Write the points of "WHAT YOU HAVE LEARNT" from the lesson Physical and Chemical Changes.
Answers
ANSWER : There are several changes we come across daily e.g dissolving sugar in water, or flattening a metal rod by beating it. These involve changes in the form of the substance. Changes can be classified as:
(i) Physical
(ii) Chemical
Physical properties
Physical properties include size, shape, colour and state (solid/liquid/gas) of a substance.
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 a chemical change.
Usually a chemical change involves a chemical reaction, which forms new products.
Example : Rusting of Iron, or burning wood.
To know the Difference Physical and Chemical Change, visit here.
Metallic Oxides
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 colour change may occur
Rusting
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. The chemical formula for rust is Fe2O3.nH2O. More the moisture in the air, quicker the formation of rust.