This experiment suggests that hydrogen ions in HCl are produced in the presence of water. The separation of H+
ion from HCl molecules cannot occur in the absence of water.
HCl + H2O → H3O+ + Cl–
Hydrogen ions cannot exist alone, but they exist after combining with water molecules. Thus hydrogen ions must always be shown as H+ (aq) or hydronium ion (H3O+ ). H+ + H2O → H3O+
We have seen that acids give H3O+ or H+ (aq) ion in water. Let us see what happens when a base is dissolved in water.
NaOH(s) + H2O(l)→ Na+ (aq) + OH– (aq)
KOH(s) + H2O (l) → K+ (aq) + OH – (aq)
Mg(OH)2 (s) + H2O (l) → Mg2+(aq) + 2OH– (aq)
Bases generate hydroxide ( OH– ) ions in water. Bases which are soluble in water are called alkalis.
Now as we have identified that all acids generate H+ (aq) and all bases generate OH– (aq), we can view the neutralisation reaction as follows –
Acid + Base → Salt + Water
H X + M OH → MX + HOH
H+ (aq) + OH–(aq) → H2O(l)
Let us see what is involved when water is mixed with an acid or a base.
The process of dissolving an acid or a base in water is a highly exothermic one. Care must be taken while mixing concentrated nitric acid or sulphuric acid with water. The acid must always be added slowly to water with constant stirring. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating. Reaction of concentrated sulphuric acid and on the bottle of sodium hydroxide pellets.
Mixing an acid or base with water results in decrease in the concentration of ions (H3O+/OH– ) per unit volume. Such a process is called dilution and the acid or the base is said to be diluted.