Monday, October 26, 2015

13 Days of Halloween: Leaf Chromatography

Don't forget to add your resources to the 13 Days of Halloween Linky Party.  There was a technical issue, but it is all fixed. 

Day 8 Leaf Chromatography

When we think of Halloween we think of leaves falling.  This is a great time to explore why leaves are so colorful in the fall.
Be sure to check out the student pages included.

What is Chromatography?

Chromatography basically means “to write with colors.”  It is the process of separating a mixture by passing it through a medium in which the different components of the mixture move at different rates.  Russian botanist M.S. Tsewtt discovered paper chromatography in 1903.  He was able to separate the pigments in plants.  Paper chromatography is now used to determine the purity and make up of many compounds.  In paper chromatography the mixture is dissolved in a liquid and the medium it will travel through is the filter paper.  How well the pigments separate is dependent on how well they travel up the paper and how soluble they are in the solvent.   

Using Leaf Chromatography to Discover the Colors in Leaves

Plants need sunlight to perform the process of photosynthesis.  The leaves contain brightly colored pigments that are essential in light absorption.   The pigment chlorophyll is the most important in light absorption.  Chlorophyll A is bluish-green and chlorophyll B is yellowish-green.  Other pigments are xanthophyll and carotenoids.  Xanthophyll is yellow and carotenoids are orange.  In peak growing season plants contain mostly chlorophyll.  This is why most plants appear green.  The other pigments are still there, but they are masked by the chlorophyll.  Using Chromatography we can separate those colors.
The pigments in leaves are not very soluble in water.  Solubility
is how well a substance can dissolve in a solvent.  The pigments in leaves are very soluble in acetone which is found in nail polish remover.  They will also dissolve in rubbing alcohol.  When the filter paper is placed into the nail polish remover the liquid begins to travel up the paper.  When the nail polish remover reaches the green plant line the pigments begin to dissolve into the liquid.  The least soluble pigments appear first on the filter paper.  They are the heavier pigments.  The more soluble the pigment the further it will travel on the paper.  The pigment that travels the furthest is the lightest of all the pigments.  The colors you see tell you what pigments are most present in that leaf.  Spinach contains lots of green and yellow.  Other leaves can be tested too.  I like spinach because it is a thick leaf with lots of moisture.  It will make a nice line on the filter paper. 
When fall approaches the leaves start to break down the chlorophyll.  This reveals the other pigments in the leaves.  The pigments were always there, but hidden by the amount of chlorophyll.
Here are some student pages for the activity. 

Remember to always follow good lab practices when working with chemicals.  Students should wear goggles.  The room should be well ventilated.

More Chromatography Fun

There are many types of chromatography you can do.  Marker chromatography is great.  I also like to explore water traveling up paper in teaching about vascular and nonvascular plants.
 Be sure to come back for Day 9 tomorrow.



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