Oxidation and reduction
- CBSE · NCERT Class 10, Ch 1 Chemical Reactions and Equations
A substance is oxidised when it takes in oxygen or gives up hydrogen, and reduced when it gives up oxygen or takes in hydrogen.
The two always happen together. If one substance gains oxygen, another substance has lost it. So a reaction with both is called a redox reaction, from reduction and oxidation.
The substance that gains oxygen is oxidised. The substance that loses oxygen is reduced. Rust on iron and rancidity in stale oily food are everyday cases of oxidation.
A small example
Pass hydrogen over hot black copper oxide: CuO + H₂ → Cu + H₂O. The black powder turns brown as copper forms. Copper oxide has lost its 1 oxygen atom, so it is reduced. Hydrogen has gained that oxygen atom to become water, so it is oxidised. One reaction, both changes.
Where marks go
Question 29 of CBSE's 2026-27 Science sample paper has three students talk about rusting and making ammonia. Drishti says both are redox reactions. The 2026-27 Science marking scheme gives 1 mark for agreeing with Drishti.
It gives 2 more marks for the reason. Oxidation and reduction go hand in hand. In ammonia, nitrogen is reduced and hydrogen is oxidised. In rust, iron is oxidised and oxygen is reduced. So the answer names what is oxidised and what is reduced in each case. See how step marking works.
Sources
Facts last checked against these sources on 30 September 2026.
About the author
Founder Member & Academic Lead
An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.
Deepti Sardana on LinkedIn
