(POS) Mixtures Lesson
Mixtures
We see mixtures all the time in our everyday lives. A stew, for example, is a mixture of different foods such as meat and vegetables; sea water is a mixture of water, salt, and other substances, and air is a mixture of gases such as carbon dioxide, oxygen, and nitrogen. A mixture is a combination of two or more substances, where these substances are not bonded (or joined) to each other and no chemical reaction occurs between the substances. Instead, the components of the mixture just intermingle and keep their original properties.
In a mixture, the substances that make up the mixture:
- Are not in a fixed ratio
Imagine, for example, that you have 250 ml of water and you add sand to the water. It doesn't matter whether you add 20 g, 40 g, 100 g, or any other mass of sand to the water; it will still be called a mixture of sand and water.
- Keep their physical properties
In the example we used of sand and water, neither of these substances changed in any way when they are mixed together. The sand is still sand and the water is still water.
- Can be separated by mechanical means
To separate something by "mechanical means", means that there is no chemical process involved. In our sand and water example, it is possible to separate the mixture by simply pouring the water through a filter. Something physical is done to the mixture, rather than something chemical.
We can group mixtures further by dividing them into those that are heterogeneous and those that are homogeneous.
A heterogeneous mixture consists of two or more substances that is non-uniform and the different components of the mixture can be seen. Cereal in milk is an example of a heterogeneous mixture. Soil is another example. Soil has pebbles, plant matter, and sand in it. Although you may add one substance to the other, they will stay separate in the mixture. We say that these heterogeneous mixtures are non-uniform, in other words, they are not exactly the same throughout.
A homogeneous mixture has the same composition throughout. In a homogeneous mixture, the different parts cannot be seen. A solution of salt dissolved in water is an example of a homogeneous mixture. When the salt dissolves, it spreads evenly through the water so that all parts of the solution are the same, and you can no longer see the salt as being separate from the water. Think also of the air we breathe as another example of a homogeneous mixture since it is made up of different gases which can't be visually distinguished from each other (i.e. you can't see the different components).
Types of Mixtures
Mixtures have different properties depending on the size of their particles. Three types of mixtures based on particle size are solutions, suspensions, and colloids, all of which are described in the table below.
Colloids
How do you determine if a substance is a colloid? One way is by the Tyndall effect. The Tyndall effect is the scattering of light by particles. This involves shining a light through the mixture: when the light cannot pass through the mixture, the mixture is considered a colloid. Milk is an example of a colloid. This is why milk appears "cloudy" - or what we think of as "milky." When light is passed through a true solution, the particles are so small that they do not obstruct the light. For example, the solution vinegar is clear and allows light to pass through easily. In comparison, when light is passed through a colloid, the larger particles act as an obstruction to the light, causing the light to scatter. The particles, however, are still small enough so that they do not settle out of the solution.
Suspensions
Suspensions are mixtures where the particles settle to the bottom of the container. This means that the particles in a suspension are large enough that gravity pulls them out of the solution. With suspensions, filtration can usually be used to separate the excess particles from the solution. A common example of a suspension is muddy water. If you had a beaker of water and stirred in a handful of fine dirt, when you let the mixture stand, dirt would settle to the bottom.
To review the information presented above, watch the video below. In the video, you will also see examples of each concept.
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