A chemical change is a change in matter that produces one or more new substances with different properties from the original substances. Chemical changes are generally irreversible. Common examples include: burning of wood, rusting of iron, cooking an egg, curdling of milk, digestion, photosynthesis, and the reaction of acids with bases. They differ from physical changes in that the chemical composition is permanently altered.
Chemical changes produce new substances with different properties from the original.
They are usually irreversible.
Examples: burning, rusting, cooking, curdling, digestion, photosynthesis, neutralisation.
Signs of chemical change: gas evolution, colour change, temperature change, precipitation, smell change.
Burning of Mg: 2Mg + O₂ → 2MgO (new white solid — MgO).
Rusting: 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ (iron oxide/hydroxide).
Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Physical changes (melting, dissolving) do not form new substances — unlike chemical changes.
Everyday chemical changes:
Burning of wood or coal: C + O₂ → CO₂ (new substance: carbon dioxide + ash)
Rusting of iron: 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ (iron changes to rust — a new substance)
Cooking an egg: Heat causes proteins to denature (irreversible change in structure)
Curdling of milk: Lactic acid produced by bacteria causes milk proteins to coagulate
Digestion of food: Enzymes break food into simpler molecules (e.g., starch → glucose)
Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (new substance: glucose)
Burning of magnesium: 2Mg + O₂ → 2MgO (new white powder formed)
Reaction of acids with bases (neutralisation): HCl + NaOH → NaCl + H₂O (new substances: salt and water)
In industry and chemistry:
Electrolysis of water: 2H₂O → 2H₂ + O₂ (water splits into new substances)
Burning of fuels: CH₄ + 2O₂ → CO₂ + 2H₂O
Souring of wine/milk: Ethanol oxidised to ethanoic acid: CH₃CH₂OH + O₂ → CH₃COOH + H₂O
Decomposition of hydrogen peroxide: 2H₂O₂ → 2H₂O + O₂
Cement hardening: CaSiO₃ and Ca(OH)₂ form new silicate compounds with water
Bleaching of cloth: HOCl + coloured compound → colourless compound (irreversible)
Reaction of baking soda with acid: NaHCO₃ + HCl → NaCl + H₂O + CO₂↑
Combustion of LPG: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Signs (indicators) of a chemical change:
Chemical change vs Physical change:
Chemical change: • New substances formed • Usually irreversible • Change in chemical composition • Examples: burning, rusting, cooking
Physical change: • No new substances formed • Usually reversible • No change in chemical composition • Examples: melting ice, tearing paper, dissolving sugar
A chemical change is a change that produces one or more new substances with different properties. Examples: burning of wood (gives CO₂ and ash), rusting of iron (gives Fe₂O₃), cooking an egg (proteins denature), photosynthesis (CO₂ + H₂O → glucose), and neutralisation (acid + base → salt + water).
Signs of a chemical change include: (1) evolution of a gas, (2) change in colour, (3) formation of a precipitate, (4) change in temperature (heat released or absorbed), (5) emission of light, (6) change in smell, (7) irreversibility of the change.
Yes, burning is a chemical change. When wood or coal burns, new substances (CO₂, H₂O, ash) are produced. The original material cannot be recovered. Heat and light are also produced — both indicators of a chemical change.
Yes, rusting of iron is a chemical change. Iron reacts with oxygen and water to form iron oxide/hydroxide (rust): 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃. The rust has completely different properties from iron, and the process is irreversible.
Chemical change: new substances are formed, usually irreversible, composition changes (e.g., burning, rusting). Physical change: no new substances, usually reversible, only shape/state changes (e.g., melting ice, tearing paper, dissolving sugar).
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