Slow reactions, all around
A steel gate hinge turns reddish-brown after one monsoon.
Step 1 of 3
- The hinge: grey, shiny, no rust yet.
- Air and water both reach it.
- The hinge turns reddish-brown and crumbles. That is rust.
Pick a thing. Switch oxygen and water on and off.
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The rule
Corrosion and rancidity are slow reactions with oxygen. Rusting needs both oxygen and moisture. Take an ingredient away and the reaction slows.
Corrosion is a metal slowly changed by air and moisture. Rust is iron slowly gaining oxygen. Copper grows a green crust. Silver turns black.
Rancidity is what happens when fats and oils react slowly with oxygen. The smell and taste turn sharp. Food spoiled this way is rancid.
| The slow reaction | It needs | What slows it |
|---|---|---|
| Rusting of iron | oxygen and moisture | paint or grease keeps both off |
| Green crust on copper | air, moisture, carbon dioxide | a coating keeps them off |
| Rancidity of oils | oxygen of the air | airtight pack, or nitrogen inside the pack |
A crisp packet is puffed with nitrogen. That pushes the air out, so less oxygen reaches the food. An antioxidant in food reacts with oxygen first. So the food reacts less.