How is the conversion of ethanol to ethanoic acid an oxidation reaction?

The conversion of ethanol to ethanoic acid involves the removal of the hydrogen atom and the addition of oxygen. Hence, it is an oxidation reaction.

$\underset{\text{Ethanol}}{\text{C}$2\text{H}5\text{OH}} +[\text{O}]\xrightarrow[\text{acidified}]{\text{K}2\text{Cr}2\text{O}7} \underset{\text{[Acetaldehyde]} }{\text{CH}3\text{CHO}} +[\text{O}]\xrightarrow[\text{acidified}]{\text{K}2\text{Cr}2\text{O}7} \underset{\text{[Ethanoic acid]} }{\text{CH}3\text{COOH}}

In the first step, an H₂ molecule is removed from ethanol to form ethanal. As the loss of hydrogen is oxidation, so the reaction is an oxidation reaction.

In the second step, an oxygen atom is added to form ethanoic acid from ethanal. As the gain of oxygen is called oxidation, the reaction is an oxidation reaction.