Chemistry Reference

An Illustrative Story
Here's a little story that may help you understand the idea behind significant digits:

Some tourists at the Museum of Natural History are marveling at the dinosaur bones.  One of them asks the guard, “Can you tell me how old these bonds are?”

The guard replies, “They’re three million, four years and six months old.”

“That’s an awfully exact number,” says the tourist.  “How do you know their age so precisely?”

The guard answers, “Well, the dinosaur bones were three million years old when I started working here, and that was four and a half years ago.”
What's the "Significance?"
Clearly, in the above story, the three million year old age assigned to the dinosaur bones is merely an approximate. It would not be surprising to find out that the bones were slightly younger or older than three million years. If it was discovered that the age of the bones were several million years older, that would be a surprising fact. The age which the curators can be certain is the age that is reported. The curators can be certain that it is three million "and something" years old. Since "and something" could be hundreds of thousands of possibilities, three million is the best "certain" age that can be given.

When scientists speak of significant digits, they are speaking of the digits in a measurement that are certain plus one estimated figure.

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