The reaction between an acid and base to form salt and water is called neutralisation reaction.
Acid + Base \(\rightarrow\) Salt + Water
e.g. HNO₃(aq) + KOH(aq) \(\rightarrow\) KNO₃ (aq) + H₂O(l)
HCl(aq) + NaOH(aq) \(\rightarrow\) NaCl(aq) + H₂O(l)

pH will decrease from 6 because during curd formation, lactic acid is produced which makes it acidic. When milk changes into curd, its pH changes and becomes less than 6.

(i) Reaction occurs in test tube A
2HCl(aq) + Mg(s) \(\rightarrow\) MgCl₂(aq) + H₂(g)
(ii) Reaction occurs in test tube B
2CH₃COOH (aq) + Mg(s) \(\rightarrow\) (CH₃COO)₂Mg(aq) + H₂(g)
Fizzing occurs in both the test tubes due to the evolution of H₂ gas but it is faster in test tube A because of the greater extent of dissociation of HCI (strong acid) as compared to B as CH₃COOH is not completely dissociated (weak acid).

lons are produced only in aqueous medium and presence of H⁺ ions are responsible for the existence of acidic properties. Hence, acids show acidic behaviour only in the presence of water and not in its absence.

(i) Dilute sulphuric acid + Zinç Granules \(\rightarrow\) Zinc sulphate + Hydrogen gas
H₂SO₄(aq) + Zn(s)  \(\rightarrow\) ZnSO₄ (aq) + H₂(g)
  Dilute          Granulated
(ii) Dilute hydrochloric acid + Magnesium \(\rightarrow\) Magnesium chloride + Hydrogen gas
2HCI(aq) + Mg(s) \(\rightarrow\) MgCl₂(aq) + H₂(g)
   Dilute
(iii) Dilute sulphuric acid + Aluminium \(\rightarrow\) Aluminium sulphate + Hydrogen gas
3H₂SO₄ (aq) + 2Al(s) \(\rightarrow\) Al₂(SO₄)₃(aq) + 3H₂(g)
        Dilute
(iv) Dilute hydrochloric acid + Iron \(\rightarrow\) Iron (II) chloride +Hydrogen gas
2HCI(aq)+ Fe (s) \(\rightarrow\)FeCl₂(aq)+ H₂(g)
    Dilute