Physical and Chemical Changes
Every day we come across many changes in our surroundings. These changes may involve one or more substances. Changes are classified into two categories namely physical and chemical changes. Colour, shape, size and physical sate of substance are called the physical properties. Melting of ice-cream, boiling of water, dissolution of sugar in water, melting of wax are some common examples of a physical change. These changes are reversible changes i.e. the product formed after the change can be converted into the reactant. A change in which a new substance with different properties is formed is called a chemical change. This change is irreversible in nature. Burning of paper, rusting of iron, curdling of milk, burning of coal are some examples of a chemical change. A chemical change is known as chemical reaction. Deposition of a reddish-brown substance on iron in the presence of moisture and air is called rust which is hydrated iron oxide. The process of rust formation is called rusting. Both Oxygen and water are essential for rusting. Rusting can be prevented by applying a coat of paint or grease or by galvanisation. Galvanisation is the process of depositing a layer of zinc on iron by dipping the iron object in molten zinc to prevent it from rusting. Crystallisation is the process of separating pure crystals of a substance from its hot and concentrated solution on cooling. It is a physical change.