Lesson 8 of 13
Slow reactions, all around
Show on a big screenA 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.
Try the lab to finish this lesson.