Question

Why if a substance loses hydrogen is said to be oxidised and if gains Hydrogen then reduced?

Answer 1

Oil rig.png
Reduction is the loss of oxygen atom from a molecule or the gaining of one or more electrons. A reduction reaction is seen from the point of view of the molecule being reduced, as when one molecule gets reduced another gets oxidised. The full reaction is known as a Redox reaction. This is a good way of remembering it.

This can be remembered with the term OIL RIG when speaking about electrons.

Oxidation Is Loss
Reduction Is Gain
In the case of Organic Chemistry it is usually the case of the gaining/loss of Oxygen/Hydrogen

In Inorganic Chemistry the term refers to the change in oxidation state of the metal center.

Oxidation is a process where a substance
Loses one or more electrons
Gains an oxygen atom or Electronegative atoms
Loses a hydrogen atom or Electropositive atoms
Gains an increase in its oxidation number
Reduction is the opposite!