What is alkali ? Definition,Propperties of alkali and Uses of alkali

Alkali - Definition, Properties and Uses

What is Alkali ?

Alkalis are all Arrhenius bases, ones which form hydroxide ions (OH⁻) when dissolved in water. 

An alkali  is a basic, ionic salt of an alkali metal or an alkaline earth metal. A solution of a soluble base has a  pH greater than 7.0. 

The word "alkali" is derived from Arabic  al qalīy / quali (or alkali), meaning the calcined ashes (from the ashes), referring to the original source of alkaline substances.

                                           " All alkali are base but all base are not a alkali. "

Properties of Alkali

Physical Properties

1.An alkali have a bitter taste.

2.Alkaline solutions are slippery or soapy when to the touch, due to the   saponification of the fatty substances on the surface of the skin.

3.Alkalis are normally water soluble,though some like barium carbonate are only soluble when reacting with an acidic aqueous solution.

4.They can conduct electricity dueto the presence of mobile ions.

5.pH of alkali are  greater than 7.( Let may start from 7.1) on pH scale

6.On the universal indicator colour changes to blue or purple.

7.It turns red litmus paper to blue.

Chemical Properties

Alkali involves neutalisation reaction and precipitation of metal hydroxide.It also react with ammonia salts and Metals.They explained in below:

1.Neutralisation Reaction

An alkali reacts with acid to form salt and water this is also known as neutralisation reaction.Examples of Neutralisation reactions are

                   NaOH(aq)+ HCl (aq) → NaCl (aq)+ H₂O(l)

$$ZnO(s)+2HCl(aq)→ZnCl2(aq)+H2O(l)

 $\text{ZnO}\left(\text{s}\right)+2\text{HCl}\left(\text{aq}\right)\to {\text{ZnCl}}_2\left(\text{aq}\right)+{\text{H}}_2\text{O}\left(\text{l}\right)$

NaoH(aq)+H2SO4(aq)→Na2SO4(aq)+2H2O(l)

2.Reaction With Ammonia Salts

When a mixture of an alkali and ammonium salt is heated, ammonia gas is given of

Ammonium chloride reacts with sodium hydroxide which gives Sodium chloride and ammonia ga

                NH4Cl(s)+NaOH(s)→NaCl(s)+NH3(g)+H2O(l)

Ammonium sulphate reacts calcium hydroxide that gives Calcium Sulphate and Ammonia gas

                (NH4)2SO4(s)+Ca(OH2)(s)→CaSO4(s)+2NH3(g)+2H2O(l)

3.Precipitation of Metal Hydroxides

Solutions of alkalis are used to precipitate the insoluble metal hydroxides from solutions of their salts.

$${\text{M}}^{n+}\left(\text{aq}\right)+{\text{nOH}}^{-}\left(\text{aq}\right)\to \text{M}\left(\text{OH}{)}_{\text{n}}\right(\text{s})$$

Example of such a reaction:

Aqueous sodium hydroxide (NaoH) and aqueous copper(II) sulphate

$$2\text{NaOH}\left(\text{aq}\right)+{\text{CuSO}}_4\left(\text{aq}\right)\to \text{Cu}\left(\text{OH}{)}_2\right(\text{s})+{\text{Na}}_2{\text{SO}}_4(\text{aq})$$

               or:      ${\text{Cu}}^{2+}\left(\text{aq}\right)+2{\text{OH}}^{-}\left(\text{aq}\right)\to \text{Cu}\left(\text{OH}{)}_2\right(\text{aq})$

A blue precipitate of copper(II) hydroxide [ Cu(OH)2 ]  is produced.

Aqueous sodium hydroxide and aqueous magnesium chloride

$$2\text{NaOH}\left(\text{aq}\right)+{\text{MgCl}}_2\left(\text{aq}\right)\to \text{Mg}\left(\text{OH}{)}_2\right(\text{s})+2\text{NaCl}(\text{aq})$$

              or:      ${\text{Mg}}^{2+}\left(\text{aq}\right)+2{\text{OH}}^{-}\left(\text{aq}\right)\to \text{Mg}\left(\text{OH}{)}_2\right(\text{s})$

A white precipate of magnesium hydroxide  [ Mg(OH)2 ] is produced. 

4.Reaction with metals

Alkalis usually do not reacts with metals.

Strong alkalis such as sodium hydroxide react with aluminium to give hydrogen gas.

                    2Al(s)+6NaOH(aq)+6H2O(l)→2Na3Al(OH)6(s)+3H2(g)

Uses of Alkali

1.Caustic soda (i.e, Sodium hydroxide) is used for the preparation of Soap.

2.Caustic potash (i.e, Potassium hydroxide) is used for the manufacture of soft soap.

3.Borax is used for the preparation of Cold cream. Due to Borax is responsible for whiteness of Cold cream.

4.Calcium hydroxide used to neutralize the acidic soil.

5.Sodium Bicarbonate is used for the treatment of acidosis as an alkaliser.