Making the bath fizzy

On a recent episode of 'Maddie's Do You Know', the small boy watched how bath bombs are made and how they fizz in the bath when they come into contact with water. It seems to have captivated him as he asked if we could make them. I don't like to say no to reasonable requests, particularly if they potentially involve fun science, so I looked into it... 

We had all but one of the ingredients that most recipes found suggested using.  What we were missing was citric acid, so I ordered a small amount, and read up on various 'recipes' . There are lots of different recipes around and ours is most closely based on the one found on the BBC good food website (although I wouldn't suggest eating it...). 

Most people recommend this as a craft for teenagers as it's a little tricky to get it exactly right, and if it goes wrong you can get a fizzy gooey mess.  Not to be deterred given the small one's interest, I thought about which steps it was sensible to deploy a 3 year-old's assistance.  I planned how we'd make them together whilst the smallest member of the family was engaged in munching a snack and watching from a safe distance, and wrote the quantities of ingredients we needed out on a piece of paper so it was easy to refer to whilst preventing said ingredients being knocked all over the table and floor.  The actual making process went as follows:

Mixture #1:
- In a microwaveable container, measure out 2 tablespoons of coconut oil.  This is solid at room temperature (or at least the temperature of the room we were doing it in) and so we briefly warmed it in the microwave to melt it.
- To the warmed coconut oil, we added a couple of drops of food colouring.  The small boy chose red, so this is what we used, and it gave a very pale pink colour.  You could definitely add a little more if you wanted, but I was cautious about ending up with a small boy bathed in excessive food colouring.
- The food colouring is soluble in water, so doesn't mix into the oil easily.  We gave it a good stir with a fork, and accepted that it had formed little blobs suspended in the oil.  In fact, my small assistant quite liked looking at the little blobs of colour and stirring them around.
- We then added about 4 drops of lavender oil.  I say about 4 as little hands did the addition, and I suspect that 'about 4' may have been more like 8.  Again, this is a bit of a matter of personal preference, but our room was pretty lavender scented for the rest of the day.

Mixture #2:
- Mix in a large container (doesn't need to be heat-safe, we used an old ice cream tub) the following: 100g bicarbonate of soda, 50g citric acid, 25g cornflour (in the US, I think this is sold as corn starch - it's the stuff you use to thicken sauces) and 25g magnesium sulphate (sold as Epsom salt).  My 3 year old likes weighing things out for cooking, and this was similar - he enjoyed watching our digital scales count up and telling me when we'd got to the right number.
- Make sure the four ingredients are well mixed - if you have clumps of each, the end result won't work as well.

Weighing the dry ingredients

Two mixtures, ready for the next step

The next step is to very slowly add mixture #1 into mixture #2.  We did it a teaspoonful at a time, and the small boy did some mixing, but I gave it what he calls a 'magic Mummy mix' each time to make sure everything was properly stirred.  We do this with baking too, as he's just not got the strength or stamina for getting gloopy things properly mixed yet.

When all of mixture #1 has been stirred in you then need to do the trickiest bit - you need to add enough water to make it stick together so you can press it into the mould, but not so much that the reaction between the citric acid and the bicarbonate of soda happens and it turns into fizzy gloop.  This is not something for a heavy-handed toddler, although if you had a spray bottle, you could possibly use that and get them to do one spray at a time.  We did it with single drops off a spoon, and the small boy just watched and told me whether he thought we'd added enough.  In hindsight, I think we could have got away with one fewer drop, it really doesn't take much.

Whilst usually spherical, there's no reason you can't use a different shape, and we used empty children's fromage frais pots for our moulds which give a near-cylindrical bath bomb as we didn't have hemispheres from any plastic packaging around (and nor did I want to buy a mould just for trying this out).  The plastic pots have a bit of flex to enable the finished product to be easily removed, but are robust enough for pressing the mixture in firmly.

Our recipe made enough for three little pots (plus a little left over) and we pushed the mixture into the pots with a teaspoon, then gave it a final squash with our fingers to make sure it was tightly packed.  To have enough structural stability to stay intact when you put them into water, you need to make sure the ingredients are tightly pressed together.

Putting the mixture into yoghurt pots

 All three turned out of the pots without breaking!

We left ours to dry in the pot for 3 hours, then turned them out and left to dry for a further half hour before trying one in the bath.  It was fun, and fizzed nicely, but Daddy had absentmindedly put soap in the bath too, so some of the fizzing was hidden under existing bubbles (we suggested postponing the testing of our science experiment to another night, but the small boy didn't agree with our suggestion, and we didn't want to waste water and energy on running another bath...)  It smelt lovely, a mix of the lavender with a hint of coconut.  We did some better product testing the next day with a box of water, and were able to watch the fizzing more closely, as the small boy watched it dissolve and periodically lifted the fizzing lump out of the water to inspect.  He liked listening to the sound it made, and watching as little bubbles appeared and popped.  Definitely more of a success than actually using it in the bath, and it had a bit more structural stability which was probably a result of having had longer to dry out.

Fizzing, and it smells lovely.  Shame about the wet sleeve...

All in all, this seemed to work nicely, although it's definitely an activity with a lot of adult participation and supervision to get it right - it'd be better done with an older child who was a little better at listening to instructions and had more fine motor skills, but it worked better than I feared it might when we set out to try it!