Every day we come across different types of changes. Turning day into night, night into a day, rising and setting of sun and moon, changing of climate, melting of ice, and lot more. These changes occur in a minute or may take a longer time. Few changes are not even noticeable.
So Many Changes
- Reversible change is that change that can be reversed by one or more methods.
- Usually, there is a change in physical properties, shape and size of the material.
- Mostly a new substance is not formed in a reversible change. For eg: folding of paper, elongation of spring etc.
- Any difference in the size or shape of an object is referred to as a change.
- Changes are either reversible or irreversible.
- The change, which is permanent and cannot be undone by any physical or chemical means is called an irreversible change.
- Formation of new substance is involved in this change.
- For example, the burning of a candle is an irreversible change as we cannot get back the candle once burnt.
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So Many Ways to Change
Expansion and Contraction
- When the temperature increases, the particle of a substance expands or becomes loose. When this happens, the material is said to undergo expansion.
- When the temperature decreases, the particles of substance contracts or becomes tight. When this happens, the material is said to undergo contraction.
- The amount of expansion or contraction varies in solids, liquids and gases.
- There are physical changes that take place when a substance or material moves from one state of matter to another.
- Water is a classic example as it can exist as either solid, liquid or gas.
- Water at very low temperatures exists as a solid known as ice. On applying heat or increasing the temperature, the ice ‘melts’ to form water. The physical change when a solid changes to liquid is called melting.
- If we keep increasing the temperature, the water now starts to boil until it fully becomes water vapour. The physical change when a liquid changes to gas is called evaporation.
- To get back the water from water vapour, it is possible by condensation – a physical change where the gas changes to a liquid. This is possible by lowering the temperature.
Anomalous expansion of water
Water on cooling contracts up to 4∘C. On further cooling, up to 0∘C, water expands rather than contracting with decrease in temperature. This means that as the temperature decreases from 4∘C to 0∘C, water expands. This behaviour is called anomalous expansion of water.
Burning is an irreversible change where a substance burns to produce new material. These new materials are ash and some gases.
For example. paper is burnt to produce ash, which is different from paper in terms of appearance and properties.
The separation of the components of a mixture or an impure substance are carried out with the following purposes :
- To remove the unuseful or harmful component.
- To obtain the useful component.
- To remove impurities for getting a pure sample.
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