Effects of oxidation: corrosion and rancidity
Corrosion is a metal being slowly damaged by moisture, air or acids around it. Rancidity is the oxidation of fats and oils, which spoils their smell and taste.
Corrosion of common metals
| Metal | What you see | Cause |
|---|---|---|
| Iron | Reddish-brown powder (rust) | Oxygen and moisture in the air |
| Silver | Black coating | Sulphur compounds in the air form silver sulphide |
| Copper | Green coating | Moist carbon dioxide in the air forms basic copper carbonate |
Why corrosion matters
- It damages car bodies, bridges, iron railings and ships.
- Replacing rusted iron costs a lot of money every year.
- Prevention: painting, oiling, greasing, galvanising (a zinc coat) and making alloys. You will study these in the chapter on metals and non-metals.
Rancidity
When fats and oils are oxidised, they become rancid: their smell and taste change. Old namkeen and chips taste bad for this reason. Rancidity is slowed by adding antioxidants, by keeping food in airtight containers, and by filling chip packets with nitrogen. Nitrogen is an unreactive gas, so it keeps oxygen away from the oil.
Corrosion vs rancidity
Both are slow, unwanted oxidation reactions.
Murukku, chips and mixture go stale and smell bad when left open, because the oil in them turns rancid. Chip packets are puffed with nitrogen, and many homes keep snacks in airtight steel dabbas.
2 marks: “What is rancidity? Write two ways to prevent it.” Oxidation of fats and oils in food, which changes their smell and taste. Prevent it by adding antioxidants, storing food in airtight containers, or flushing packets with nitrogen.
2 marks: “Why do chips makers fill the packets with nitrogen?” Nitrogen is unreactive and keeps oxygen out, so the oil in the chips is not oxidised and does not turn rancid.
3 marks: “What is corrosion? Name the coatings formed on iron, silver and copper.” The slow damage of a metal by air, moisture or acids. Iron: reddish-brown rust. Silver: black silver sulphide. Copper: green basic copper carbonate.
Rust is Red, Silver goes Black, Copper goes Green, and fats go Rancid.