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Functional groups are critical components in organic molecules as they determine the chemical reactivity and properties of the molecules. By replacing hydrogen atoms in hydrocarbons with other atoms or groups of atoms, functional groups introduce diversity in structure and function. This alteration can significantly affect how the molecule interacts with other substances, including its solubility, acidity, and ability to form chemical bonds.

For example, the presence of a hydroxyl group makes a molecule alcohol, which is polar and soluble in water, while a carboxyl group transforms a hydrocarbon into a carboxylic acid, affecting its acidity. Thus, the ability of functional groups to replace hydrogen atoms with more reactive elements or groups empowers organic molecules to participate in varied biochemical reactions that are essential for life processes.