We followed Super Awesome Sylvia’s suggestion recently and made a “no-heat lava lamp”.

We used

  • a smallish glass bottle (so we didn’t require a huge amount of oil)
  • vegetable oil
  • baking soda (sodium bicarbonate, NaHCO3)
  • citric acid (C6H8O7)
  • water (H2O)
  • food colouring (water soluble)

We put a couple of cm of water in the bottom, filled the bottle mostly with the oil, then added a few drops of food colouring.

We mixed equal quantities of baking soda and citric acid (1/2 tsp of each). Small quantities of this mixture were then dropped into the bottle.

Below is the result:

As you can see, the water and food colouring mix, but the oil and water do not mix, and the oil sits in a layer above the coloured water. The powder mixture falls through the oil to the coloured water at the bottom, where a reaction occurs and coloured bubbles rise to the surface. After the bubbles reach the surface, the coloured water drops to the bottom again.

So why does it work? The effect relies on several properties of the ingredients involved:

Polarity

The food colouring is water soluble because, like the water, it is a polar molecule. This means that the charge within the molecules is not evenly distributed, so part is positive and part negative, even though the molecule is neutral overall. Polar molecules generally mix well with other polar molecules, but not so well with non-polar molecules. Oil is a non-polar molecule, so the water and food colouring do not dissolve into the oil.

Density

The oil is less dense than the water. That is why regardless of which order you pour them into the bottle, after a little bit of time the water will be on the bottom and the oil will be on the top.

The bicarbonate of soda and citric acid are also denser than the oil and therefore sink through it.

The carbon dioxide that is produced, being a gas, is quite light and therefore rises up through both the water and oil layers into the air above. Once the CO2 is released into the air the water/food colouring mix drops back down again through the less dense oil layer.

The lava lamps that you can buy as a light work because the blobs of colour they contain are formed from a wax that becomes less dense as it heats up from the light at the base. The blobs of colour then sink when the wax cools again near the top, away from the bulb.

Chemical reaction

The bicarbonate of soda and the citric acid react with the water. When they reach the water they react with it to produce sodium citrate and carbon dioxide.

The ever-popular baking soda volcano works in a similar way, in that the vinegar (a mild acid) reacts with the bicarbonate of soda to produce carbon dioxide gas.

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