Q.Classify the following reaction as addition, elimination or substitution: CH3-CH2-OH + HBr -> CH3-CH2-Br + H2O.

[!TLDR] Br replaces OH on the same carbon skeleton, with nothing added and no multiple bond created, so this is substitution. [! …

In $\text{CH}_3-\text{CH}_2-\text{OH} + \text{HBr} \rightarrow \text{CH}_3-\text{CH}_2-\text{Br} + \text{H}_2\text{O}$, the hydroxyl group of ethanol is replaced by a bromine atom, while the rest of the carbon skeleton ($\text{CH}_3-\text{CH}_2-$) and its degree of saturation stay completely unchanged -- no multiple bond is created or destroyed, and one group ($-\text{OH}$) is simply swapped for another ($-\text{Br}$). This atom-for-atom replacement, with the rest of the molecule left intact, is the defining feature of a substitution reaction. …

Check whether one atom or group is simply replaced by another, with the rest of the carbon skeleton and its saturation level unchanged; if so, and no multiple …

Confusing this with elimination; elimination would remove two atoms/groups from adjacent carbons and create a double bond, whereas here only one group i …