Substitution Reaction Definition

A substitution reaction is a fundamental type of chemical reaction where one atom or a group of atoms within a molecule is fundamentally replaced by another atom or group.

Analysis of Options

• Option 1: Removal of atoms to form simpler molecules - This describes decomposition or elimination reactions, not substitution.
• Option 2: Addition of atoms to double bonded molecules - This describes an addition reaction, typically seen with alkenes and alkynes.
• Option 3: Replacement of a carbon atom with hydrogen - While this is a specific type of substitution (like reduction), it doesn't define the general concept. Substitution can involve many different atoms or groups being replaced.
• Option 4: One atom or group of atoms being replaced by another - This accurately captures the essence of a substitution reaction. The key is the 'replacement' aspect.

Therefore, the definition of a substitution reaction is the process where one chemical entity is substituted for another in a molecule.