One of my goals is to create simple science projects and demos that parents, caregivers, and teachers can easily perform using everyday supplies. I love this color changing lava lamp because it does exactly that. It illustrates so many concepts of chemistry, has ingredients you might already have (or can easily grab from the grocery store), and it is quite frankly AWESOME. So pour yourself some red cabbage juice, oil, and Alka-Seltzer and watch the science happen!
Alka-Seltzer lava lamps are all over the kid-activity-internets nowadays. You take oil and colored water, drop in an Alka-Seltzer tablet, and watch as the colored water droplets are propelled into the oil, dance around, then settle back down to the water layer, just like the old lamp-heated lava lamps. We took it a step further and used red cabbage juice instead of water and dye, which allows for a mesmerizing color change.
Red cabbage has a high concentration of anthocyanins, a group of molecules which happen to change color at different pH values. Anthocyanins are also present in many other plant products like red wine, blueberries, and red autumn leaves.
When you introduce an acid or base to an anthocyanin, a reaction occurs where you are slightly changing the molecule by taking or giving a “proton,” also known as a hydrogen ion. (Acids can give protons and bases take protons). Many molecules can donate or accept protons, and some molecules, like anthocyanin, change color when this occurs (most do not). We call these “indicators” because they indicate or let us know a pH change happened by changing color. You might remember this from high school or college chemistry when you were titrating a base with clear/pink phenolphthalein as the indicator.
Anthocyanins are incredibly amazing indicators in that they can take on a wide range of colors depending on the pH because they have several sites on the molecule that will cause a color change when a proton is present or missing. Here is an info graphic from the amazing site Compound Interest (www.compoundchem.com). Source
Alka-Seltzer is a solid tablet of aspirin, sodium bicarbonate, and citric acid. When a tablet is dropped into water, the sodium bicarbonate (AKA baking soda) and citric acid dissolve in the water then react together to form carbon dioxide, among other products. The carbon dioxide bubbles out of the glass, creating the iconic fizz. However, not all of the carbon dioxide actually bubbles out, some of it becomes dissolved in the water. There, it reacts with water to form carbonic acid, which lowers the pH (increases the acidity) of the solution.
In the color changing lava lamp, the red cabbage juice is acting as an indicator during the carbon dioxide dissolution and reaction to form carbonic acid, creating the beautiful color change. The effervescence, which tosses the colorful droplets up into the oil layer, is an added bonus to this wonderful demo.
- Red cabbage
- Blender/ juicer/ pot to boil water (basically just some way to get cabbage juice)
- Canola Oil (or another lightly colored oil)
- Washing Soda/ Baking Soda (something to increase the alkalinity of the cabbage juice to make it blue, I’ve even used powder dishwasher detergent)
- Alka-Seltzer tablets
- Tall, clear glass or bottle
- Juice the cabbage in a juicer OR blend some leaves with some water and strain the juice OR boil some water with some leaves and strain the juice. The result should be a dark purple liquid whichever way you do it.
- Take about a cup of the juice in a new container and slowly add your washing soda (or whatever you choose to make it alkaline), mixing thoroughly until the solution is a blue. Baking soda will turn it indigo and washing soda will make it teal. If it takes more than about 1/8 tsp, dilute the juice with water and try again. This is because if the cabbage juice is too concentrated, one Alka-Seltzer tablet won’t produce enough carbonic acid to change to the color.
- Fill up your tall, clear glass about 2/3 of the way with canola oil.
- Add some of your blue cabbage juice to the tall glass. It should sink to form a bottom layer and be about 1/4 of the amount of the canola oil. Make sure to leave a little room at the top of the glass because it may foam up.
- Drop in a half tablet of Alka-Seltzer.
- It’s not changing color.
- If the cabbage juice is too concentrated, it may not change color with the first tablet. If this happens, just dilute your cabbage juice stock by adding water, then make a new lava lamp, keeping volumes the same.
- If you add too much baking/washing soda, it may not change color with the first tablet. If this happens, either add a little bit of vinegar or take a new sample of purple cabbage juice and add baking/washing soda slowly this time, stoping when it turns blue to ensure not too much excess base, then make a new lava lamp.
- No droplets are making it to the oil layer.
- If you have too much cabbage juice in the bottom layer, the effervescence may not have enough oomf to make it to the oil layer. Take a turkey baster or eye dropper and take some cabbage juice out, or use a full tablet.
- Too much effervescence.
- Use a smaller piece of tablet.
- A friend on Twitter (Tom Kuntzleman @pchemstud) had an idea to use acidic cabbage juice by adding vinegar and observe the color change with the Alka-Seltzer. Here, the dissolved carbon dioxide is acting as a base (bicarbonate AKA hydrogen carbonate) and accepting protons from the acidic solution. This increases the pH and changes the color from red to pink (rather than decreasing the pH to go from blue to pink in the above example). Together, these two demos would be a great example of how an amphiprotic molecule (such as carbonic acid) can act as an acid or base depending on the pH of the solution. This is also fun to try with little kids who have no clue what protons are- it was a big hit with my pink-loving daughter.
- We also tried the lava lamps with glitter thrown in there. I highly recommend it, as does my three-year-old.
This demo is loaded with scientific concepts ranging from simple miscibility and density, to complex multi-component acid/base chemistry. Let me know in the comments or social media if you try this out. I’d absolutely love to hear your take on it, whether through a lens of child’s caregiver or chemistry instructor!