Introduction
Matter undergoes certain changes as a result of the application of energy. Glaciers melt in summer and rivers freeze in winter. Water from saltpans on the sea coast dry up, leaving behind salt. Water from the sea evaporates to from water vapor, which form into clouds and then condense to form rain. A candle upon burning gives light, heat, water vapor and carbon dioxide. Hydrogen burns in air to form water.
Physical Change
A physical change is one in which the shape, size, appearance or state, of a substance may alter, but its chemical composition remains same. No new substance is formed. It is usually a change, which is reversible. By reversing the process, the original substance can be obtained.
Chemical Change
A change in which the composition of a substance is altered is called as chemical change. As a result, the original properties get changed and one or more new substances are formed.
Main Features of Physical and Chemical Changes
Energy Changes in a Chemical Reaction
Chemical reactions are often accompanied by evolution or absorption of energy.
Types of Chemical Changes
Decomposition It is a chemical reaction in which molecules of a substance break down to form simpler molecules of two or more new substances. Decomposition of a substance, due to heat is called thermal decomposition, while decomposition due to electricity, is called electrolytic decomposition.
Action of Heat on Substances
Heat is a very powerful form of energy. Although there are other forms of energy like light and electricity, heat is the most commonly used agent for initiating all chemical changes.
Summary
In a physical change the shape, size, appearance or state of a substance may alter, but it is a temporary change, which is usually reversible. There is no new substance formed, no change in the mass of the substance and no energy changes are involved.
