Every substance has unique or characteristic properties. These properties can be classified into two categories – physical properties and chemical properties.

Physical properties are those properties which can be measured or observed without changing the identity or the composition of the substance.

Some examples of physical properties are colour, odour, melting point, boiling point, density etc.

The measurement or observation of chemical properties requires a chemical change to occur. The examples of chemical properties are characteristic reactions of different substances; these include acidity or basicity, combustibility etc.

Measurement of physical properties :

Many properties of matter such as length, area, volume, etc.are quantitative in nature. Any quantitative observation or measurement is represented by a number followed by units in which it is measured. For example, the length of a room can be represented as 6 m; here 6 is the number and m denotes metre – the unit in which the length is measured.

The International System of Units (SI) :

The SI system has seven base units and they are listed in Table 1.1. These units pertain to the seven fundamental scientific quantities. The other physical quantities such as speed, volume, density etc. can be derived from these quantities.

Mass and Weight :

Mass of a substance is the amount of matter present in it while weight is the force exerted by gravity on an object. The mass of a substance is constant whereas its weight may vary from one place to another due to change in gravity. You should be careful in using these terms.

Volume:

Volume has the units of (length)³. So in the SI system, the volume has units of m³. But again, in chemistry laboratories, smaller volumes are used. Hence, the volume is often denoted in cm³ or dm³ units.

A common unit, litre (L) which is not an SI unit, is used for measurement of the volume of liquids.

1 L = 1000 mL , 1000 cm³ = 1 dm³ . In the laboratory, the volume of liquids or solutions can be measured by graduated cylinder, burette, pipette etc. A volumetric flask is used to prepare a known volume of a solution.

Density :

The density of a substance is its amount of mass per unit volume. So SI units of density can be obtained as follows:

$\mathrm{S.I\, unit\, of\, density= \frac{S.I\, unit \, of \, mass}{S.I\, unit \, of \, volume}}$

= kg m^-3

This unit is quite large and a chemist often expresses density in g cm^-3. where mass is expressed in grams and volume is expressed in cm^-3.

Temperature :

There are three common scales to measure temperature -^oC (degree Celsius), -^oF (degree Fahrenheit) and K (Kelvin). Here, K is the SI unit. The thermometers based on these scales. Generally, the thermometer with the Celsius scale is calibrated from 0^o to 100^o where these two temperatures are the freezing point and the boiling point of water respectively. The Fahrenheit scale is represented between 32^oto 212^o.

The temperatures on two scales are related to each other by the following relationship:

^oF = (9/5)(^oC) + 32

The Kelvin scale is related to the Celsius scale as follows :

K =^oC + 273.15

It is interesting to note that temperature below 0 °C (i.e. negative values) are possible in the Celsius scale but in the Kelvin scale, the negative temperature is not possible.