How does capillary action make chromatography possible?
Imagine for a moment a tree in your yard or a local park. How does that tree get the nutrients it needs to live and grow? It is true that it obtains energy from sunlight, but it also acquires water through its root system. Does this occur due to absorption, or is there a more specific way to describe what is happening? How does the water go up into the tree, moving against gravity? Why doesn’t the water more or less drain back down and out of the tree? Trees use capillary action to move fluids around their systems. With this activity you will explore how capillary action makes chromatography possible, why it is such an important scientific process, and a few of the ways it can be utilised by chemists and engineers for scientific exploration.
You’re probably familiar with capillaries as they pertain to your body, but by definition they are any tube that has an internal diameter of hair-like thinness. Capillaries are a fundamental structure for life on earth as things like circulation and growth depend on that method of movement. Capillary action is when liquids can flow in a narrow space, even against gravity. If you have ever dipped a paintbrush in paint or water, the paint fills the paintbrush because of capillary action. Since gravity is so crucial for our planet to function, there must be a way for things to run counter to that force so that life can still thrive.
In our experiment today, you’ll learn that chromatography is the separation of a mixture (our marker ink) by passing it in a solution (water) through a medium (coffee filter) in which the components of the mixture (the various colours that make up any given marker colour) move at different rates. Capillary action is important to the process of chromatography because it allows a liquid to move through a medium. The first controlled chromatography tests were very similar to the experiment we will do today- dye makers used chromatography to test the mixtures of dye they were using with paper or string. In 1901, chemist Mikhail S. Tsvet used chromatography to isolate chlorophyll in plants. Today, we will use chromatography to learn what marker ink is made of.
You should see the water move up the coffee filter and the different colours in the ink start to separate. There are two things working together to separate the colours from each other in the ink. First, the water and ink are flowing up the filter because of capillary action. The fibres in the coffee filter are packed close together so the water can flow up them just like the bristles on a paintbrush.
Second, the different densities of the dyes allow them to separate into different bands along the filter. As we learned earlier, chromatography is the separation of a mixture (our marker ink) by passing it in a solution (water) through a medium (coffee filter) in which the components of the mixture (the various colours that make up any given marker colour) move at different rates.
In our experiment, as the water travels up the coffee filter, it carries the ink particles with it. Because the ink particles are a mix of lots of different colours, and the different colours have different densities, some are carried further than others, so you can see the colours separate. Some colours are actually mixtures of lots of different inks, while some are just one or two inks mixed together. See if you can discover which colours make the best patterns.
Remember to clean up when you are done. Pour the water down the sink and wash the container. If you want to keep the coffee filters and use them to make an art project, or a decoration, you can certainly do so, or you can throw them away when you are done.
How often were you able to correctly guess what colors would show up based on the color of the marker? Were any marker colors more impressive than others? Why might they be different? Do you think that the same marker color but from a different brand of marker would give the same result, or a different one?
Sometimes scientists need to separate things which are combined together, like you did with the inks in these markers. Can you think of or look up any times when it might be useful for them to use this same kind of method? Try to find or think of examples where a scientist might need to separate things that are combined, but where this method wouldn’t work. Why wouldn’t it work for those?
This experiment was selected for Science at Home because it teaches NGSS Disciplinary Core Ideas, which have broad importance within or across multiple science or engineering disciplines.
Learn more about how this experiment is based in NGSS Disciplinary Core Ideas.