Mention the changes that are always, desirable, undesirable and periodic.

Desirable change: The changes which are desired(need) to be changed are called desirable changes. e.g. change milk into curd.

Undesirable change: The changes which are not desired to change are called undesirable changes., e.g. floods and epidemics are undesirable changes.

Periodic: Changes that are repeated at regular intervals of time are called periodic changes, e.g. change of day and night.