In Section 2.1 we have seen that all acids have similar chemical properties. What leads to this similarity in properties? We saw in Activity 2.3 that all acids generate hydrogen gas on reacting with metals, so hydrogen seems to be common to all acids. Let us perform an Activity to investigate whether all compounds containing hydrogen are acidic.
The bulb will start glowing in the case of acids, as shown in Fig. 2.3. But you will observe that glucose and alcohol solutions do not conduct electricity. Glowing of the bulb indicates that there is a flow of electric
current through the solution. The electric current is carried through the acidic solution by ions.
Acids contain H+ ion as cation and anion such as Cl– in HCl, \( NO_{3}^{-} \) in \( HN{{O}_{3}},\,\,SO_{4}^{2-} \) in \( {{H}_{2}}S{{O}_{4}},\,\,C{{H}_{3}}CO{{O}^{-}} \) in CH3COOH. Since the cation present in acids is H+, this suggests that acids produce hydrogen ions, H+(aq), in solution, which are responsible for their acidic properties.
Repeat the same Activity using alkalis such as sodium hydroxide, calcium hydroxide, etc. What can you conclude from the results of this Activity?
Do acids produce ions only in aqueous solution? Let us test this.
Note to teachers: If the climate is very humid, you will have to pass the gas produced through a guard tube (drying tube) containing calcium chloride to dry the gas.
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.
\( HCl+{{H}_{2}}O\to {{H}_{3}}{{O}^{+}}+C{{l}^{-}} \)
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+).
\( {{H}^{+}}+{{H}_{2}}O\to {{H}_{3}}{{O}^{+}} \)
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.
\( NaOH(s)\xrightarrow{{{H}_{2}}O}N{{a}^{+}}(aq)+O{{H}^{-}}(aq) \)
\( KOH(s)\xrightarrow{{{H}_{2}}O}{{K}^{+}}(aq)+O{{H}^{-}}(aq) \)
\( Mg{{(OH)}_{2}}(s)\xrightarrow{{{H}_{2}}O}M{{g}^{2+}}(aq)+2O{{H}^{-}}(aq) \)
Bases generate hydroxide (OH–) ions in water. Bases which are soluble in water are called alkalis.
All bases do not dissolve in water. An alkali is a base that dissolves in water. They are soapy to touch, bitter and corrosive. Never taste or touch them as they may cause harm. Which of the bases in the Table 2.1 are 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 –
\( Acid+Base\to Salt+Water \)
\( HX+MOH\to Mx+HOH \)
\( {{H}^{+}}(aq)+O{{H}^{-}}(aq)\leftarrow {{H}_{2}}O(\ell ) \)
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. Look out for the warning sign (shown in Fig. 2.5) on the can 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.
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