Physical and Chemical Changes
A change is transformation of a substance from one form to another. It may alter the physical or chemical or both the properties of the substance. Changes can be classified as desirable or undesirable, natural or man-made, periodic or non-periodic, reversible or irreversible, slow or fast and physical or chemical changes.
Desirable change is a change that is useful to us.
Undesirable change is a change that is not useful to us.
Natural change is a change that occurs naturally.
Man-made change is a change that occurs due to the action of human beings.
Reversible change is a change that can be reversed by reversing the conditions that brought about the change.
Irreversible change is a change that cannot be reversed.
Slow change is a change that takes longer time to complete.
Fast change is a change that takes place in very short interval of time.
Periodic change is a change that is repeated after a regular interval of time.
Non-periodic change is a change that is not repeated after a regular interval of time.
Physical change is a change in which the chemical composition of a substance remains the same and no new substance is formed. In this change, only the physical properties such as shape, size, colour and state of the substance change. It is a temporary change and usually can be reversed. There may or may not be any exchange of energy during this change.
Chemical change is a change in which new substance is formed whose composition and properties are entirely different from those of original substance. It is a permanent change and cannot be reversed. In this change, exchange of energy takes place in the form of heat, light or sound.
Rusting is a chemical process in which iron reacts with oxygen in presence of moisture to produce a reddish brown substance called rust.
Chemical reaction is a process that brings about a chemical change. Reactant is the original substance that undergoes chemical reaction. Product is the new substance formed as a result of chemical reaction.
A chemical change is always accompanied by some characteristics such as evolution of gas, release or absorption of energy in the form of heat, light and sound, change of colour and change in odour etc.