Why is steel stronger than iron?

Answer: Adding carbon atoms makes it harder

Compressing it under extreme pressure makes it harder — Wrong. While pressure can affect metal properties, steel's strength comes from carbon addition. Carbon atoms in iron's crystal lattice prevent deformation, creating a harder, stronger material than pure iron.

Adding carbon atoms makes it harder ✓ — Correct! Steel is iron with a small amount of carbon added (typically 0.2–2%). The carbon atoms fit into iron's crystal structure and stop layers from sliding past each other, making steel much harder and stronger than pure iron. Different carbon levels and heat treatments produce steels with different properties (mild steel, tool steel, stainless steel).

Heating it to a higher temperature makes it harder — Wrong. Heating iron alone doesn't turn it into steel; without added carbon, it stays relatively soft. Heat treatment can adjust the properties of steel that already contains carbon, but it isn't what makes steel stronger than pure iron.