulb will be more brightly lit when the conductivity tester is placed in a solution containing a strong electrolyte and then placed in a solution containing a weak electrolyte. If the electrodes are placed in a solution containing a nonelectrolyte, the electric current will be unable to flow, and the light bulb will not produce light. The use of a conductivity tester in three different solutions is shown in the figure below.
Example 4: Identifying Which of Three Substances Is a Strong Electrolyte
Three different solutions are shown. Which solution contains a strong electrolyte?
An electrolyte is a substance that conducts electricity when dissolved in water. This is due to the production of ions when the solute dissociates or ionizes to form the solution. The process of dissolving glucose (), (), and () into water can be represented by the following equations:
While glucose does dissolve in water, it does not ionize to produce ions in the solution. Therefore, we can eliminate beaker A.
Ammonium hydroxide (), and sodium hydroxide () are both electrolytes. Electrolytes may be considered strong or weak. Strong electrolytes completely dissociate when dissolved, while weak electrolytes only partially dissociate when dissolved. The difference between the two types of electrolytes can be seen in the reaction equations as an equilibrium arrow will be used when a weak electrolyte dissolves, while a forward arrow will be used when a strong electrolyte dissolves.
We can see from the reaction equations that () is a weak electrolyte, while () is a strong electrolyte. The solution that contains a strong electrolyte is beaker C.
When two or more substances combine to form a mixture that has a nonuniform composition, the mixture can be classified as heterogeneous. The prefix “hetero-” means different and indicates that the composition and properties of the mixture are different throughout.
Definition: Heterogeneous Mixture
A heterogeneous mixture is one with a nonuniform composition throughout.
Sulfur mixed with iron filings is an example of a heterogeneous mixture. The sulfur and iron filings remain separate, and the mixture does not appear to be the same throughout.
Oil mixed with water is another example of a heterogeneous mixture.
Often, with heterogeneous mixtures, the various substances in the mixture are visible with the naked eye; however, this is not true of all heterogeneous mixtures. Colloids are heterogeneous mixtures whose composition appears the same throughout when examined with the naked eye. As such, they are frequently mistaken for homogeneous mixtures. However, when examined with a magnifying glass or under a microscope, it is apparent that colloids do not have a uniform composition. Examples of colloids include fog, meringue, and milk.
A colloid is a type of heterogeneous mixture in which particles are dispersed throughout a medium, resulting in a mixture that appears to have a homogeneous composition when examined with the naked eye.
Suspensions are a type of heterogeneous mixture that consist of very large particles suspended in a medium that, given time, will settle to the bottom of the container. Examples of suspensions include flour, chalk powder, and dirt stirred into water.
A suspension is a type of heterogeneous mixture in which large particles are suspended in a medium. The large particles will settle to the bottom of the container over time.
The figure below compares solutions, colloids, and suspensions.
Example 5: Identifying the Type of Mixture Produced When Sugar Is Added to Water
Which of the following is correct when we add sugar to water?
- They form a heterogeneous mixture and are called a colloid.
- They form a homogeneous mixture and are called a colloid.
- They form a heterogeneous mixture and are called a solution.
- They form a heterogeneous mixture and are called a suspension.
- They form a homogeneous mixture and are called a solution.
A suspension is a heterogeneous mixture consisting of large particles suspended in a medium. The particles do not dissolve in the medium and, given time, will settle to the bottom of the container. A colloid is a heterogeneous mixture consisting of small particles dispersed in a medium. The particles do not dissolve in the medium and will not settle to the bottom of the container. Suspensions and colloids appear cloudy.
A solution is a homogeneous mixture consisting of particles dissolved in a medium. The particles will not settle to the bottom of the container and they cannot be separated by filtration. Solutions may be colored but are always clear.
Based on the definitions of suspension, colloid, and solution, we can eliminate answer choices B and C. When sugar is mixed with water, the sugar dissolves in the water, producing a clear mixture. This mixture is a solution. The correct answer is answer choice E.
The diagram below summarizes the terms related to mixtures.
- A mixture is composed of two or more substances that are physically blended.
- Mixtures may be classified as homogeneous or heterogeneous.
- Homogeneous mixtures are also called solutions and are composed of a solute and a solvent.
- The amount of solute present in a solution can be expressed by the terms unsaturated, saturated, and supersaturated.
- Solutes that produce ions when dissolved are called electrolytes.
- Colloids look like solutions but consist of larger particles dispersed in a dispersing medium.
- Suspensions consist of very large particles in a medium that will settle over time.