A Fizzy Lava Lamp

​This bubbly experiment combines principles of chemistry, density and buoyancy to create a colorful Lava Lamp look-alike that may be even more fun than the real thing!

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What you'll need

• Tall plastic beverage cup or jar
• Vegetable oil
• Food colors
• Alka-Seltzer tablets (or equivalent)

experimental procedure

1. Fill your cup about 3/4 full with vegetable oil.
2. Slowly add water to the cup and watch the droplets sink through the oil until there is about a 1" layer of water in the bottom of the cup.
3. Add a couple of drops of food coloring to the cup.  They should sink through the oil and burst once they reach the water layer.
4. Break an Alka-Seltzer tablet into 4 or 5 smaller pieces.
5. Drop one of your Alka-Seltzer pieces into the cup and enjoy the show!  When the bubbles stop, drop another piece.

what's happening

 You may have heard someone say that water and oil don't mix.  The molecules (the arrangement of atoms that make up these chemicals) that make up water are very different from molecules that make chemicals like vegetable oil.  Water molecules (or H2O) attract other water molecules and oil molecules can (weakly) attract other oil molecules, but water molecules strongly repel oil molecules, kind of like the way magnets repel each other if you point the north pole of one towards the south pole of the other.  Thus when you add oil to water they will separate into different layers, and the oil layer will float to the surface because oil is less dense than water.  This is the scientific term which means that a cup of water has more mass (and is therefore heavier) than the same cup filled with oil.  The force that causes the oil to float above the water layer is called the buoyant force or just buoyancy.  Since the food coloring is dissolved in water, these drops also will not mix with oil and sink to the bottom of your cup where they burst and mix with the water to color it.   

The solid Alka-Seltzer pieces are more dense than the oil or the water, so they sink all the way to the bottom.  As soon as they reach the water they begin to dissolve and a chemical reaction takes place between two of the ingredients that produces carbon dioxide gas bubbles.  As these bubbles rise they pull some of the water along with them and have enough buoyancy to rise through the oil layer, but once they reach the surface the bubbles pop and the gas escapes.  Now the water droplets are too heavy to float, and they fall back through the oil where they may pick up another gas bubble to rise again.

This process resembles the rising and falling bubbles in a Lava Lamp, but it works a little differently.  A real Lava Lamp also contains two immiscible liquids (i.e. they don't mix) with different densities, but it uses the energy from a light bulb underneath to warm the lower liquid.  As it warms it expands and its density decreases, allowing bubbles  to rise up through the upper liquid.  As these bubbles float higher (and farther away from the heat source) they cool down, become denser once again, and fall back into the lower layer where the process can repeat.  The upper liquid also warms during this process, but it was carefully chosen so that its density does not decrease as much, so that it still provides enough buoyant force to push the bubbles from the lower liquid upwards.  See the reference link below for a more detailed explanation.

variations and related activities

• Instead of a tall cup, you can use a soda bottle for this experiment, then simply screw on the cap to store it between uses.
• There is another different version of this Lava Lamp which uses salt instead of Alka-Seltzer tablets (see reference link below).  For this variation you first fill your tall glass about 3/4 full of water, then add enough oil to form a layer on top of the water about 1" thick.  Now add a couple spoonfuls of salt which sinks to the bottom of the glass (because it's much more dense than the oil or water), but as it does it drags some of the oil with it.  Now watch as the salt begins to dissolve in the water, releasing the attached oil to float to the surface.  This version is not as colorful (unless you like the yellowish color of the oil), but you can jazz it up it if you have oil-soluble food colors.
• Another really cool buoyancy experiment is the Dancing Raisins (see reference link below)

references and links to more information

Alternative Lava Lamp experiments:

• https://sciencebob.com/blobs-in-a-bottle-2/   (Lava Lamp bottle)
• https://youtu.be/XGgC-S9trD4   (uses salt instead of Alka-Seltzer)

How real Lava Lamps work:

• https://home.howstuffworks.com/lava-lamp.htm

​More about the differences between and oil and water molecules:

• https://www.thoughtco.com/why-oil-and-water-dont-mix-609193
• https://www.thoughtco.com/examples-of-polar-and-nonpolar-molecules-608516
• https://www.scientificamerican.com/article/mix-it-up-with-oil-and-water/

Try some cool density column experiments:

• https://www.stevespanglerscience.com/lab/experiments/density-tower-magic-with-science/
• https://www.stevespanglerscience.com/lab/experiments/oil-and-water/

More about density and buoyancy:

• https://gosciencegirls.com/density-science-for-kids/
• https://mocomi.com/buoyancy/
• https://sciencing.com/teach-buoyancy-grade-school-children-8159938.html
• https://www.khanacademy.org/science/physics/fluids/buoyant-force-and-archimedes-principle/a/buoyant-force-and-archimedes-principle-article

More cool buoyancy experiments:

• https://funlearningforkids.com/dancing-raisins-science-experiment-kids/
• ​https://www.coolscience.org/cool-physics/cartesian-divers  (our Cartesian Divers experiment)

Subject Tags

• Bubbles
  
• Density
• ​Kitchen Science
• Oils
• ​Water

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