Significant Figures Often beginning chemistry students have a hard time understanding significant figures. With calculators showing up to 10 digits, students perpetually report too many significant figures. Presented is a very simple demonstration that can be used in class or lab to show how many significant figures to report when using addition or ~"btraction. The demonstration requires a 500-mL beaker with graduations of 50 mL and a 100-mL graduated cylinder with eraduatians of 1.0 mL. Fill the beaker with about 270 mL water and the cvlinder with about 65 mL. Have several students &ermine thevolumeof water in each as accurately aspossihleand write the answers of eachon the board. The beaker can be read to within 10 mLand thecylinder to0.1 mL. Have the class add up the two numbers on the board. A calculator may show the answer to be 337.3 mL. Obviously, this number is being reported more accurately than warranted. To show why that answer has t w many significant figures, pour the water from the cylinder into the beaker. You will have just'kdded" the two numbers together. Now have the same students determine the new volume of water in the beaker as accurately as possible. The accuracy will only he within 10 mL, not 0.1 mL. Kenton B. Abel Wllllam M. Hemmerlln Pacific Union College Angwin, CA 94508
Volume 67
Number 3
March 1990
213
