2.1
Measuring Physical Quantities
be alble to:
Outcome- You should
Learning
name appropriate apparatus for measuring mass,
time, temperature and
TeS
of liquids and gases.
The modern metric system used to measure quantities of differ
Science Bitess
The metric system
ings
Units (S.I. Units). We use
is called the International System of
is measured in kilor
system in everyday life. For example, speed
5
kilograms and 10 kilograms.
hour and rice comes in sacks of
metric
In order to communicate scientific information easily, scientisit
e S.
units as a
common
standard for their
S
per
measurements.
in Singapore
Singapore has used
several systems of
measurement. An
example is the imperial
system of measurement
(e-g. length measured
in feet). In 1968,
Measuring Mass
The S.I. unit for mass is the kilogram (kg). Smaller masses are measured
in grams (E). In industry, the tonne (t) is used to measure the masses of
heavy objects like
1
1
kg =
vehicles and
large cargo
boxes.
1000 g
tonne (t) = 1000 kg
Singapore adopted the
metric system because
many of its trading
partners were using
this system. In 1979.
Singapore produced
a series of stamps to
The mass of a substance is measured with a beam balance or an electronic
balance (Figure 2.1). Often, very smalil masses of substances need to be
measured in Chemistry experiments. In such cases, an electronic balance,
with an accuracy of up to t0.01 g (two decimal places), is used for precise
and fast weighing.
familiarise people with
the metric system.
5493
Beam balance
Electronic balance
Flgure 2.1 Apparatus for measuring mass
24
Chapter 2
Measuring Time
The S.I. unit for time is the second (s). Other
and the hour (h), are used to measure
units, such
longer
1h
1
the minute
intervals of time.
as
Take Note
(min)
In Chemistry experiments
time is usually recorded
to the nearest second.
60 min
min = 60 s
laboratory, a stopwatch or stopclock is used to measure time. Two
types stopwatches are shown in Figure 2.2. The accuracy of an analogue
In the
of
stopwatch is £0.1 s, while the accuracy of a digital stopwatch is +0.01 s.
knob
used to start, stop and
reset the stopwatch
MIN
SEC 1/C
The digital stopwatch
reads 12 seconds
and 72 hundredths of
secondary dial
measures time
in minutes
15
LAP
a second.
main dial
measures time
in seconds
Digital stopwatch
Analogue stopwatch
Figure 2.2 Apparatus for measuring time
Measuring Temperature
The S.I. unit for temperature is the kelvin (K). Another unit, the degree
Celsius (°C), is also commonly used.
Temperature in K Temperature in °C+273
(Note that there
is
no
degree sign (°) before 'K'.)
or an alcohol thermometer
In the school laboratory, a mercury thermometer
Each division on the
2.3).
can be used to measure temperature (Figure
one
degree Celsius (1°C). The
scale of these thermometers represents
t0.5°C.
is
thermometers
accuracy of these types of
connected to
A temperature sensor can be
This
a data logger to measure temperature.
temperature
of
accurate
readings
gives more
thermometer. We
than a mercury or alcohol
record and study
to
can use a data logger
of a substance
the changing temperature
over a period of time.
being heated or cooled
used for taking
Data loggers are commonly
alcohol
thread
data logger
measurements outdoors.
w
for
Figure 2.3 Apparatus
bulb
measuring temperature
temperature
sensor
Alcohol thermometer
Measurement and Experimental Techniques
25
Measuring Volume
The S.I. unit for volume is the cublc metre (m>). The cublc centimetra .
and the cublc decimetre
1m
1 dm
(dm")
are
(cm)
also used.
1000 dm
1000 cm
How do we measure volumes of liquids?
Scientific experiments often involve liquids. Figure 2.4 shows
Some
apparatus commonly used to measure the volume of a liquid. The
erent
apparatus have different degrees of accuracy (Table 2.1). The apparatus w
choose for an experiment depends on the volumes we are measurine and
how accurate we need the volumes to be.
Bottled drinks are commonly
sold in litres (/) and millilitres (m/).
measuring cylinder
20C
Domletm
1 litre is equal to 1 dms.
00
Figure 2.4 Apparatus for
measuring volumes of liquids
pipette
burette
Table 2.1
Degree
of accuracy of
apparatus used
to measure volumes of
Apparatus
measuring cylinder
burette
liquids
Accuracy
measures to the nearest 0.5
cm3, e.g. 41.5 cmor 99.0 cm
accurately measures out the volume of a liquid to the
scale marked
(graduated) in 0.1 cm3 divisions
Used to deliver
different volumes of lqulds, e.g.
pipette
26
Chepter 2
nearest 0.05 cm
24.00
cm
or
38.95 cm
accurately measures out flxed volumes of liqulds, e.g. 20.0 cm or 25.0cm
How do we read the volume of a llquld?
When water or a solution is placed in a container, it forms a curved surface
called a meniscus. To read the volume of a liquid, align your eyes to the
liquid level (Figure 2.5).
correct reading
correct reading
= 25.60 cm3
25
25.60 cm3
25
Read off the
scale at the top
Read off the scale
at the bottom of
the meniscus.
of the meniscus.
| 26-
26
Let's
Explore!
From what you have
Figure 2.5
Reading the meniscus
in
a
learnt so far, have you
burette
found the answers to
Let's Explore (a) and (b)?
How do we measure volumes of gases?
used to measure the volume of
maximum volume of 1000 cm3.
syringe is
A gas
measures a
a
gas. The gas syringe
plunger
barrel
gas collected
gas from
0
10
20
30
40
50
60
70
90
80
external
100
(a) At the start of an experiment,
pushed in fully to expel any gas in
the
the
0
10
20
30
40
50
60
70
80
90
100
source
lnmubtaiatttttansbtsutssesul=
external source (e.g. gas
(b) As the gas fror
produced from a chemical reaction) enters the syringe,
plunger is
syringe.
it pushes the plunger outwards.
Figure 2.6 Measuring the volume of a gas
Test Yourself 2
KEy Ideas
What apparatus would you use to measure
Quantity
mass
time
temperature
volume
Apparatus
for
measurement
Unit
(a)
the time taken to run 100 m;
(b)
exactly 25.0 cm3 of sodium
(c)
23.6 cm3 of dilute sulfuric acid;
hydroxide solution;
electronic balance
stopwatch
thermometer
for liquids: burette, pipette
for gases: gas syringe
S
C
(d)
65 cm of carbon dioxide gas;
(e)
80.0g of water?
cm
Measurement and Experimental Techniques
27
2.2
LInk
Selecting Suitable Apparatus for
Experiments
Finding out the speed
of
reaction involves
a
Learning
measuring the time
taken fora reaction to
take place and a
change
in mass of reactants
or the volume of
gas
to: -
experimental science. You may need to plan and perform
Chemistry is
experiments to learn more about reactions. It is thus important to be able
le
produced. Find out what
to select suitable apparatus for your experiments.
apparatus and methods
are
Outcome- You should be able
select suitable apparatus for collecting a sample of gas.
used to measure the
speed of a reaction in
In some experiments, gases are given off. The gases can be collected and
Chapter 18.
identified to help us find out more about the reactions that have occurred.
The gases may also be collected for use in another reaction.
Methods for Collecting Gases
Take Note
As
a
How we collect a gas depends on the physical properties of the gas:
general guide,
solubility- how soluble the gas is in water
density- how dense the gas is compared to air
a gas is less densse
than air if its molecular
mass is less than 30.
The three methods of collecting gases are shown in Figure 2.7.
gas
delivery
gasjar
tube
gas
gas
delivery
gas jar
gas jar
tube
gas
delivery
tube
water
gas-
gas
Displacement of water
Downward dellvery is used to
is suitable for collecting
gases that are insoluble
or slightly soluble in water.
collect gases that are soluble
in water and denser than ai,
such as chlorine and
hydrogen
Carbon dioxide, hydrogen
chloride. (The collection of
or oxygen can be collected
by this method.
carried out in
poisonous gases should be
Displacement of water
a
fume
Displacement of air
Ifwe
Chapter 2
Displacement of air
upward delivery
2.7 Methods used to collect
gasees
need to collect and measure
the volume of
gas syringe.
28
less dense than air,
such as ammonia.
cupboard.)
downward deliveryy
Flgure
Upward delivery is used
to collect gases that
are soluble in water and
a
gas,
we can also
u
e
a
Tohle
Ta
some
2.2 summarises the solubilities and densities of
methods for collecting the gases.
some
gases and
Table 2.2 Solubilities, densities and methods of collection of some gases
Gas
Solubility in water
Density compared to air
hydrogen
not soluble
less dense
oxygen
very slightly soluble
slightly denser
carbon dioxide
slightly soluble
denser
chlorine
soluble
denser
hydrogen chloride
very soluble
denser
sulfur dioxide
very soluble
denser
ammonia
extremely soluble
less dense
Method of collection
displacement of water
downward delivery
upward delivery
Drying a Gas
in an experiment. We can dry a gas by
Sometimes, we need to use a dry gas
2.8 shows three sets of apparatus
passing it through a drying agent. Figure
and the chemicals used to dry gases.
dry ammonia
moist hydrogen
dry hydrogen
chloride
dry
moist
hydrogen
chloride
Note:
hydrogen
quicklime
the tube
fused caicium
chloride
introducing
the gas is
concentrated
immersed in
sulfuric acid
the acid while
moist
ammonia
the exiting
tube is not
Concentrated sulfuric acid
is used
chlorine
gases, including
chloride. However,
and hydrogen
ammonia
it cannot be used to dry
to
dry
most
because
it reacts with
Fused calcium chloride
(calcium chioride that has
been heated to remove all
traces of water) can be used
Quicklime (calcium oxide) is
used to dry ammonia.
to dry most gases.
ammonia.
Figure 2.8 Experimental set-ups used for drying gases
(ICT Link
Key Ideas
The method of
collecting
gas.
(a) Displacement of
slightly
(b)
2.
a
gas depends
water is used for
soluble in water.
delivery is used for
Downward
on
the
density
and
solubility of the
collecting gases that
collecting gases that
are
insoluble
are soluble
or
in water
and denser than ai.
that are soluble in water and
(c) Upward delivery is used for collecting gases
less dense than air.
We can collect a dry sample of gas by passing the gas through a drying agent
such as concentrated sulfuric acid, quicklime or fused calcium chloride.
Search the Internet,
using keywords like
video' and 'hydrogen gas
collection', for a video
on how hydrogen gas is
collected over water.
OLet's Explore!
From what you have
learnt so far, have you
found the answers to
Let's Explore (c)?
Measurement and Experimental Techniques
29
Test Yourself 2.2
Worked Example
Thought Process
The
apparatus shown was used
gas. What are X and Y?
to
collect
dry
a
as ammonia reacts with
Option A is incorrect
concentrated sulfuric acid. Options C and D are
incorrect because water would dissolve ammonia
gas
and carbon dioxide. Carbon dioxide does not react with
concentrated sulfuric acid. It is denser than air and is
collected by downward delivery.
Answer
x
B
Questlons
Y
concentrated sulfuric acid
ammonia
concentrated suifuric acid carbon dioxide
B
water
water
carbon dioxide
ammonia
1.
Ammonia, chlorine and hydrogen chloride cannot
be collected by displacement of water. Why?
2.
Methane is insoluble in water and is less dense
than air. Describe, with the aid of a diagram,
how you would collect a dry sample of methane
gas.
Map it
Chemistry Experiments
involve measurement of
Physical quantities
such as
Mass
Time
Temperature
Volume
involve
Units: gram (g)
Units: second (s),
kilogram (kg),
minute (min), hour (h)
tonne (t)
Apparatus: beam
Units: degree Celsius
kelvin (K)
Apparatus: stopwatch
Apparatus: thermometer,
temperature sensor
connected to data logger
balance, electronic
balance
Selecting suitable apparatus
for
Collecting gases
Displacement of water
Displacement of air
-Upward delivery
Downward delivery
Chapter 2
Units: cubic centimetre
(cm3), cubic metre (m°).
cubic decimetre (dm)
Apparatus: measuring
cylinder, burette,
pipette, gas syringe
by
30
(°C),
Get it Right
State
whether each of the following statements is true or false. Then, correct the false statement(s). Revise the
elevant sections) if you got the answer wrong or are unsure of the concepts.
relevar
True/
Statement
Section(s)
to revise
False
(a)
A pipette is used to accurately measure out 25.0 cm' of a liquid.
(b)
The volume reading of liquid in the burette is 13.50 cm.
2.1
2.1
2.2
(c)
All gases can be collected by displacement of water.
(d)
Concentrated sulfuric acid can be used to dry ammonia gas.
2.2
Answers are available at the back of the book.
Let's Review
Section A: Multiple-Choice Questions
1.
2.
measuring mass?
What is the S.I. unit for
Gram
Pound
c
4.
wanted to dry
Sim, a Chemistry teacher,
of apparatus
set
Which
some nitrogen gas.
Mr
should he use?
Kilogram
B
Tonne
D
gas
gas
gas
gas
L
What would a chemist use to accurately
measure out 25.5 cm3 of dilute
hydrochloric acid?
A
Burette
3
C
Beaker
D
Pipette
Measuring cylinder
concentrated
concentrated
sulfuric acid
sulfuric acid
D
C
3.
the
Gas X and gas Y can be collected using
can you infer
What
below.
shown
apparatus
gas
gas>
gas
gas:
about X?
delivery tube
gasX
gas jars-
concentrated
concentrated
sulfuric acid
sulfuric acid
gas Y
X is less dense than air
X is slightly soluble in water.
D
X is denser than Y.
X is less soluble in water than Y.
Measurement and Experimental Techniques
Section
1.
B:
Structured Questions
the
()
From
common laboratory
below, select the best
of
the
following
tasks.
apparatus once,
(Note: Diagrams
more
and
ammonia
apparatus
to perform eacn
You may use any of
thee
than once or not at
shown
apparatus
(1)
all.
are not
Copy the diagram shown above
complete it to show how
be dried and collected.
drawn to scale.)
can
Suggest why the volume of gas
collected
at first is not
measured
immediately.
Ammonia is less dense than air and
very
soluble in water. Draw a labelled diagram
to show how ammonia can be
(b)
collected h
displacement of air. (Only the collection
apparatus is required.)
100 cm3
100 cm3
measuring beaker
cylinder
Boiling
tube
Section C: Free-Response Question
1.
Watch glass
50 cm gas
20 cm3
syringe
pipette
working in a laboratory that
Imagine you
makes tablets for relieving indigestion. The
tablets work by reacting with water or acid in
the stomach. Carbon dioxide gas is
produced
as a result. You are asked to
determine if taking
the tablets with alcohol will affect the
speed
are
at
which the tablets work.
(a)
50 cm3
burette
2.
(a)
(b)
(c)
(d)
(e)
(
Holding
(a)
Ammonia gas
Measuring 35 cm3 of hydrogen
Measuring 21.10 cm3 of vinegar
Boiling 40 cm3 of a non-flammable
Holding 2 gof a solid for weighing liquid
5 cm3
can
heating
be formed
by
adding
concentrated ammonia solution to
sodium
hydroxide pellets in a round-bottomed flask,
as
shown in the
diagram
State the aim of your
investigation.
Describe how you will carry out
your
investigation. Include diagrams of your
experimental set-up. You can use any
of the apparatus below.
Measuring 80 cm of alcohol
of water for
(b)
o: s
2(100 cm) conical flasks
2 balloons
2
rubber bands
2 indigestion tablets
Water
Alcohol
Measuring cylinder
Stopwatch
Electronic balance
below.
add concentrated
ammonia solution
thistle funnel
round-bottomed flask
sodium hydroxide pellets
The ammonia formed is dried by
passing it
through solid calcium oxide (quicklime). Dry
ammonia is then collected in
32
Chapter 2
a
gas syringe.
Workbook Link
Worksheet 2