Chemical Reactions - ICSE CLASS 9

 ICSE CHEMISTRY CLASS 9 CHAPTER 2 CHEMICAL REACTIONS

Chemical Reaction

It is a chemical change in which rearrangement of atoms takes place which leads to the formation of new substance/s.

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Chemical reaction is associated with the following external indications.

1. Evolution of a gas

2. Colour change

3. Release of  heat

4. Formation of a precipitate

1. Reactions associated with Evolution of a gas

When dilute acid is added to an active metal releases a colourless gas.

example:

Zn + 2HCl → ZnCl₂ + H₂ (gas)

2. Reactions which involve a colour change

A Copper carbonate (green) on heating turns to Copper oxide ( black) 

CuCO₃ (Green) → CuO (black) + CO₂

3. Reactions which liberates heat

When quick lime is dissolved in water gives Calcium hydroxide along with heat energy.

CaO + H₂O →  Ca(OH)₂ + Heat

4. Reactions in which precipitate is formed

Silver nitrate solution when added to Sodium chloride solution gives a white precipitate of Silver chloride.

AgNO₃ + NaCl →  NaNO₃ + AgCl↓

The conditions required for a  chemical reaction to take place

1. Chemical Reactions require heating to take place

CuCO₃ (Green) → CuO (black) + CO₂

2. Chemical reaction takes place on passage of electric current

2H₂O → 2H₂ + O₂ 

water undergoes decomposition on passage of electricity

3. Some Chemical reaction takes place when subjected to sound

C₂H₂→  2C + H₂

Ethyne decomposes into Carbon and Hydrogen when it is subjected to sound.

4. Some Chemical reaction which take place by the absorption of light

2AgCl →  2Ag + Cl₂

Silver salts when exposed to light undergo photolytic decomposition. Hence they are stored in dark coloured bottles.