what is physical change, chemical change, slow change, fast change, periodic change, non-periodic change,  desirable change and undesirable change ?

1) Some changes affect only the physical properties of the substance that is undergoing change. Physical properties include shape, size, colour and state of matter.

• Some properties of physical changes are:

A physical change is temporary.
No new substance is formed when a physical change takes place.
Forms of energy, such as heat, light and electricity, are neither emitted nor absorbed in a physical change.
A physical change is generally reversible. 

2)Some changes are in the form of chemical reactions, which lead to the formation of new substances. These are called chemical changes. These changes may be accompanied by the emission of heat, light or sound, a change in colour, or the formation of a gas.