REVISION

REVISION

CH-6 PHYSICAL AND CHEMICAL CHANGES

PHYSICAL CHANGE

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. 

CHEMICAL CHANGE

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. 

Activity 1

• Formation of metal oxides are examples of chemical changes. They are formed by the reaction of oxygen in air. 

  – Burning of Magnesium ribbon: 

  – 2Mg + O2 → 2MgO 

  – The product formed is the oxide of magnesium, which is in the form of ash. It does not look anything like the magnesium ribbon used for burning. 

Reaction of metallic oxides with water 

• Reaction of metal oxides with water form metal hydroxides.  Example dissolving Magnesium oxide in water, by stirring the ash very well with water.  MgO + H2O → Mg(OH)2  The product formed is basic in nature and turns red litmus paper → blue 

Reaction between baking soda and vinegar 

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 

Observations that indicate a chemical change 

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 

RUST

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.