The kinds of matter that cannot be converted into simpler substances in ordinary chemical reactions are elements.

Here's why:

* Elements are the simplest pure substances. They are made up of only one type of atom.

* Chemical reactions involve the rearrangement of atoms and the breaking and forming of chemical bonds.

* Elements cannot be broken down into simpler substances by chemical reactions because they consist of only one type of atom.

For example, water (H₂O) can be broken down into hydrogen (H) and oxygen (O) through electrolysis, a chemical process. However, hydrogen and oxygen themselves are elements and cannot be further broken down by ordinary chemical means.

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