NCERT Summary - Changes Around Us
Category : 6th Class
Changes Around Us
(i) Slow and fast changes
(ii) Desirable and undesirable changes
(iii) Periodic and non-periodic changes
(iv) Reversible and irreversible changes
(v) Physical and chemical changes
(i) Slow and fast changes: Slow changes take place over days, months or years. For example, rusting of iron nails, germination of seeds, ripening of fruits, etc.
Fast changes occur within a short span of time. For example, spinning of a top, burning of a matchstick, curdling of milk by adding lemon juice, etc.
(ii) Desirable and undesirable changes: The changes that are beneficial or desirable for us are called desirable changes. For example formation of curd from milk, formation of manure from cow dung and dead plants. But there are some changes that may be undesirable or harmful. These are called undesirable changes.
For example-the burning of a factory, flooding of a river, rotting of food stuff, etc.
(iii) Periodic and non-periodic changes: Changes that occur again and again after a fixed interval of time and their recurrence can be predicted are called periodic changes. For example - winter, summer, autumn, spring and rainy seasons recur each year, the waxing and waning of the moon nights recur each month, generation of high and low tides in the sea, etc.
Changes that do not repeat themselves at regular intervals of time and cannot be predicted are called nonperiodic changes. For example - train accidents, the occurrence of earthquakes, landslides, sneezing, etc.
(iv) Reversible and irreversible changes: If a change can be reversed, it is called a reversible change. For example, ice changes into water on heating whereas on cooling water changes back to ice. Similarly, when we put a weight on a rubber band or a spring, it stretches; but when we remove the weight, it comes back to its original shape.
If a change cannot be reversed, it is called irreversible change. For example, when coal is burnt, it changes into ash and smoke and we cannot get back coal from ash and smoke. Ageing, changing of milk into curd are some other examples.
(v) Physical and chemical changes
A change in which new substances are formed is called a chemical change. For example, when paper bums, carbon dioxide, water vapour and ash are produced. None of them have properties of paper.
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