VII B SCIENCE 4.PHYSICAL AND CHEMICAL CHANGES( TOPIC THOUGHT AND HOMEWORK)

 TOPIC :CHEMICAL CHANGES 

DEFINITION:

A change in which a substance undergoes a change in its chemical properties is called a chemical change. New substances are formed. For rusting, the presence of both oxygen and water (or water vapour) is essential.

• 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.

 

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

EXAMPLES 

• Rusting of iron in presence of moisture and oxygen.
• Burning of wood.
• Milk becoming curd.
• Formation of caramel from sugar by heating.
• Baking of cookies and cakes.
• Cooking any food.
• Acid-base reaction.
• Digestion of food.

 

 

 

• LINK: https://youtu.be/IvdhVqyyZdk

Crystallisation

 
– The process of separation of salts from their solution is called as crystallisation. It is a purification technique that purifies seawater or separates crystals from impure samples. It is a physical change.

To know more about Crystallisation

LINK:https://youtu.be/ohtrnIODkAM

Crystallisation: A physical change

The process of deriving large crystals of pure substances from their solutions is known as crystallisation. It is a physical change. Crystals of salt are obtained by the evaporation of sea water.

Similarly crystals of copper sulphate can be obtained by adding copper sulphate powder to the boiling solution of water and dilute sulphuric acid. This mixture when filtered and allowed to cool produces crystals of copper sulphate.

 

The process of crystallization is an example of a physical change. By obtaining crystals, we are able to purify certain substances. Pure sugar, urea, copper sulphate,  alum and table salt form crystals.
The salt obtained by evaporation consists of many impurities such as sand, magnesium chloride etc. The impure salt can be purified by the crystallization process.

• For crystallization a large amount of salt is dissolved in water and allowed to boil to obtain a saturated solution.
• Filter the saturated solution to remove impurities.

 

 

 

HOME WORK:READ THE TOPIC OF CHEMICAL CHANGES FROM TEXTBOOK.