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Physical and Chemical Changes

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Physical and Chemical Changes - Lesson Summary

Change is all around us and happening constantly. Some changes are physical and some are chemical.

Physical change:  A change in the physical properties such as shape, size, color and state of a substance is a physical change.
Examples of physical changes are melting of ice, boiling of water, freezing of water, Condensation, Crystallization and heating of metals to red hot, etc.
Characteristics of a physical change:
     •  Physical changes are  generally reversible.
     •  Temporary in nature.
     •  No new substances are formed when a physical change takes place.
     •  Energy in the form of heat, light and temperature is not emitted.

Chemical Change:  A chemical change occurs when one or more substances are converted into one or more new substances.
Examples: Formation of rust on the iron bench or articles of iron kept in the open for a long time, Digestion of food, burning of magnesium ribbon, the reaction between vinegar and baking soda, etc.

Characteristics of a Chemical change:
     •  A chemical change is permanent.
     •  A new substance is formed.
     •  Forms of energy, such as heat, light or electricity, may be emitted or absorbed during a chemical reaction.
     •  A chemical change is generally irreversible

Rusting: If articles made of iron are left in open air for some time, they get coated with a brown powdery substance, called rust. This process of iron changing into rust is called rusting.
Rusting of iron occurs only if iron comes in contact with moist air.
The process of rusting can be represented as:
                            Fe + O(from air) +H2O (water) → Fe2O(rust)

Prevention of Rusting:  Rusting can be prevented by adopting any of these methods.
     •  Galvanisation - It is the process of depositing a layer of zinc on iron.
     •  Alloy formation - Stainless steel is an alloy.
     •  Painting an iron surface.   

Some chemical changes in laboratory
Burning of magnesium ribbon
When magnesium ribbon burns in oxygen it forms magnesium oxide.
             2Mg + O2 → 2MgO
If water is added to magnesium oxide magnesium hydroxide is produced.
              MgO + H2O → Mg(OH)2
The change from magnesium to magnesium oxide and then to magnesium hydroxide is an example of a chemical change

Reaction between vinegar and baking soda
Add a pinch of baking soda to this test tube which contains vinegar. You would observe a strong effervescence along with a hissing sound. This is due to the evolution of carbon dioxide gas.

Vinegar (acetic acid) + Baking soda (sodium hydrogen carbonate) → Carbon dioxide + water + Other substances
This reaction between vinegar and baking soda is a chemical change.

When carbon dioxide gas is passed through freshly prepared limewater turns it milky.

Carbon dioxide (CO2) + Lime water [Ca(OH)2] → Calcium Carbonate (CaCO3) + Water (H2O)
This reaction between carbon dioxide and lime water is a chemical change.

Reaction between copper sulphate solution & iron nail
Put an iron nail in blue colour copper sulphate solution and wait for half an hour. The solution turns light green and the iron nail acquires a brown colour deposit on it, which is of copper.

Copper sulphate solution (blue) + Iron → Iron sulphate solution (green) + Copper (brown deposit)
This change in colour of the solution is a chemical change.


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