Making a Lava Lamp

Photo by Dean Hochman

Many people still remember the captivating lamps where colorful liquids move up and down and change shape in slow motion. For a while now lava lamps have been rather out of fashion and don’t quite fit with the current mainstream home design. But what if you could journey back to the 1990’s for a while and stare at the moving bubbles again without having to commit to a lava lamp for the next few years? And better yet, make it a fun and educational activity for children and adults alike?

Here, we will show you how you can make your own temporary lava lamp with household items. It is a fun and easy experiment suitable for those in need to entertain their kids in these corona isolation times and those who want to let out their inner child and be mesmerized by science.

The experiment

Follow the laboratory protocol below. Whether kids are involved or whether this is a games-night alternative, encourage all participants to think about what is happening and why. How come the two liquids won’t mix? Why is there movement only when we add salt or a fizzing tablet? How is this different to a real lava lamp and what causes that difference? As every other scientist does, you could write down your observations and conclusions.

Labjournal entry

How does it work?

Most of us know that oil and water repel each other. No matter how many fizzy tablets we add to our lava lamp or how well we shake the bottle, the water and oil will separate into millions of small droplets. This is because water is made from polar molecules, meaning it has two differently charged sides: one negative and one positive. As with magnets, opposite sides of water molecules attract each other and assemble to the liquid phase we like to drink and wash our dishes with. In contrast, oil is non polar and has no interest to assemble with water.

Lava Lamp using salt

Lava Lamp using fizzy tablets

Density

That said, why do bubbles of one liquid travel into the other liquid in our lava lamp even though we are not shaking the bottle? It all has to do with manipulation of the liquids’ densities. On its own, water has a higher density than oil, meaning that 50 mL of water will weigh more than 50 mL of oil. This is because water is made up of more tightly packed molecules – more molecules in the same space makes the liquid heavier. You could compare it to an over packed metro during rush hour, whereas the oil would be the emptier metro around midnight.

Salt and fizzing tablets

However, if we add salt to our lava lamp, it travels through the oil where some molecules attach to the salt crystals. The salt-oil combination is denser and heavier than water and drops to the bottom. When the salt has dissolved, the oil regains its lower density and travels upward.

Similarly, the addition of fizzy tablets changes the density of water in our lava lamp. At first, the fizzing tablet sinks to the bottom and dissolves in the water phase. While dissolving, carbon dioxide (CO2) is freed and sticks to some water molecules. This combination has a lower density than oil or water alone and CO2-water bubbles rise up. Once the CO2 has escaped from the bottle the now denser water droplets descend again.

Lava lamps from the 90’s

Original lava lamps from the 1990’s use the same principles but with a slightly different implementation. Two liquids of similar densities, but with very different polarities, are placed in a container above a heat source: a light bulb. The slightly denser liquid B will drop to the bottom of the lamp, where the light bulb heats up the liquid causing it to expand. This means that the molecules of liquid B are now less densely packed than those of liquid A and blobs of liquid B float upward. As it moves further away from the heat source, liquid B cools again and, now denser, drops back down toward the light bulb. There the cycle repeats, hypnotizing the observer in a meditative and now hopefully also scientific manner.

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