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Question

(1)

If you leave a piece of iron in the open for a few days, it acquires a film of brownish substance, called rust.​
(a) Do you think rust is different from iron?

(2)

If you leave a piece of iron in the open for a few days, it acquires a film of brownish substance, called rust.​

(b) Can you change rust back into iron by some simple method?

(3)

If you leave a piece of iron in the open for a few days, it acquires a film of brownish substance, called rust.​

(c) Do you think formation of rust from iron is a chemical change?

(4)

If you leave a piece of iron in the open for a few days, it acquires a film of brownish substance, called rust.​

(d) Give two other examples of a similar type of change.


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Solution

(1)
Rust is Iron oxide which is formed during rusting of Iron and has different chemical properties than Iron. Hence Rust is different from Iron.
Iron (Fe)+O2+water(H2O)
Fe2O3(rust)

(2)
Rust formation is a chemical change as the composition of the iron changes to ferric oxide and it can not be reversed using simple methods
Iron (Fe)+O2+water(H2O)
Fe2O3(rust)

(3)
Formation of rust is a chemical change because a new substance ferric oxide (rust) is formed during this process and its chemical composition is different from that of iron.
Iron (Fe)+O2+water(H2O)
Fe2O3(rust)

(4)

Examples of changes in the colour during a chemical change:​

  • Iron nail in copper sulphate solution turns green in colour-
    Iron (Fe)+CuSO4(blue)
    FeSO4(greenish)+Copper(Cu)
    Passing carbon dioxide gas into lime water turns milky due to formation of calcium carbonate (white).

CO2 Lime water [Ca(OH)2]
Calcium Carbonate (CaCO3)[white]+ water (H2O)

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