Bath bombs are awesome balls of fizzy goodness, with some interesting science behind them! They were invented in 1989 by Mo Constantine, one of Lush’s founders. Bath bombs contain the chemical sodium bicarbonate, also known as baking soda, as their key ingredient.
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Some of you might remember that baking soda isn’t good for your skin because it’s a base, with a high pH. High pH (alkaline or basic) products disturb the skin’s acid mantle, which protects your living tissue from the environment, particularly bacteria, like acne-causing Propionibacterium acnes.
But don’t fret! The second key ingredient in a bath bomb is a solid acid, such as citric acid or tartaric acid (cream of tartar). This lowers the pH by reacting with the baking soda when water is added to the mixture. Unless the maker of the bath bombs has really messed up their proportions, the final pH should be reasonably neutral. Until the water dissolves the acid and baking soda and allows them to mix at a microscopic level, nothing happens.
Aside from neutralisation, the acid + base reaction with sodium carbonate also produces tiny bubbles of carbon dioxide gas, which is what causes the fizzing:
Citric acid + sodium bicarbonate → sodium citrate + water + carbon dioxide
C6H8O7(s) + 2NaHCO3(s) → Na2C6H6O7(aq) + 2H2O(l) + 2CO2(g)
This is almost the same reaction as the one commonly used in volcano science projects to create foaming “lava” (they usually use vinegar as the acid). Of course, in bath bombs, there’s also fragrance and colours and glitter – the fizzing helps the bath bomb disperse faster, and combined with heat from the hot water, spreads the scent faster and makes the whole bathroom smell amazing.
Bath bombs can slowly absorb water from the air, using up the acid and sodium carbonate and releasing carbon dioxide prematurely – this is why bath bombs get less fizzy as they get old! Make sure you keep your bath bombs in a dry place until you’re ready to use them.