A chemical reaction is a process in which one or more substances (called reactants) are converted into new substances (called products) with different physical and chemical properties.
During a chemical reaction:
- Old bonds between atoms break.
- New bonds are formed.
- A new substance is produced.
Chemical reactions are represented using chemical equations and are often accompanied by observable characteristics that help identify them.
1. Evolution of a Gas
Some chemical reactions are identified by the production of a gas, which may be observed as bubbles or detected by its odour.
Example 1: Sodium sulfite reacts with dilute hydrochloric acid to produce sulfur dioxide gas.
Na2SO3 + 2HCl ⇢ 2NaCl + H2O + SO2
Example 2: Zinc reacts with sulfuric acid to produce hydrogen gas.
Zn + H2SO4 ⇢ ZnSO4 + H2
2. Formation of a Precipitate
A precipitate is an insoluble solid that separates out from a solution during a chemical reaction. Its formation is a clear indicator that a chemical change has occurred.
Example 1: Copper sulfate reacts with sodium hydroxide to form a blue precipitate of copper hydroxide.
CuSO4 + 2NaOH ⇢ Na2SO4 + Cu(OH)2
Example 2: Silver nitrate reacts with sodium chloride to form a white precipitate of silver chloride.
AgNO3 + NaCl ⇢ NaNO3 + AgCl
3. Change in Colour
A visible change in colour during a reaction indicates the formation of a new substance with different chemical properties.
Example 1: Citric acid reacts with potassium permanganate solution, causing a colour change from purple to colourless, as citric acid acts as a reducing agent.
2MnO4− + 5C6H8O7 + 6H+→2Mn2++ 5C6H6O7 + 8H2O
Example 2: Sulfur dioxide reacts with acidified potassium dichromate solution, causing a colour change from orange to green.
SO2 + K2Cr2O7 + 3H2SO4 ⇢ K2SO4 +Cr2(SO4)3 + 3H2O
4. Change in Temperature
Chemical reactions are often accompanied by a change in temperature. Reactions that release heat are called exothermic, while those that absorb heat are called endothermic.
Example 1 (Exothermic): Quicklime reacts with water to form slaked lime, releasing a large amount of heat.
CaO + H2O ⇢ Ca(OH)2 + Heat
Example 2 (Endothermic): Barium hydroxide reacts with ammonium chloride, absorbing heat and causing a decrease in temperature.
Ba(OH)2 + 2NH4Cl→BaCl2 + 2NH3 + 2H2O
5. Change in State
Some chemical reactions result in a change in the physical state of the substances involved, indicating the formation of new products.
Example: During the combustion of candle wax, the solid wax transforms into carbon dioxide gas and water vapour, accompanied by the release of heat and light.
CXHY + O2 ⇢ CO2 + H2O + Heat and Light
