Properties such as shape, size, state of the substance and its colour are known as physical properties. Physical properties are affected by the physical change.
For Example, when ice changes to water, there is a change in the physical state of ice, i.e. it changes from solid to liquid. If we cool the water again, ice will be formed making it a reversible reaction. Also, no new substance is formed in the above physical change.
A change in which one or more new substances are formed is known as chemical change.
For Example, when the iron is exposed to air and moisture, rust formation takes place. Rust is nothing but Iron Oxide; a new substance formed out of the reaction. The colour of the surface of the iron also changes. Hence, rusting of iron is a chemical change.
Rusting of Iron
When substances made of iron are exposed to oxygen and moisture (water), rusting takes place. Rusting removes a layer of material from the surface and makes the substance weak. Rusting is a chemical change.
Chemical reaction taking place during rusting is shown below.
Rusting is undesirable and methods are used to avoid rusting.
The process of depositing zinc on the iron to prevent rusting is known as galvanisation. A striking example of the use of galvanisation is the water pipes used in houses.
A physical process of obtaining large crystals of a pure substance from its solution is known as crystallization. Crystallization is a physical change.
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