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Chemical reactions are processes where substances, known as reactants, transform into different substances called products. There are various types of chemical reactions, each with unique characteristics and implications.

Two common types are substitution and addition reactions, which are essential for creating a vast array of chemical compounds. In substitution reactions, an atom or a group of atoms in a molecule is replaced by another atom or group, whereas in addition reactions, atoms add to a molecule without removing any. By understanding these fundamental differences, students can better predict the outcomes of chemical reactions and their potential applications in areas such as pharmaceuticals, materials science, and organic synthesis.

Functional groups are specific groups of atoms within molecules that have characteristic properties. They are the reactive parts of a molecule, dictating how the molecule will behave in chemical reactions.

Each functional group has a distinct set of reactivity patterns. In substitution reactions, the functional group involved is crucial because it determines what kind of new atoms or groups can replace it. Understanding the nature of functional groups is fundamental to recognizing how different molecules will interact during chemical reactions. For instance, a hydroxyl group (-OH) may be substituted by a chlorine atom in an alcohol to form a chloroalkane. Students should focus on learning common functional groups, such as alcohols, amines, and carboxylic acids, to predict and understand these changes effectively.

Chemical reactions can lead to changes in the size of molecules. These changes are critical when considering the properties and applications of the resulting substances.

In a substitution reaction, the molecular size usually remains the same because one atom or group replaces another of a similar size. Conversely, addition reactions result in larger molecules as more atoms or groups join the initial molecule, increasing its complexity. This concept is exemplified by the polymerization processes where small monomers add together to form large polymers. The understanding of molecular size changes is vital in product design, synthesis routes, and even functional group modifications to achieve desired chemical and physical properties.