Changes Around Us Class 6 Notes - Chapter 6

According to the CBSE Syllabus 2023-24, this chapter has been removed from NCERT Class 6 Science textbook.

Every day we come across different types of changes. Turning day into night, night into day, rising and setting of the sun and moon, changing of climate, melting of ice, and a 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

• Reversible change is that change that can be reversed by one or more methods.
• Usually, there is a change in the physical properties, shape and size of the material.
• Mostly a new substance is not formed in a reversible change. For example, folding of paper, elongation of spring etc.

Change

• Any difference in the size or shape of an object is referred to as a change.
• Changes are either reversible or irreversible.

Irreversible change

• The change, which is permanent and cannot be undone by any physical or chemical means, is called an irreversible change.
• The formation of new substances 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.

For more information on Reversible and Irreversible Changes, watch the below video

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Reversible and Irreversible Changes

Chemical Changes: Reversible and Irreversible Changes

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 the substance contract or become 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 change 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 a decrease in temperature. This means that as the temperature decreases from 4∘C to 0∘C, water expands. This behaviour is called the anomalous expansion of water.

Burning

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.

Separation

The separation of the components of a mixture or an impure substance is carried out for the following purposes :

1. To remove the unuseful or harmful components.
2. To obtain the useful component.
3. To remove impurities for getting a pure sample.

Learn more about Changes around us from the topics given below: