Experimental Techniques and Measurement

A. Measurement

The unit used in physical quantities in the experiment chemistry are those the metric system. The metric unit use for scientific measurement are called the International System of Unit (SI unit). We have seven base units physical quantities which all other units are derived :

Table 1.1 Physical Quantities

Physical Quantities	Name of Units	Abbreviation
Mass	Kilogram	Kg
Length	Meter	m
Time	Second	s
Temperature	Kelvin	K
Amount of Substance	Moles	mole
Electricity	Ampere	A
Luminous Intensity	candela	cd

In the experiment chemistry in laboratory, specific apparatus are needed to measure :

Time
Temperature
Mass
Volume

A.1 Measurement of Time

The fundamental unit of time is second (s). In the other unit of time are minutes (min) and hour (h). Time is measured using stopwatch. In the laboratory has 2 type of stopwatch which is analogue and digital stopwatch.

Picture 1. 1 Analogue and Digital Stopwatch

A.2 Measurement of Temperature

Two temperature scales uses in physical sciences are Kelvin (K) and Celsius (⁰C). The Kelvin scales uses in the SI unit for temperature (K) whereas the Celsius scales uses in the non SI units for temperature (⁰C). Kelvin scales start from 0 K and Celsius start from -273 ⁰C. The lowest possible temperature 0K is known as absolute zero .The temperature of a substance is measured using a thermometer :

Mercury in glass thermometer (-10 ⁰C to 110 ⁰C or 0 ⁰C to 360 ⁰C)
Alcohol in glass thermometer
Electronic thermometer (temperature sensor)

Table 1.2 Differences between Mercury and Alcohol Thermometer

Mercury Thermometer	Alcohol Thermometer
Freezing point : -39 ⁰C Boiling point : 257 ⁰C	Freezing point : -115 ⁰C Boiling point : 78 ⁰C
It does not wet the tube	It wet the tube
Opaque Easy to read	Colorless It needs to be dyed
poisonous	Safe liquid
expensive	cheap
Conduct heat well, responds faster to temperature changes	Responds more slowly than thermometer mercury

Temperature sensor with data logger give more accurate reading of temperature than thermometer mercury and alcohol. Data logging is useful for recording temperature changes continuously over a period of time. The data logger can record the changing temperature of a substance that is being heated or cooled.

Picture 1.2 Temperature Mercury, Alcohol and Electronic

A.3 Measurement of Mass

The SI units used for measuring mass is the kilogram (kg). Often, very small masses of substances are expressed in gram (g) and milligram (mg). the mass of substance is measured with beam balance and electronic balance. Beam balance give mass reading accurate to 0.1 gram whereas electronic balance can give very accurate mass readings up to 0.01 gram.

Picture 1.3 Apparatus for Measuring Mass

A.4 Measurement of Volume

The basic SI units for volume is cubic meter (m³). The volume of a substance is measured using the following apparatus :
Beaker and conical flask are used to measure approximate volumes of liquid e.g. 100 cm³
Measuring cylinder are used to measure up to the nearest e.g. 99 cm³. More accurate than beaker
Burette measure accurate volume of a liquid to the nearest 0.05 cm³
Syringe are used to measure volume of a gas. A gas syringe can measure up to 100 cm³ of a liquid.
Volumetric flask are used to contain an accurate volume of a liquid. These flask have capacities ranging from 50 cm³ to 5 dm³

Picture 1.4 Apparatus for Measuring Volume

How should we read the volume of liquid ?

When water or solution is placed in a glass container, it forms a curved surface called meniscus. A meniscus may be concave or convex. To read the volume of liquid, align your eyes to the liquid level. If the meniscus is concave, read off the scale at the bottom of meniscus. If the meniscus is convex, read off the scale at the meniscus instead.

Picture 1.5 Reading Meniscus in a Burette

B. Collecting Gases and Measuring Volume of Gases

In the experiment, gases are given off. The gases can be collected and identified to help us find out more about the reactions that have occurred.

What method are used for collecting gases?

The method for collecting the gases its depends of the properties of the gas. Two type properties of gas are

Solubility – how soluble the gas in the water
Density – how dense the gas is compared to air

Table 1.3 The solubility and Density of some common gases

Gas	Solubility in water	Density compared to air
Ammonia	Extremely soluble	Less dense
Carbon dioxide	Slightly soluble	Denser
Chlorine	Soluble	Denser
Hydrogen	Not soluble	Less dense
Hydrogen chloride	Very soluble	Denser
Oxygen	Very slightly soluble	Slightly denser
Sulphur dioxide	Very soluble	denser

Three method for collecting the gases :

Displacement of water

This method is suitable for collecting the gases that are insoluble or slightly soluble in the water. The gas that usually collected by this method are carbon dioxide, hydrogen or oxygen

Picture 1.6 Displacement of water

Downward Delivery

Downward delivery should be used to collect gases that are soluble or insoluble in the water but the gas its supposed denser than air, such as chlorine and hydrogen chloride.

Picture 1.7 Downward Delivery

Upward Delivery

Upward delivery is used to collect the gases that are soluble or insoluble in the water and less dense than air, such as ammonia.

Picture 1.8 Upward Delivery

How do we collect a dry sample of gas?

Sometimes, we need to use a dry gas in an experiment. We can dry a gas by passing it through a drying agent. Some commonly used drying agents are concentrated sulfuric acid, quicklime (calcium oxide) and fused calcium chloride.