Measuring, Precision vs.
Accuracy, and Intro to
Significant Figures
What do we measure?
Volume – the amount of space taken up
by a substance
Mass – the amount of matter in an object
Temperature – measure of how much
thermal energy a substance has compared
to a reference point
Length – straight line distance between 2
points
What do you measure with?
Units of Measurement
You cannot just record a number for a
measurement. A unit must also be indicated.

For example, a measurement is taken for the
length of the laboratory table and recorded as 183
is not correct. A correct recording would be 183
centimeters (cm).
All measurements should be made in metric
units, or converted to metric.
Measuring Liquids – Volume!
When you measure the volume of
a liquid it is important to make
sure that you use the appropriate
kind and size of instrument.
When you put a sample of a liquid
into one of these instruments you
will notice a curve at the surface of
the liquid. This curve, which is
called a meniscus.
Adjust your body and head so
that you are eye level with the
instrument and the meniscus.
Read the volume of the liquid
at the bottom of the meniscus.
Measuring Mass
The triple beam balance
Carrying the Balance
1. Be sure all movable masses are
back to the zero point.
2. Place one hand under the balance
and the other hand on the support
(arm) to carry the balance.
Using the Balance
1. Zero the balance before you
determine the mass of any
substance.
a) Slide all of the movable
masses back to the zero
point.
b) Check to see that the
pointer swings freely along
the scale.
Mass cont.
Never put a hot object directly on the balance pan.
Any dry chemical that is to be massed should be
placed on paper or in a container.
Never pour chemicals directly on the balance pan.
Remember to mass the paper or container before adding the
substance.
Once you have placed the object to be massed on
the pan, move the riders along the beams
beginning with the largest mass first. If the
beams are notched, make sure all riders are in
a notch before you take a reading.
What is the mass?
Answer!
Measuring Temperature
We will use degrees Celsius in this class!
Never stir with a thermometer.
Never read a temperature with a
thermometer leaning against a side of a
beaker.
NO WAY! ->
Measuring Length
Meter sticks are carefully calibrated in
centimeters with a millimeter least count.
That is, the millimeter is the smallest
subdivision on the meter stick.

This means the millimeter is the unit of the
smallest reading that can be made without
estimating.
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Figure 1. This meter stick is
calibrated in centimeters
(shown as the numbered
major divisions) with a
least count of millimeters.
Reading instruments to significant
figures
The least count of this meter stick
is one millimeter, so we know
with absolute certainty that the
object is between 41.6 cm and
41.7 cm.
We then estimate the object's
length to the fractional part
(doubtful figure) of the least
count subdivision… 41.64 cm.
Let’s do this again…What is the
mass?
Answer = 216.49 cm
Sig Figs
Significant digits or significant figures
are digits read from the measuring
instrument plus one doubtful digit
estimated by the observer.
Examples:


203.4 cm: least count of ruler was 1 cm,
tenth of centimeter was estimated.
4.07 cm: least count was 0.1 cm.
hundredth of centimeter was estimated.
Rules for Sig Figs
Digits from 1-9 are always significant.
Zeros between non-zeros are always significant
Zeros at the beginning of a number are never significant,
they merely indicate the position of the decimal point
Zeros that fall both at the end of the number AND after
the decimal point are always significant
When a number ends in zeros but contains NO decimal
point the zeros may or may not be significant….if this is
the case, we use scientific notation!
Examples
EXAMPLES
# OF SIG. DIG.
COMMENT
453 kg
3
All non-zero digits are
always significant.
2004 L
4
Zeros between 2 sig.
dig. are significant.
5.00
3
Additional zeros to the
right of decimal and a
sig. dig. are
significant.
0.007
1
Placeholders are not
Practice problems
What is the number of sig figs in each of the
following:
a)
b)
c)
d)
e)
1282 kg
0.00296 s
8.070 mm
0.0105 L
9.7750 x 10-4 m
Accuracy and Precision
Accuracy-the closeness of a measurement
is to the true value of what is measured
Precision- a gauge of how exact a
measurement is
Which is more precise…a glug ruler or a
0.01 glug ruler? Why?
With which ruler can you report more
significant figures?
Accuracy vs. Precision
Clock Example
Digital vs. analog