Pro-Tip the second one is from Lush and not the one that I made!!
This is an acid-base (alkali) neutralisation reaction. A neutralisation reaction occurs when a weak acid and an alkali react to create a salt and water as products. The part that makes them fizz is the same as an Alka Seltzer or soluble aspirin tablet: sodium bicarbonate and citric acid. These chemicals are inert when they are dry powders, but in water they dissolve and the citric acid reacts with the sodium bicarbonate (alkali) to form sodium citrate and carbon dioxide . The sodium citrate stays in solution and you don’t really notice it, but the carbon dioxide bubbles out as a gas that helps the bath bomb break up. This lets the detergents, perfumes and oils that make up the rest of the bath bomb mix with the bathwater.
Citric Acid crystals (can sometimes be bought from a chemist or brewery shop).
Bicarbonate of Soda
Essential oils e.g. almond oil (although even vegetable oil will do)
Essential aromatherapy oils or even food essences like vanilla/strawberry essence for cakes e.g. lavender/chamomile/mint or perfume
a water spray
Food colouring of your choice (powdered is best as it doesn’t set off the reaction)
Moulds (You can use Ice Cube moulds or silicon cake moulds-bendy is best)
Optional: Pestle and Mortar/bath salts/edible or normal glitter
1) The dry mixture of citric acid and bicarbonate of soda (sodium bicarbonate) needs to be 3 parts sodium bicarbonate to 1 part citric acid. Mix them together with a dry spoon. At this point, you might also want to grind the ingredients with a pestle and mortar to make them finer. You can also add bath mineral salts to bulk out the mixture if you grind them up at this stage.
2) Add a few drops/sprinkles of food colouring and perfume/oils dependent on the size of your mixture but mix these in as quickly as possible so that you don’t set off the reaction with a liquid (making it aqueous)!
3) Add the vegetable oil/essential base oil in the exact same way- VERY carefully. If the reaction starts to fizz, try and halt it by quickly adding and mixing in more dry ingredients.
4) Mix in your edible glitter if you have some.
5) Very carefully spray a small amount of water in to the mixture. The aim here it to have it slightly damp so that it could clump together if you squeezed it really hard, NOT wet. You will kickstart the reaction if you make the reactants fully wet. After every spray, mix the water in as quickly as possible until the reactants are damp. It will not take much- this is often where people make mistakes.
6) Press your mixture into the mould as hard as you can. If the mixture is fizzing or rising, you need to either start again, or add some more dry ingredients to it as quickly as possible. This means that the mixture is too wet.
7) Either leave your bath bomb to dry for up to 24 hours or (if you’ve put the bath bombs in a silicon mould, you can also put them in the oven on a very low heat for 15/20 mins to dry them out).
You can even try and layer your bathbombs for more funky colours, or add bits to the bottom of your mould before you start (such as edible confetti, etc) for extra decoration.
- Feel the bathbomb going off in your hand- the reaction feels cold. It’s endothermic (absorbs heat).
- If you crush up the citric acid and bicarbonate of soda even finer with a pestle and mortar, does the rate of reaction increase (does the bath-bomb dissolve faster) because of an increase in surface area?