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.

Answer 1

(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.
$\text{Iron (Fe)+}{O}_{2_{+}}\text{water}\left(H_{2}O\right)$
$\to F{e}_2{O}_3\text{(rust)}$

(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
$\text{Iron (Fe)+}{O}_{2_{+}}\text{water}\left(H_{2}O\right)$
$\to F{e}_2{O}_3\text{(rust)}$

(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.
$\text{Iron (Fe)+}{O}_{2_{+}}\text{water}\left(H_{2}O\right)$
$\to F{e}_2{O}_3\text{(rust)}$

(4)

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

• Iron nail in copper sulphate solution turns green in colour-
  $\text{Iron (Fe)}+CuS{O}_4\text{(blue)}\to$
  $FeS{O}_4\text{(greenish)+Copper}\left(Cu\right)$
  $\bullet$ Passing carbon dioxide gas into lime water turns milky due to formation of calcium carbonate (white).

$\bullet$ $C{O}_2\text{Lime water}\left[Ca\right(OH{)}_2]$
$\to \text{Calcium Carbonate}\left(CaC{O}_3\right)\left[\text{white}\right]+\text{water}\left(H_{2}O\right)$