Rainbow Separation

Introduction:

In the real world, it is often difficult to tell whether something is made up of just one pure material (substance) or whether it is a mixture of different materials.  Many materials that appear to be pure  are actually mixtures made up of two or more different substances.  For example which of the following do you think are pure materials and which one do you think are mixtures?

Water (pure)      Milk (mixture)      Gold (pure)     Sugar (pure)  Air (mixture)     Blood (mixture)

Chromatography is one method that scientists use to separate mixtures.  Some mixtures of things like paint and ink contain two or more materials that each have their own color.  While these colors (dyes) are often hidden when the mixture is formed, they can be isolated based on the fact that each material in the mixture has its own unique properties.

Materials:

Filter paper or coffee filters         A variety of water soluble pens (including black)

Plastic cups              Scissors             Water

Exploration:      Student Activity Sheet

 In this activity, your job is to determine if different types of black ink are made up of a mixture of materials or just one pure substance. 

Procedure:

1) Set up two plastic cups on a level surface. Fill one cup with water to about1/2 inch from the top. 

2) Place one of the filter papers on a clean dry surface and locate the middle of the paper and poke a small hole in the paper. Using a black pen, make a series of dots around the hole as shown below in the first diagram.  Using a second piece of filter paper and a different black pen, make a series of similar dots.  

Start                                  Finish 

3)   To form the wick, fold a quarter piece of filter paper in half and then fold the filter paper in half again.  Fold it a couple of more times until it forms the “wick” that will be used for the experiment. Repeat this procedure to make the second wick.

4) Insert the wicks into hole next to the pattern of dots. Place the wick and the filter into the cup of water so that the wick extends into the water and the filter rests on top of the cup. Make sure that the rim of the cup is completely dry before completing this step. 

 5) Observe the ink dots as the water absorbs up the wick and across the filters. Record your observations on the data sheet. Pull it out of the water just before the water front reaches the edge of the filter.  

6) If time permits, create your own pattern of dots using multiple markers of different colors. Try to make the most colorful and creative pattern possible. After you're finished display the patterns in your room.  

Guiding Questions:  

1. According to your results, was the ink in your black markers made up of a mixture or was it a pure substance?

2. What do you think it was that caused the colors to separate from each other?

3. Do you think that most of the things on our planet our pure substances or mixtures? Explain your thinking. 

4. Name two things that we didn't discuss that you think are pure substances and two things that you think are mixtures.  

Explanation: If possible, have each group write their data on the board. Allow each group of students to explain and compare their results.  Then use their ideas to broaden their understanding of this topic. 

The key concept of this activity is that most materials are made up of mixtures and mixtures can be separated into their individual parts. Although they appear as one pure material, most black inks are comprised of a mixture of dyes -- sometimes as many as eight or nine.  For younger students, this is a sufficient objective for this activity. Other mixtures include butter, cement, dirt, and Coke. Other pure substances include iron, oxygen, and carbon dioxide. 

For older students, you can discuss their ideas in regards to why the mixture of colors separated as they moved through the filter.  Encourage multiple ideas. As the water is absorbed outward on the filter and flows past the black spots, some of these dyes are more attracted to the filter paper than the water. The ones that attract more strongly to the paper stay more or less where they are spotted, while the ones that attract more strongly  to the water move out faster. In a way, chromatography can be thought of as a race where the faster pigments separate from the slower ones.  If the dots don't move at all, then the ink may be insoluble in water and then you cannot determine whether it is a mixture or pure substance. 

Extension and Integration:  Have students brainstorm about when and why it might be important to separate the different parts of a mixture.  For example, what are the different parts of the blood and what good does it do to separate these parts? In addition, environmental scientists are often involved in trying to remove harmful pollutants that have become mixed into our natural surroundings. This lesson can easily be integrated as an art project where students attempt to create various patterns and designs. 

 1. According to your results, are the inks from  the black                          Assessment and Closure: Each group of students should turn in lab write-up and chromatography filter for this lesson. The teacher should have students briefly summarize the key concepts of this lesson.