In a physical change, no new substance is formed. In a chemical change, new substance is formed.

Law of conservation of mass.
Mass can neither be created nor destroyed during a chemical reaction.

The following reaction is an example of a double displacement reaction:
2NaOH (aq)     +    H₂SO₄ (aq)      \(\rightarrow\)  Na₂SO₄(aq)       +   2H₂O (l)
Sodium hydroxide  Sulphuric acid          Sodium sulphate     Water

The colour of copper sulphate solution changes when an iron nail is dipped in it because iron being more reactive than copper, displaces copper metal from aqueous copper sulphate solution. Thus, blue colour of copper sulphate fades away to give green colour solution of ferrous sulphate.
Fe (s) + CuSO₄(aq) \(\rightarrow\) FeSO₄(aq) + Cu(s)
Grey       Blue                  Green         Brown       

(i) BaCl₂(aq) + Na₂SO₄ (aq) \(\rightarrow\) BaSO₄(s)\(\downarrow\) + 2NaCl(aq)
(ii) NaOH (aq) + HCl (aq) \(\rightarrow\) NaCl (aq) + H₂O(l)

(a) Zinc sulphate and copper is formed when zinc reacts with copper sulphate,
\(\underset{Zinc}{Zn( s)} + \underset{Copper\ sulphate}{CuSO_4} (aq) \rightarrow \underset{Zinc\ sulphate}{ZnSO_4(aq)} + \underset{Copper}{Cu}\)
(b) Hydrochloric acid forms aluminium chloride and hydrogen gas when reacts with aluminium metal.
\(2AI + 6HCI\rightarrow 2AlCl_3 + 3H_2\)
(c) When silver metal is added to copper sulphate solution, no reaction takes place because silver is less reactive than copper

(i) CaCO₃(s) + 2HCl(aq) \(\longrightarrow \) CaCl₂(s)+ H₂O(I) + CO₂(g)
(ii) 2Al(s) + 6HCl(aq)\(\longrightarrow \) 2AlCI₃(aq) + 3H₂(g)
(iii) MnO₂ + 4HCl(aq) \(\longrightarrow \) MnCl₂ + 2H₂0(I) +Cl₂(g)

(i) Substance X-Calcium Hydroxide.
Ca(OH)₂(aq) + CO₂(g)\(\longrightarrow \) CaCO₃(s) + H₂Cl
                                             (White ppt.)
(ii) Calcium hydroxide is obtained by reaction of calcium oxide and water.
CaO(s) + H₂O(l) \(\longrightarrow \)Ca(OH)₂(aq) + Heat

(i) Before heating, it is pale green.
After heating, it is brown or reddish brown.
Two observations are:
(a) Change in state and colour
(b) Evolution of gas
(ii) Decomposition reaction
2FeSO₄(s) \(\overset { Heat }{ \rightarrow } \) Fe₂O₃(s) + S0₂(g) + SO₃(g)