Chemically, what makes a food product go stale?

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Products like bread, cookies, cereal, etc. go bad if left unopened (or eventually go bad with time). What is the chemical reaction taking place? What determines how long it takes for something to go stale?

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Tiny water molecules blanket and permiate the structure of the food.
Due to evaporation the water eventually leaves the product in some form.
That makes the food harder or stale.

The food contains fats. Both added directly in the form of butter or oil but many plants contain plenty of fat, especially seeds like grain where fat is used as energy storage for the new plants. When fat gets in contact with oxygen what can happen is that one of the hydrogen bonded to the carbon chain can react with this oxygen in a reaction chain creating lipid peroxide. This is a similar reaction to what makes vinegar out of ethanol. This reaction is also accelerating as one of the byproducts of the reaction is a lipid radical which is also a previous intermediate step bypassing the initial reaction. The lipid peroxide being created is known as rancid fat and tastes quite bitter.

You know how stale chips/crisps become soft, but stale bread becomes hard?

This is because food has an optimal level of water content. If too much water leaves, the food tends to go hard. If too much water is absorbed, it tends to go soft.