In short, not a lot - basically just one ingredient. In reality, its a bit more complex but we will get to that later on.
Soaps are made by a chemical reaction between an acid an alkali, in a process called saponification. The acid in the equation is the fats and oils used in the bars (in our case olive oil, sweet almond oil, cocoa butter, castor oil and coconut oil) each one bringing its own unique properties to the bar. The alkali used to react with these fats to make bar soap is sodium hydroxide, which when mixed with water creates a liquid we soap makers call 'Lye'. When the acid and alkali are mixed together, the chemical reaction immediately starts and the mixture takes on a custard like appearance and over a period of about 24 hours becomes a solid bar (its still not ready though, it has to cure for weeks first - we will explain that in another blog at some point). Amazingly this results in a really gentle bar that is naturally high in moisturising glycerine.
To make liquid soap you still use the same acids (oils), it is the alkali that is different - you use potassium hydroxide instead. We have never made it, we bought some potassium hydroxide to give it a go and it is still sat in the cupboard! Saponification still takes place in this process but the resulting soap is more fluid. The amount of water in the recipe also varies compared to bar soap to help achieve the desired consistency. Because of the additional water content, it is sometimes necessary to add emulsifiers and preservatives - something bar soap does not contain.
Up until now its been fairly straightforward, but this in when things get a bit blurry - soap isn't always soap! Primarily due to cost, a lot of bars and liquids you purchase nowadays are made from synthetic detergents (milder forms of the stuff you wash your clothes in) - these do have their benefits, and downsides, but that is for another day. There is a really easy way to see if what you are buying is made using the traditional saponification method or is made using detergents without reading through a list of confusing ingredients - its all in the language. To be called a 'Soap' or 'Liquid Soap' the product must have been made using oils and an alkali through the process of saponification, whereas detergent based products will have names such as 'Wash Bar', 'Liquid Hand Wash', 'Shower Gel', 'Cleansing Bar' etc.
In conclusion though bar and liquid soap are very different in form and consistency they are actually surprisingly similar in makeup. The real difference comes when you compare true soap (in either form) with its synthetic cousin. Bar soap can be made from 100% natural ingredients, liquid soap can be made from predominantly natural ingredients (possibly with the help of some synthetic preservatives), and all other options can be made with a mixture of the two, or entirely from synthetics. In short, if natural ingredients are important to you remember to look out for products with 'soap' in the official name!
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