Science: Significant Figures - Rules and Practice

What are significant digits? Significant digits indicate how much care was taken in making a measurement. They also indicate how much precision is available in the tool used to make a measurement. For example, the triple beam balance, when used correctly, will allow you to measure an object's mass to the hundredth of a gram. Many students, however, will be careless and record the mass only to the tenths place.

Example: A student was asked to find the mass of a penny using a triple beam balance. Which measurement shows the mass recorded as accurately as the balance will allow? How many significant digits are in the measurement?

mass of copper penny = 3.15 g

or

mass of penny = 3.2 g

Answer:  Since the balance can measure to hundredths, 3.15 g shows that the tool was used with care. The correct measurement has 3 significant figures. The sloppy measurement has only 2 significant figures.

 

1. If a number does not have a decimal point, then zeros at the end of the number are place holders. These digits are not significant.

 

Example: 55,000 meters

 

This number was measured to 2 significant digits. The zeros at the end of this measurement are just place holders.

 

2. If the number begins with a decimal, the zeros at the end are significant.

 

Example: 0.88000 meters

 

This number has 5 significant digits. The zeros at the end tell us that the measurement was done with a precise tool.