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Pink noodles in a bowl.

The noodles change colour as you add the lemon juice!

Want to bring a little magic to your plate? Have some fun with these colour-changing “unicorn noodles”, perfect for a birthday party!

Safety: hot hazard iconfood safety hazard iconUse clean hands and clean equipment while cooking. This activity uses a stove and boiling water. Ask an adult to help.

You will need

  • One quarter of a red cabbage
  • Pot
  • Water
  • Tongs
  • Packet of rice noodles/glass noodles
  • Strainer
  • Lemon/lemon juice
  • Other ingredients to taste/serve with (e.g. soy sauce, vegetables)

What to do

  1. Purple cabbage in a saucepan of water.Put the cabbage into the pot and cover with water.
  2. Boil until water is fully dark blue/purple. This should take about 10-20 minutes.
  3. Purple liquid in a saucepan.Ask an adult to take out the cabbage using tongs.
  4. Boil your noodles in the purple water according to the instructions on the packet.
  5. Ask an adult to strain the noodles and then serve them in a bowl.
  6. Squeeze/drip a small amount of lemon juice on the noodles and watch them change colour!
  7. Pink and purple noodles in a bowl.Add your other ingredients and serve.
  8. You can keep a small amount of noodles to the side to experiment with. Try putting baking soda on them – it won’t taste good, but it will look cool!

 

What’s happening?

How cool are these colour changing noodles! There’s some really interesting chemistry going on, and it’s all about acids and bases.

The purple colour in the cabbage comes from a collection of molecules called anthocyanins. They’re a class of pigments (colour chemicals) found in lots of flowers, fruits and vegetables. And importantly, they can also tell you how acidic something is.

Anthocyanins are pH indicators, which means they change colour depending on how acidic they are. pH stands for “potential of Hydrogen”, and is a scale used to indicate how acidic or basic a liquid is. The lower the pH, the more acidic it is. Pure water is 7, lemon juice is about 2.5 and extremely concentrated hydrochloric acid can reach about -1. On the other end of the scale, bicarb soda is about 8.4 and caustic soda can reach 14 or higher.

Red cabbage indicator is quite useful because it changes colour gradually over a large pH range. Slowly adding lemon juice makes the noodles more and more acidic, so they look redder. Adding baking soda will do the opposite, making them more basic and turning them blue-green.

Since anthocyanins are so common, you can learn quite a lot about soils by looking at the colour of fruits, vegetables and flowers growing in them. Hydrangea flowers are particularly sensitive to soil pH. Some gardeners add acids and bases to the soil to get just the right colours in their hydrangea!

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