Significant Figures and Measurement Recording in Chemistry

Significant Figures and Measurement Recording

Introduction to Significant Figures

In chemistry, significant figures (also called significant digits) are the digits in a measurement that are known with certainty plus one digit that is estimated. The number of significant figures reflects the precision of a measurement. The more significant figures, the more precise the measurement.

• Precision refers to how close repeated measurements are to each other.

• For example, a reading of 25.00 mL is more precise than just 25 mL because it includes more significant digits.

Recording Measurements

When recording a measurement, always include all certain digits plus one estimated digit, which is typically the last decimal place. This estimated digit is based on your best judgment (sometimes called the "eyeball test").

• Record all known numbers plus one additional decimal place.

• The last digit is always an estimate.

Example: Determining Significant Figures in a Measurement

Suppose you are measuring the length of a square using a ruler. The correct way to record the measurement is to include all certain digits and one estimated digit.

• Example Measurement: The length of the square is measured as 3.20 cm.

• This value has three significant figures: 3, 2, and the estimated 0 in the hundredths place.

Practice Question 1

Read the length of the pencil to the correct number of significant figures.

• The correct answer is 9.0 cm.

• This value has two significant figures: 9 (certain) and 0 (estimated).

Practice Question 2

What is the correct reading for the liquid in the burette provided below?

• The correct answer is 6.80 mL.

• This value has three significant figures: 6, 8, and the estimated 0 in the hundredths place.

Key Points for Recording Measurements

• Always record one more decimal place than the smallest marking on the instrument.

• The last digit is always an estimate, reflecting the uncertainty in the measurement.

• Significant figures communicate the precision of your measurement to others.

Definitions

• Significant Figures: All the digits in a measurement that are known with certainty plus one estimated digit.

• Precision: The degree to which repeated measurements under unchanged conditions show the same results.

• Estimated Digit: The last digit in a measurement, which is uncertain and based on the observer's estimation.

Example Table: Number of Significant Figures in Measurements

Measurement	Number of Significant Figures	Explanation
3 cm	1	Only the digit 3 is certain; no decimal or estimated digit.
3.2 cm	2	3 is certain, 2 is estimated.
3.20 cm	3	3 and 2 are certain, 0 is estimated.
3.200 cm	4	3, 2, and 0 are certain, last 0 is estimated.

Summary

• Always include all certain digits and one estimated digit when recording measurements.

• The number of significant figures indicates the precision of the measurement.

• Properly recording significant figures is essential for accurate scientific communication.