Classification of Matter
How Are Different Types of Mixtures Distinguished from Each Other?
Homogeneous Mixtures
Mixtures may be homogeneousglossary term (opens in a new window) or
heterogeneousglossary term (opens in a new window). In a homogeneous
mixtureglossary term (opens in a new window), the components of the mixture are uniform throughout. Any part of the mixture will be identical in composition to any other part of the mixture. The mixture is homogeneous because the components of the mixture are distributed evenly.
A
solutionglossary term (opens in a new window) is a homogeneous mixture composed of two or more substances, but only exhibiting one
phase of matterglossary term (opens in a new window). Solutions may be in the gaseous,
liquidglossary term (opens in a new window), or
solidglossary term (opens in a new window) states of matter. Air is a gaseous solution. Seawater is a liquid solution. Steel is a solid solution.
Different combinations are possible from these three states of matter. A solution can consist of a
gasglossary term (opens in a new window) dissolved in another
gasglossary term (opens in a new window), in a liquid, or in a solid. A solution also may consist of a liquid dissolved in a gas, in another liquid, or in a solid. Finally, a solution may consist of a solid dissolved in a gas, in a liquid, or in another solid. The substance that is dissolved is called the soluteglossary term (opens in a new window). The substance that does the dissolving is called the solventglossary term (opens in a new window). In a glass of salt water, the solute is salt. The solvent is water.
An
alloyglossary term (opens in a new window) is a solid solution composed of two or more elements. Steel is a solid solution made up of iron and other substances, such as carbon, nickel, and chromium. Bronze is also a solid solution. It is an alloy made of copper mixed with tin. Brass is an alloy of copper and zinc. The dime, the U.S. 10-cent coin, is made of an alloy of copper and nickel.
Teacher Note: Misconception
Students may confuse the terms “dissolving” and “melting.” In reality, melting involves a change in state from solid to liquid, while dissolving involves the uniform dispersion of one substance in another.
Heterogeneous Mixtures
In a heterogeneous mixture, the components of the mixture are not uniform throughout the mixture. That is, any part of the mixture will not be identical in composition to any other part of the mixture. The mixture is heterogeneous because the components of the mixture are distributed unevenly.
Soil is a solid heterogeneous mixture. It is made up of fine and large particles of decayed plant and animal parts, clay, sand, pebbles, and rocks. A sample of soil in one place may not resemble a sample of soil in another place. For example, beach soil is much different in composition from forest soil.
Salad dressings are also liquid heterogeneous mixtures. They may consist of vinegar, oils, and solid materials, such as spices. One sample of salad dressing will be different from another sample of the same dressing. This is because the materials in the dressing are not uniformly distributed.
Oil and water do not easily dissolve in one another. So, they do not form a homogeneous mixture. If such a mixture is shaken vigorously, it will look as if the oil and water are permanently distributed uniformly. But if this mixture is allowed to stand for a while, the oil will separate and rise to the top of the mixture. This occurs because oil is less dense than water and will float on top of the water. The parts of this mixture are temporarily suspended, or “hanging in place.” A mixture in which the parts are temporarily suspended is called a
suspensionglossary term (opens in a new window).
Soil mixed with water is another example of a suspension. If this mixture is placed in a beaker, it will look like muddy water at first. But as time passes, soil particles will settle out at the bottom of the beaker, and less murky water will be found at the top.
If the particles in a liquid mixture are very small, they may not spontaneously separate. They will remain suspended. But they do not form a solution. Such a mixture can be thought of as being somewhere between a solution and a suspension. Mixtures like these are called colloids. If a
colloidglossary term (opens in a new window) consists of a liquid dispersed in another liquid, the mixture is called an
emulsionglossary term (opens in a new window).
One difference between solutions, colloids, and suspensions is the size of the solid particles in the mixture.
Particle size is one characteristic of different types of mixtures. What other properties are used to classify mixtures?
Note: The symbol nm stands for nanometer. One nanometer equals one billionth of a meter.
How can a person determine whether a mixture is a colloid or a suspension? There are a few methods. One method is to separate the parts of a suspension by passing the mixture through ordinary filter paper, such as a coffee filter. The parts of a solution or colloid will pass right through ordinary filter paper.
Another way to identify a colloid is based on particle size. Many colloids appear homogeneous because the particles are too small to be seen without a microscope. However, when a beam of light passes through a colloid, it is scattered by the colloidal particles and the beam’s path can be seen clearly in the colloid. You may have seen a car’s headlight beam in the fog at night. This is known as the Tyndall effect. When a light beam shines through a colloidal liquid, the beam’s path can be seen clearly in the liquid. Colloids and suspensions can exhibit the Tyndall effect. Since solutions do not show the Tyndall effect, it is helpful to determine if a mixture is a suspension, colloid, or solution.
Examples of colloids include whipped cream (gas in a liquid), mayonnaise (liquid in a liquid), smoke (solid in a gas), butter (liquid in a solid), paint (solid in a liquid), and fog (liquid in a gas).
Teacher Note
Students should complete the writing prompt as they work through the Exploration “Classification of Matter.” The conclusion portion of the writing prompt can be used as a summative assessment. Extend this writing activity by having students construct sketches accompanied by written labels and descriptions that classify a variety of everyday objects or substances found somewhere other than in a kitchen.
Teacher Note: Practices
This item requires students to use multiple types of models at the molecular level to provide mechanistic accounts of the type of matter each model represents. As such, this item provides a summative assessment to test student understanding of the ways that chemists classify matter. Extend this item by having students make their own sketches of matter at the molecular level as examples of each type of matter along with written statements explaining why they represent the type of matter that they have been labeled as representing.
