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Blue foam spilling out of a lemon.Kick off a colourful explosion in your kitchen with simple ingredients!
Safety: sharp hazard iconYou’ll need to use a sharp knife in this activity. Ask an adult to help.

You will need

  • Lemons
  • Baking soda
  • Food colouring (optional)
  • Knife and cutting board
  • Spoon
  • Baking tray

What to do

  1. Cutting a lemon with a knife.Cut off the bottom of the lemon so that the lemon stands upright on the tray. Ask an adult for help with this step.
  2. Scoop out the inside of the lemonCreate the volcano cone by cutting off the top of the lemon. Ask an adult for help with this step.
  3. Break up the inside of the lemon with a spoon.Use the spoon to break up the inside of the lemon.
  4. Add food colouring to the inside of the lemon.Add some food colouring of your choosing to the lemon.
  5. Adding baking soda into the lemon.Pour baking soda, about a tablespoon at a time, into the lemon.
  6. Stir up the baking soda to keep the reaction going.Keep the reaction going by using your spoon to stir up the baking soda and to break up the lemon.
What’s happening?

This bubbly explosion is powered by combining an acid with a base. An acid is a substance that gives off hydrogen ions (H+), which we taste as sour. A base is a substance that takes up H+ ions and does something else with them – bases often taste bitter. When we combine an acid with a base, it triggers an acid-base reaction where both substances change as they exchange H+ ions.

In this activity, you added basic baking soda to the acidic lemon, which kicks off an acid-base reaction. In chemical terms, the lemon juice supplied H+ ions which were taken up by the baking soda (also called sodium bicarbonate). This chemical reaction caused the baking soda to break down, releasing its carbon atoms as carbon dioxide gas. We can write this as a chemical equation:

H+ + NaHCO3 = Na+ + H2O + CO2

The rapid formation of carbon dioxide gas causes the lemon juice and food colouring mixture to foam and bubble out of the top of the lemon. Breaking up the lemon with your spoon releases more lemon juice, which then reacts with the baking soda.

This bubbly reaction is very similar to the acid-base reaction that happens when cooking with baking soda. Check out the sense-sational baking soda scones (https://blog.doublehelix.csiro.au/sense-sational-baking-soda-scones/) experiment to witness the way a similar acid-base reaction causes scones to rise.

A lava-ly note about volcanoes

Your lemon volcano’s “lava” is generated by a chemical reaction, but Earth’s volcanoes spurt out lava because of pressure underneath the surface. Here’s how that works: the Earth’s crust is made of tectonic plates, which move and shift independently of one another. When one of these plates gets trapped under another plate, it melts the nearby rocks into molten magma. Magma is lighter than the surrounding rocks and so it rises and collects in chambers underneath the Earth’s surface. When enough magma collects in one place, the pressure causes the magma to erupt out of volcanos. We call the magma that reaches Earth’s surface this way, lava.

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