Eli5: Why chemicals in products, like shampoo or cleaners, don’t go reaction among themselves and form whole different compound?
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Shampoos do kind of change, but very slowly, on the scale of years.
As others have said, what gets put into most consumer products especially cosmetic products like shampoo, is chosen for not being very reactive in the first place. Any chemical that tracts easily with other compounds in the bottle is also going to be reactive with your body, probably in bad ways.
Then it is standard to add preservatives and stabilizers to the mix. Paraben is a well known and common example. It acts as a preservative. And there are often some form of wax added that thickens the product to prevent settling.
To-the-trade, professional use only shampoos and conditioners tend to have less of these extra chemicals or far less. It can make for a more effective or more predictable product when combined with other salon chemicals. But it makes for a much shorter shelf life.
Because they already did that to make it, stuff isn’t just randomly chemically changing all the time
Chemicals don’t just react randomly, they follow a set of rules and behave in predictable ways. They know what they put in a shampoo and what things it can do, if reacting spontaneously in an unwanted way was in that list, they wouldn’t put it there.
Lots of reasons depending on the chemicals in question.
In some cases, the ingredient is inert and just doesn’t react. In other cases there is a reaction of sorts, which is desired. In still other cases, the ingredient in question is already the product of two or more other components.
In the end, too, there could be active reagents that simply don’t have anything else in the product with which they react.
Remember, too, that in a compound chemical, the specific makeup of the compound may inherently not react with other things because of makeups. For instance, hydrogen may react with something, but hydrogenated so and so will not because the H atoms are already bound with whatever they’re in.
A good way to see the last point is the common analogy of sodium and chlorine. Elemental sodium is HIGHLY reactive to water. Elemental chlorine is highly toxic. Sodium chloride is table salt and provides us with necessary nutrients for proper human function.
And lastly, some compounds may be off-handed (right instead of a natural left or left instead of a natural right), which often gives them similar or identical properties, but makes them unable to react with another compound as they might in their natural handedness. This is incredibly common in pharmaceuticals.
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