Section 5: Solids, liquids and Gases

You need to know these only

a) Units
b) Density and pressure
c) Change of state
d) Ideal gas molecules

a) Units:

Examiner's expection: 5.1 Students will be assessed on their ability to Following quantities and their units needs to be understand:

use the following units: degrees Celsius (oC), kelvin (K), joule (J), kilogram (kg), kilogram/metre3 (kg/m3), metre (m), metre2 (m2 ), metre3 (m3), metre/second (m/s), metre/second2 (m/s2 ), newton (N), pascal (Pa).

S.NoQuantity with DefinitionunitSymbol represention of unit


Definition: Average Kinetic Energy of molecules.

Note: The word molecule can be replaced with either particles or atoms, both are acceptable. However, the word molecule is used rather frequently to refer atoms or particles.

It has two units

  • Celsius
  • Kelvin

Note: Kelvin is a scientific unit and must be used in all scientific texts unless told to do otherwise.

  • 0C
  • K

Note: do not put degree superscript "0" before Kelvin.



Definition: Ability to do work.

Joule J


Definition: It is a ratio of mass per unit volume.


In exams, examiners prefer formula to word definition. After all, formula defines that term more elaborately.

Density = mass / volume

Unit of mass is Kilogram.

Unit of Volume is meter cubed.

So, unit of density would be

Density= mass / volume

Symbolic representation of kilogram is Kg and that of volume is m3.


Density= kg/m3

4 Length Meter m