How do leaves change colour? Autumn experiment

Chromatography samples in colour order and the leaves the samples are taken from fanned below.
Discover the science of autumn with this great experiment. (Photo: Rachel Hoskins/WTML)

I don’t know about you, but seeing the leaves change colour is my favourite part of the year. It feels special every autumn! But what’s the science behind leaves changing?

How do leaves change colour?

Plants produce a green chemical pigment called chlorophyll. They use it to harness energy from sunshine and make food through a process called photosynthesis. This works best in spring and summer when leaves are at their greenest.

The bright green chlorophyll covers other chemical pigments in leaves. So, as the weather gets cooler and trees sense winter coming, they stop producing chlorophyll. As the green fades, other colours are revealed.

Now, let’s see for ourselves in an exciting experiment you can do at home!

Mix of autumn colours on the trees at Nidd Gorge
Head out to the woods to see how many colours you can spot. This vibrant mix is at Nidd Gorge. (Photo: Mark Sunderland/WTML)

Leaf chromatography experiment

You can use a method called chromatography which is a way of separating mixtures of chemicals, just like the chemical pigments you find in leaves.

You will need:

  • Leaves. Choose both green and different coloured leaves to compare results from the same tree.
  • A glass or jar for each sample and something to cover them with.
  • Sticky labels.
  • Rubbing alcohol (also known as surgical spirit). You’ll use this to extract the pigments from your leaves. Apparently, you can also use clear spirits like vodka!
  • A spoon to mash your leaves up.
  • Kitchen roll or coffee filter paper.
  • Twigs. Make sure you have a small one for each sample.
  • A deep dish like an oven dish, baking tray or plastic box.
  • A notepad and pencil to record your results.

Instructions:

1)  First, find out which trees your leaves come from. You can use our leaf ID sheet, or our free tree ID app. We chose birch, Norway maple, English oak and cherry plum.

Plum cherry, birch, oak and Norway maple leaves ripped up in glasses and in full below
You can use as many samples as you like. make sure to keep track of which leaf is in which jar. (Photo: Rachel Hoskins/WTML)

2)  Rip up your leaves into small pieces. Put each type in a different jar – eg green birch in one, and yellow birch in another. Remember to label them so you can tell them apart later on. Pour in enough rubbing alcohol to cover the leaves and use a spoon to mash them up a bit. Repeat this for each type of leaf.

3)  Pour some warm water into a deep dish and stand your jars in it. Cover them so the alcohol doesn’t evaporate (this can stop your experiment working). Leave your samples for at least half an hour. The more patient you are, the better your results. We checked in every 15 minutes to see how ours were doing.

Chromatography samples in a plastic box full of water with the full leaves below.
We put our samples in a plastic box and covered them with a tray. (Photo: Rachel Hoskins/WTML)

4)  When the liquid is brightly coloured, take out the leaves with a spoon and throw them away.

5)  Cut your kitchen roll or filter paper into long strips around 3 centimetres wide. Roll each strip around a twig, and place the twigs on top of your jars so the end of the paper is touching the liquid. (Make sure it doesn’t touch the side of your glass, as this could affect the result.)

Danielle putting strips of kitchen roll  into chromatography samples. There are 8 glasses with a small amount of brightly coloured green and yellow samples in them and a range of autumn leaves below.
Our samples were nice and bright. We noticed that some leaves gave much stronger colours than others. Make sure to note down what you see. (Photo: Rachel Hoskins/WTML)

 6)  The paper will start sucking up the liquid quickly, but you’ll want to leave your samples for at least 45 minutes to let the pigments separate. It was really exciting to check in on our samples and see how the colour was climbing and slowly separating.

 7)  When you can clearly see different colours on your strips, carefully remove them and see how many pigments you can spot in each leaf. You might notice that green leaves have yellow or red stripes at the top, these are the other pigments under all the chlorophyll!

Experiment conclusions

Chromatography test results. A green norway maple has green on the strip and a red norway maple has red, yellow and green on the strip.
We had some great results from our leaves. (Photo: Rachel Hoskins/WTML)

By using rubbing alcohol and energy from the warm water, we extracted the chemical pigments from leaves. The separated pigments on the kitchen roll show us that leaf colour is determined by the chemical pigments, the strongest being green chlorophyll. (The green on our kitchen roll faded quickly, but was clearly visible when we took the strips out of the solution.) In yellow and red leaves, we can see that most if not all chlorophyll has left the leaf.

How did your experiment go? Tag your pictures #NatureDetectives!

What happened in your experiment?

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