Chapter 7 · Chemical Reactions
7.2Evidence of a Chemical Reaction
8 min · two checks
Predict
Which observations suggest a reaction, and which can fool you?
The idea
- List four common clues of a chemical reaction.
- Give one look-alike that is only physical.
Useful clues: a color change that is not simple mixing, a precipitate (a solid forming when solutions meet), gas bubbles from solutions that are not being heated to boiling, and heat or light released or a mixture that gets cold because the process absorbed energy. None of these rewrites the definition. They tell you to look for new substances.
Look-alikes are everywhere. Water bubbles when it boils. A colored crystal seems to disappear as it dissolves, and the color spreads. A cold pack can be endothermic dissolving rather than a reaction that makes a new covalent compound — though dissolving of some salts is still a chemical process at the ion level. When you write a lab observation, write what you saw, then decide whether identity changed.
Keep these
- Clues: color, precipitate, gas, energy change.
- Clues are not proof by themselves.
- Boiling and ordinary dissolving are the usual false alarms.