Reaction or change of state?
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Why
When ice melts, it is still water. When wood burns, no wood is left. Both change a substance, but in very different ways. On this page you learn to tell them apart.
Rule
- Change of state
In a change of state such as melting or evaporating the substance stays the same. Only the state of matter changes.
Ask yourself: is the substance still the same afterwards? If so, it is a change of state. You can usually reverse it: water vapour becomes water again when it cools. If a new substance was formed, it was a chemical reaction.
Example
A candle burns
Both happen at a burning candle. The wax below the flame melts and becomes liquid: that is a change of state, it stays wax. The evaporated wax burns in the flame to other substances, for example carbon dioxide and water vapour: that is a chemical reaction.
Practice
Exercise
Solution
Wood burns. right answer
Burning forms new substances, for example carbon dioxide and ash. It is a chemical reaction.
Hydrogen burns to water. right answer
Hydrogen and oxygen become water, a new substance. It is a chemical reaction.
Ice melts.
Melted ice is still water. Only the state changes.
Water evaporates.
Water vapour is still water. It is only a change of state.
When melting and evaporating the substance stays the same, only the state changes. When burning, new substances are formed.
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Exercise
Solution
It is a change of state; the wax stays wax. right answer
When it cools it becomes solid again. The substance has stayed the same.
It is a chemical reaction because the wax changes.
Only the state has changed. No new substance was formed.
It is a chemical reaction because heat is supplied.
Heat alone does not show a reaction. What counts is whether a new substance is formed.
When melting, the wax stays the same; only the state changes.
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Exercise
Solution
Right answer: The statement is false.
The brown substance has different properties from sugar: colour and smell are new, and it does not turn back into sugar when it cools. It is a chemical reaction.
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