THE SI UNIT SYSTEM OF MEASUREMENT
The system of measurement used nowadays is known as the SI system of units. SI stands for International System.
BASIC UNITS
Basic units are a set of unrelated units that form the basis of the SI system of units. These quantities cannot be expressed in terms of other quantities.
|
Quantity |
Unit name |
Unit Symbol |
|
Length |
Metre |
M |
|
Mass |
Kilogram |
Kg |
|
Time |
Second |
S |
|
Temperature |
Kelvin |
K |
DERIVED UNITS
These are units obtained from combination of basic units through multiplication and division. Some of them are shown below.
|
Quantity |
Name of unit |
Unit symbol |
|
Area |
square metre |
m2 |
|
Volume |
cubic metre |
m3 |
|
Speed |
metre per second |
m/s |
|
Density |
kilogram per cubic metre |
kg/m3 |
SI PREFIXES FOR UNITS OF MEASUREMENTS
The SI prefix is an affix that is added to the name of a basic unit. It tells you whether the unit is a multiple or a fraction of the basic unit. Examples of SI prefixes are shown in the table.
|
Prefix |
Fraction or multiple |
|
nano |
10−9 |
|
micro |
10−6 |
|
mili |
10−3 |
|
kilo |
103 |
|
mega |
106 |
|
giga |
109 |
MEASURING PHYSICAL QUANTITIES
Physical quantities must be measured as accurately as possible. Different measuring instruments are used to measure the physical quantities.
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MEASURING MASS
The mass of an object is measured using a balance. The triple beam balance is the commonly used balance in the chemistry laboratory. It is shown in the figure below.
HOW TO USE THE TRIPPLE BEAM BALANCE
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Put the balance on a flat hard surface where no wind is blowing.
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Put all the masses to zero marks.
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Adjust the zeroing screw until the pointer is at zero.
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Place the object whose mass you want to measure on the pan carefully.
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Move the big mass (100g) first, gradually until the beam balance topples over. Then push the mass one step back.
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Move the second mass (10g) gradually until the beam balance topples over again and push it one step back.
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Move the smaller mass (1g) gradually until the pointer is at zero mark again.
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Read the mass by adding the numerical values of positions of all the three masses. An example is shown.
The mass of the object = (300 + 40 + 5) = 345 g
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MEASURING VOLUME
The instrument for measuring volume of a liquid is the measuring cylinder. The measuring cylinders are in different sizes such as 25 ml, 50 ml, 100 ml, 500 ml, and 1000 ml.
HOW TO READ THE VOLUME
The volume of the liquid in the measuring cylinder must be taken from the bottom of meniscus. The meniscus is the curve on the liquid caused by the liquid being slightly attracted to the glass. The meniscus must be viewed at eye level and not from an angle as shown below.
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MEASURING TEMPERATURE
Temperature is measured using an instrument called thermometer. There are different kinds of thermometers but the commonly used in the chemistry laboratory is the liquid–in–glass thermometer.
This kind of thermometer has a glass tube sealed at both ends and is partly filled with liquid like mercury or alcohol. When the liquid in the bulb is heated, it expands and the top of the liquid moves up the tube.
To measure the temperature of a liquid, suspend the tip of the thermometer below the surface of the liquid. Allow the liquid in the thermometer to expand. When it has finished expanding and it is no longer moving up the column, you can read the thermometer.
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MEASURING TIME
Time is measured using an instrument called stopwatch. A stopwatch is handheld and come in different versions. The most commonly used in the chemistry laboratory is the digital stop watch.