Choosing what to watch
Not every reaction is easy to measure. It is straightforward only when the reaction gives us an outward sign to watch. So the first decision is what to observe.
| What is watched | How it is measured |
|---|---|
| A gas being given off | Measuring the volume of gas collected, or weighing the mass lost as the gas escapes |
| A solid reactant disappearing | Timing with a stopwatch how long it takes to disappear completely |
| A colour changing | Timing how long the colour takes to fade or to appear |
| A precipitate forming | Timing how long the clouding solution takes to hide a mark placed underneath it |
The limestone and acid reaction offers the first two of these — carbon dioxide leaves as bubbles, and the limestone steadily disappears. That is why it is used again and again as a classroom experiment.
From here on we look at what changes this rate. That is what section 17.1 is about.