Joined in a fixed ratio
Table salt is safe to eat. Its two elements are not. One is a soft metal. One is a poison gas.
Step 1 of 3
- Hydrogen (circle) and oxygen (square), ready to join.
- Two hydrogen join one oxygen: four groups form.
- One hydrogen and five oxygen have no partner. Left over.
A compound is made when different elements join in a fixed ratio. Water has hydrogen and oxygen, 2 to 1. That is 2 hydrogen atoms for every oxygen atom. Sodium chloride is table salt. It has sodium and chlorine, 1 to 1.
A property is how a substance looks or behaves.
The rule
A compound is made when elements join in a fixed ratio. Its properties differ from those of its elements. Physical methods cannot separate its elements.
That is because its elements are joined, not just mixed.
Slide the hydrogen. Count the water groups.
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