Chemical Changes & Reactions | ICSE Class 9 Chemistry

Chemical Changes & Reactions

Chapter 2 | Section 1: Conditions & Characteristics

1. Conditions Necessary for Chemical Changes

A chemical reaction occurs only when reactants are in close contact or under specific conditions:

Mixing/Contact: Reactions occur when substances are brought together. E.g., Iodine and Phosphorus react when mixed.
Solution: Some reactions only occur in an aqueous state. E.g., $AgNO_3(aq) + NaCl(aq) \rightarrow AgCl \downarrow + NaNO_3(aq)$.
Heat: Many reactions require thermal energy to start. E.g., $2HgO \xrightarrow{\Delta} 2Hg + O_2 \uparrow$.
Light (Photochemical Reactions): Reactions triggered by light. E.g., $H_2 + Cl_2 \xrightarrow{\text{Sunlight}} 2HCl$.
Electricity (Electrochemical Reactions): Decomposition using electric current. E.g., $2H_2O \xrightarrow{\text{Elec.}} 2H_2 + O_2$.
Pressure: Some gaseous reactions require high pressure. E.g., Haber's process for Ammonia.
Catalyst: A substance that alters the rate of reaction without being consumed.

A chemical reaction is often accompanied by observable changes:

Evolution of Gas: E.g., Zinc reacting with dil. $H_2SO_4$ evolves Hydrogen gas ($H_2 \uparrow$).
Change of Color: E.g., Heating Copper Carbonate (green) turns it into Copper Oxide (black).
Formation of Precipitate: An insoluble solid settles out. E.g., White precipitate of $BaSO_4$.
Change of State: Burning a candle (solid wax) produces $CO_2$ gas and water vapour.
Energy Change: Exothermic (heat released) or Endothermic (heat absorbed).