Chemical reaction lava lamp

I've seen various versions of an acid and alkali reaction lava lamp, but all have used medicines e.g. alka seltzer or effervescent vitamins to create carbon dioxide bubbles. I thought we could avoid both buying something we wouldn't otherwise use and not use medicines we didn't need, but achieve the same effect.

Weighing the bicarbonate of soda

We have previously made bath bombs, which the small boy really enjoyed. I thought we could try making some very small bath bombs with a similar recipe to use in place of the alka seltzer; the bath bombs contain both acid and base which mix when they dissolve in water and produce carbon dioxide bubbles. Instead of adding alka seltzer to vinegar, we could therefore just drop these into water.

Mixing ingredients

As before the small boy enjoyed donning his lab coat and mixing the ingredients for the bath bombs with some careful supervision. I scaled down the recipe and omitted the colour and scent this time. The steps were as follows:
- Weigh out dry ingredients into a tub: 50g bicarbonate of soda, 25g citric acid, 12.5g cornflour, 12.5g Epsom salts. Mix thoroughly.
- Warm 1tbsp coconut oil until liquid.
- Mix the oil into the other ingredients.
- Sprinkle in a tiny bit of water (literally 5 or so drops) and mix quickly. It doesn't look like it's all stuck together.
- Press into moulds. We used a chocolate mould from IKEA which was flexible and made very small bath bombs (it makes pretty stingy sized chocolates!).
We left these to dry for a couple of hours.

Mini bath bombs

In a conventional lava lamp, the coloured, less dense, material is heated at the bottom and rises to the top, cools and sinks. In this version, the coloured material is water and it is carried up through the less dense oil by bubbles of carbon dioxide gas. When the bubbles pop at the surface, the more dense coloured water sinks back through the oil.

Our 'lava lamp'

To make the faux lava lamp, put some vegetable oil in a clear plastic bottle. Pour in about a third of the volume of water with food colouring of your choice - my son chose red. You can do it the other way round, but I took the opportunity to ask the small boy whether the water would go to the bottom or stay at the top. He's previously experimented with mixing oil and water in our lab set and in making colour mixing pots, but I was curious whether he would have remembered. He had indeed and confidently told me it would go to the bottom. He was pleased to see he was right! The next step is to add the mini bath bombs; we used two as it takes a while for them to start reacting quickly and my son wanted to throw a second one in so it worked better! You could also crumble them into smaller pieces to increase the surface area and speed up the reaction, but the way we did it made them last for about 5 minutes which probably provided more entertainment!

Both boys (3 years and almost 1) enjoyed watching the bubbles and blobs of red sinking slowly through the oil (see video above). The smallest was mesmerised until he decided to pick the bottle up and give it a shake! His brother wasn't very impressed, but it didn't take very long to settle back down with the water layer at the bottom and it kept bubbling nicely. Having it in a bottle with a lid on tightly meant I didn't have to worry about the little one picking it up, but note that it does make the pressure build up inside the bottle (not an issue if it has previously held carbonated drinks but just be careful when you open it!).

All in all, this worked nicely and using a chemical reaction he had done before probably meant the small boy better understood what he saw more than if we had bought some tablets where he didn't know what was in them.