NEET Chemistry Formula Sheet 2027

1.4 MEASUREMENT OF PROPERTIES AND ATOMIC MASS

The properties of a substance may be grouped into physical properties, which can be observed without changing the identity of the substance, and chemical properties, which appear only when the substance takes part in a chemical change. Most properties of interest to the chemist – mass, volume, temperature, density – are quantitative, and a measured value must always be written as a number together with its unit.

Density is the mass contained in unit volume of a substance. The SI unit, kg m⁻³, is inconveniently large for laboratory work, so densities are commonly quoted in g cm⁻³. A high density indicates that the particles of the substance are packed closely together.

Atoms are far too small to be weighed individually on a balance, and for this reason their masses are compared with that of a standard atom. The standard now accepted is the carbon-12 isotope, and one atomic mass unit is defined as exactly one-twelfth of the mass of one atom of carbon-12.

The molecular mass of a substance is obtained by adding the atomic masses of all the atoms present in one molecule of it, each atomic mass being multiplied by the number of atoms of that element in the molecule.