REVISION

Any change to the physical properties of a substance is called a physical change.  Physical changes are usually reversible as no new substance is formed. It is the same substance but with changed physical properties. 

A change in which one or more new substances are formed is called as a chemical change.  Usually a chemical change involves a chemical reaction, which forms new products.  Example : Rusting of Iron, or burning wood. 

When a pinch of baking soda is added to vinegar, we hear a hissing sound and observe the formation of bubbles. 

– Vinegar (Acetic Acid) + Baking Soda (Sodium bicarbonate) → CO2 (Carbon dioxide) + Other products 

The carbon dioxide produced during the reaction of Vinegar and baking soda, when passed through lime water gives calcium carbonate, as follows: 

– CO2 + Ca(OH)2 (lime water) → Calcium Carbonate (CaCO3) + H2O 

– The calcium carbonate turns lime water milky 

Heat or light is absorbed or given out during a chemical reaction. 

Production of sound 

Production of gases or precipitates 

Production of smell 

A color change may occur 

When substances made of Iron are exposed to oxygen and moisture in the atmosphere, it forms a red layer, which is called rust. 

The formation of rust can be represented by the following reaction:  4Fe + 3O2 → 2Fe2O3.