Transcript Document

Practicing in Measuring
Volume, Mass and Length
Volume
• Graduated cylinders
Glass
Plastic
Clear fluid problems
Procedure for finding volume of irregular
objects
• Burettes and Pipettes
• Beakers
• You should be
able to measure
the volume of
liquids in a
graduated
cylinder. How
precisely you can
measure volume
depends on the
size and type of
graduated
cylinder you use.
Generally, you
should be able to
estimate between
the etched or
printed lines.
On this 100 milliliter
cylinder, the numbers
are 10, 20, 30, etc., so
there is a 10 milliliter
increment between
them. Since there are
10 divisions between
consecutive numbers,
each division
represents one milliliter.
Therefore, you should
be able to estimate to
tenths of a milliliter by
reading between the
lines.
It is important to notice what
each line or interval on
the graduated cylinder
represents.
Different kinds of graduated
cylinders are set up
differently. A 10 milliliter
cylinder, for example,
usually has one tenth of a
milliliter for each
graduation, but some
have two-tenths milliliter
for each graduation.
The way to check this is to
count the divisions
between consecutive
numbers.
8.8 mL
On some cylinders, there may only be five divisions
between numbers. Or there may be ten divisions
for a 2 milliliter increment. In these cases, each of
the divisions represents 0.2 milliliters, rather than
0.1. You need to be aware of that when you're
using the cylinders like these, and adjust your
between-line-estimates accordingly.
?
We need to see the rest
of the graduated cylinder
to know
what increments
these lines are in.
A characteristic of
liquids in glass
containers is that they
curve at the edges.
You measure the level
at the horizontal
center or inside part
of the meniscus.
Read the bottom of the
meniscus.
5.9 mL
6.8 mL
In some plastic
cylinders
water has a
flat surface. In
that case top
or bottom
doesn't matter,
but we can still
say use the
center rather
than the
edges.
~ 293 mL
The visibility of the
meniscus can be
enhanced by using a
card with a dark stripe
on it, placed behind the
cylinder.
Adjusting the placement of
the card can give you
either a white meniscus
against a black
background or a black
meniscus against a white
background.
~ 48 mL
16 mL
12 mL
21.63
36.5 mL
http://www.ptloma.edu/Physics/conversions.
htm
Use this site to make conversions from the
English system to the Metric system.
Interactive quizzes to see if you understand measuring distance
volume
temperature
http://www.bbc.co.uk/skillswise/numbers/measuring/distance/quiz.shtml
Choose level ‘A’ first, then proceed to level ‘B’ and ‘C’.
http://www.bbc.co.uk/skillswise/numbers/measuring/lwc/quiz.shtml
http://www.bbc.co.uk/skillswise/numbers/measuring/temperature/quiz.shtml
Finding volumes of regular objects
and irregular objects.
75 – 50 = 25 mL
1) Measure the volume
of liquid in a partially
filled graduated
cylinder,
2) Add the solid (making
sure it is submerged)
and note how the level
of the liquid goes up,
3) Measure the combined
volume of the solid and
liquid. The difference
between the initial and
final volumes is the
volume of the solid.
Burettes
Change in a burette
47 – 38 = 9 mL
~ 39.24 mL
The black/white card
Pipettes
Water in a pipette adheres to the side of the tube and forms a curved
surface called a meniscus. By common practice, all readings are made
at the bottom of the meniscus. The units for this pipette are milliliters
(ml).
0.25 mL
At start of experiment
0.23 - 0.07 = 0.15 mL
Ten minutes later:
Beakers with only one scale of
numbers.
~ 159 mL
43 or 44 mL
Beakers with two scales of
numbers.
~ 2050 mL
Mass
• Kinds of balances
• The standards
• The way to read the scales
The same worldwide.
~ 100.41 grams
Mass Measurement
The triple beam balance is commonly used to measure mass in the biology lab. T
his device is named for its three long beams on which sliding bars called riders (or tares)
are used to determine the mass of an object placed on its platform.
It is very important that the riders on the rear beams are in the notch for the whole
number of grams and not in between notches. The front beam is a sliding scale
graduated in grams. The rider on this beam can be positioned anywhere on the scale.
Masses on a triple-beam balance can be read to tenths of a gram and estimated
to hundredths of a gram.
The picture shows the measurement of a mass in progress.
Without estimation, the mass of the object appears to be 373.3 grams (g).
~ 373.3 grams
http://www.explorelearning.com/ind
ex.cfm?method=cResource.dspVie
w&ResourceID=385 Practice
measuring with this interactive site.
Length
• Using a meter stick.
• Converting metric units to other metric
units.
• Microscope ‘Field of View’.
• Estimating size of microscopic organisms.
Problem: How long is leaf A?
The tip of the leaf is at about 6.5 cm, but note the measurement started at 1 cm.
Therefore, Leaf A is 5.5 cm or 55 mm. in length.
Converting metric units to other
metric units.
Microscope ‘Field of View’.
Finding the Size of a Microscope Field of View
In the pictured field of view, it can be observed that there
are approximately 3 1/2 divisions equal to a length of 3.5
mm.
Therefore this field of view is equal to 3.5 mm
or 3,500 micrometers.
View of a metric ruler
Viewed under LOW power
View of a metric ruler
Viewed under MEDIUM power
Finding the Size of Multiple Cells in a Field of View
The two cells in this field take up a field of view of one
millimeter. Therefore, the size of the specimen is equal
to 1 mm/2 cells or 0.5 mm per cell. There is 500
micrometers in 0.5 mm., so the average size of each cell
is 500 micrometers.
Accuracy and Precision
http://www.brainpop.com/science/scientificinquriry/precisionandaccuracy/
A short video about this topic.
The usefulness of measurement is
enhanced by knowledge of its level of
certainty.
Multiple measurements of the same
property are like multiple shots at the
same target.
The pattern of the shots tells you
something about the measurement
and its ability to describe the 'true'
value of the property being sought.
The patterns depict possible outcomes of
different experiments to measure the
same property.
Expt IV is of course the best, because it
give very reproducible results (precise)
and also results that are very close to
the true value or bull's-eye (accurate).
Experiment III is precise but not
accurate. It exhibits systematic error,
which is very difficult to estimate at
times.