Definitions [3]
Define a chemical change. Give two examples of the above changes.
A permanent change in which new substances are formed whose composition and properties are completely different from those of the original
substances.
Chemical change:
(i) Burning of wood.
(ii) Breathing.
Define physical change. Give two examples for the above changes.
Physical change: A temporary change in which no new substance is formed, the composition of substance remains the same, though its state, shape and size may change.
Example of physical change:
- Change of water into its vapours.
- Heating of iron rod.
Definition: Corrosion
Corrosion is the gradual damage of metals caused by their reaction with components of the atmosphere, such as oxygen and moisture.
Key Points
Key Points: Crystallisation Method
- Based on difference in solubility of the compound and impurities at different temperatures.
- Compound more soluble in hot solvent → dissolved at high temperature → cooled → pure crystals form → impurities remain in solution (mother liquor).
- Common solvents: water, alcohol, ether, chloroform, acetone, benzene, petroleum ether.
- Example: Benzoic acid (slightly soluble in cold water, completely soluble in hot water) → separated from naphthalene.
- Fractional crystallisation: Used when two compounds have different solubilities in the same solvent → less soluble crystallises first on cooling (e.g., K₂CO₃ and KCl).
Concepts [4]
- Classification of Change: Physical Changes
- Experiment of Chemical Change
- Corrosion of Metals
- Crystallisation Method
