ICSE Chemistry Class 8 Notes Chapter 2 Physical and Chemical Changes
→ Matter changes its state when energy is added or removed from it. i.e. on heating or on cooling. Mostly matter expands on heating and contracts on cooling.
→ Physical change : “Composition does not change, energy is not absorbed not produced, temporary, reversible, no new substance is formed.” e.g. breaking of glass, change of ice into water, ringing of electric bell, heating of platinum wire and magnetisation.
→ Chemical change : “A permanent change, change in composition, energy is evolved or needed, new substance with new properties is formed, irreversible”, e.g. burning of wax, ripening of fruits, respiration, sprouting of seeds, photosynthesis.
→ Reversible change : “A change which can be reversed by change in conditions.” i.e. melting of ghee, melting of iron, melting of ice, change of water vapours to water on cooling.
→ Irreversible change : “A change which cannot be reversed by changing conditions.” i.e. burning of paper, rusting of iron.
→ Slow change : “A change which occurs veiy slowly over days or years.” e.g. change of seed into plant.
→ Fast change: “A change which takes place rapidly in seconds.” e.g. burning of paper.
→ Periodic change : “A change which occurs periodically at regular intervals.” e.g. swinging of pendulum, rise of day and fall of night.
→ Non-periodic change : “A change which does not take place periodically at regular interval.” e.g. change of weather, occurance of earth quake.
→ Desirable changes : “Changes that we want to happen and are brought about by man or nature.” e.g. lighting of electric bulb at night, making of glass, fall of rain on hot day.
→ Undesirable changes : “Changes which we do not wish to happen or which produce useless results.” e.g. falling ill of man, breaking of glass, accident to take place.
→ Comparison of physical change and chemical change. Physical change : No change in composition, temporary, no change in mass, no gain in energy or loss of energy. Chemical change : Change in composition, mass and permanent change irreversible, energy is either absorbed or evolved.