Classifying Mixtures



Classifying Mixtures

Introduction

It is easy to see that some materials are mixtures because you can easily see what they are composed of. For example, soil or dirt is a mixture of various substances, including small rocks and decomposed animal and plant matter. You can see this by picking up a handful of soil and look at it closely. Milk, on the other hand, does not appear to be a mixture, but it is. Milk is composed mainly of fats, proteins, milk sugar, and water. Both milk and soil are examples of heterogeneous mixtures because their composition is not uniform.

Salt (sodium chloride) and water form a homogeneous mixture. The sodium and chloride ions become distributed evenly throughout the water molecules, and the mixture appears uniform.

Mixtures can be separated into 3 categories: solutions, suspensions and colloids. A solution is a homogeneous mixture of two or more substances, a suspension is a heterogeneous mixture containing particles that will settle out of solution, and a colloid is a heterogeneous mixture that contains particles that do not settle out of solution. Many colloids appear homogeneous because the individual particles cannot be seen. However, the particles are large enough to scatter light. You may have noticed that a car headlight bright beam is far more noticeable in fog than on a clear night. This effect is called “The Tyndall Effect,” and it occurs when light is scattered by colloidal particles dispersed in a transparent medium. The Table below summarizes the differences between these 3 types of mixtures.

|Properties of Solutions, Colloids, and Suspensions |

|Solution |Colloid |Suspension |

|Homogeneous |Heterogeneous |Heterogeneous |

| |(Looks Homogeneous) | |

|Particle: atoms, ions, molecules |Particles: large molecules |Particles: large particles or aggregates |

|Does not separate |Does not separate |Separates with standing |

|Cannot be separated with filtration |Cannot be separated with filtration |Can be separated with filtration |

|Does not scatter light |Scatters light (Tyndall effect) |May scatter light, but it is not |

| | |transparent |

In this laboratory exercise you will make various kinds of mixtures in water and determine whether each mixture can be classified as a solution, a suspension or a colloid.

Key Question

How is matter classified?

Pre-Lab Vocabulary

Define each word below using this handout, a chemistry book in the lab or a dictionary.

Mixture: _______________________________________________________________

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Heterogeneous: _________________________________________________________

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Homogeneous: __________________________________________________________

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Solution: _______________________________________________________________

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Suspension: _____________________________________________________________

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Colloid: ________________________________________________________________

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Tyndall Effect: __________________________________________________________

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Transparent: ____________________________________________________________

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Materials

8 large test tubes

250 mL beaker

glass stir rod

test tube rack

flashlight

marker to write on glass

cooking oil

gelatin

red food coloring

borax detergent

liquid starch

sugar

milk

dirt

Procedure

1. Using the marker, label your test tubes with the following labels: milk, gelatin, sugar,

dirt, starch, oil, food coloring, and borax.

2. Put approximately 100 mL of water in your 250 mL beaker, and place on your hot

plate to bring to a boil.

3. Use the Table below to make each water mixture to study: Once each mixture is

assembled, make sure it is stirred and mixed thoroughly.

|Mixture |mL of Water |Solute to Add to Water |Other additions |

|1. milk |5.0 |5.0 mL of milk | |

|2. gelatin |2.5 |0.12 g gelatin |Add 7.5 mL of hot water |

|3. sugar |10.0 |0.48 g sugar | |

|4. dirt |10.0 |0.20 g dirt | |

|5. starch |5.0 |5.0 mL of liquid starch | |

|6. oil |8.0 |2.0 mL of cooking oil | |

|7. food coloring |10.0 |2 drops food coloring | |

|8. borax |10.0 |0.08 g borax detergent | |

4. Look at the milk, dirt, cooking oil, gelatin and starch mixtures under a microscope

and draw a picture of what you see in the microscope in your Data Table.

5. Use a flashlight and shine it through each mixture in a dark box or darkened room. A

solution will not scatter light, a colloid will scatter light (you will be able to see the

flashlight beam travel through the mixture) and a suspension may scatter light, but the

light beam will not be transparent. Record your observations for each mixture in the

Data Table.

6. Using your observations and the properties of each type of mixture summarized in the

Table on page 1 of this handout, classify each mixture as a solution, colloid or

suspension in your Data Table.

Question

1. How did you decide if a mixture was a colloid or a suspension?

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|Data Table |

| |Homogeneous or Heterogeneous? |Does it scatter light? |Solution, Suspension or Colloid? |

|Mixture | | | |

|1. milk | | | |

| | | | |

| | | | |

|2. gelatin | | | |

| | | | |

| | | | |

|3. sugar | | | |

| | | | |

| | | | |

|4. dirt | | | |

| | | | |

| | | | |

|5. starch | | | |

| | | | |

| | | | |

|6. oil | | | |

| | | | |

| | | | |

|7. food coloring | | | |

| | | | |

| | | | |

|8. borax | | | |

| | | | |

| | | | |

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