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INTRODUCTION:
| PHYSICAL CHANGE | CHEMICAL CHANGE |
| Processes in which no new chemical substances are formed are called physical changes.
Example: 1. Melting of ice 2. Evaporation of water | Processes in which a new chemical substance with different properties is formed are called physical changes. Example: 1. Rusting of iron 2. Souring of milk in summer 3. Burning of coke in air 4. Cooking of food |
- CHEMICAL REACTION: The process in which new substances with new properties are formed from one or more substances is called Chemical Reaction
Examples of Chemical reaction:
- Digestion of food
- Respiration
- Rusting of iron
- Burning of Magnesium ribbon
- Formation of curd
REACTANT: The substances which take part in a chemical reaction are called Reactants.
- They are written on the L.H.S (Left Hand Side) with a plus sign (+) between them.
PRODUCTS: The substances which are formed in a chemical reaction are called Products.
- They are written on the R.H.S (Right Hand Side) with a plus sign (+) between them.
2Mg + O2 à 2 MgO
Reactants Product
- CHARACTERISTICS of Chemical reaction
OR
How can we say that a chemical reaction has taken place?
The observable changes that take place in a chemical reaction are called characteristics of a chemical reaction. Such changes help us to identify whether a chemical reaction has taken place OR not.
- Change in state – Some chemicals reaction occurs with a change in state of the reactants.
Ex: SOLID wax burns to from a water vapour and carbon dioxide which are gaseous.
- Change in colour - – Some chemical reactions are accompanied with a change in colour.
Ex:
- Reaction between Lead nitrate Pb(NO3)2 and Potassium iodide (KI)
Lead nitrate Pb(NO3)2 and Potassium iodide (KI) are initially colourless but on mixing the two a YELLOW precipitate of LEAD IODIDE is formed.
- Change in temperature
Ex:
When water is added to quick lime (CaO), taken in a beaker, slaked lime Ca(OH)2, is formed and the beaker is found to be warm.
- Evolution of gas
Ex:
- When Zinc pieces (Zn) are added to dilute sulphuric acid or hydrochloric acid, hydrogen (H2) gas is evolved.
- When Calcium carbonate (CaCO3) is heated it decomposes into Calcium oxide (CaO) with the evolution of carbon dioxide gas (CO2).
- Formation of a precipitate
Some chemical reactions are accompanied by the formation of a precipitate.
Ex:
- In the reaction between Lead nitrate Pb(NO3)2 and Potassium iodide (KI) a YELLOW precipitate of LEAD IODIDE is formed.
- When BARIUM CHLORIDE (BaCl2) solution is mixed with SODIUM SULPHATE Na2SO4 a WHITE precipitate of BARIUM SULPHATE BaSO4 is formed.
