Question

compounds & formulas we drink water. we breathe out carbon dioxide. we use salt in our foods. what do these substances have in common?

Explanation:

The problem is about identifying the commonality among substances like water, carbon dioxide, and salt. These are all chemical compounds. Water has the formula $H_2O$, carbon dioxide is $CO_2$, and salt (sodium chloride) is $NaCl$. All are made of two or more different elements chemically bonded, so they are compounds.

Answer:

These substances (water, carbon dioxide, salt) are all chemical compounds. Water ($H_2O$), carbon dioxide ($CO_2$), and salt (sodium chloride, $NaCl$) are each composed of two or more different elements that are chemically combined in a fixed ratio.