Saturday, September 18, 2021

Acids, Bases and Salts | Class 10 CBSE | Web Notes | Part 2 | What do all Acids and Bases have in common?



All acids have similar chemical properties.
E.g. All acids generate hydrogen gas on reacting with metals, so hydrogen seems to be common to all acids.

But all compounds containing hydrogen are not acidic. It can be proved by the following experiment.
  • Take solutions of glucose, alcohol, HCl, H2SO4, etc.
  • Fix two nails on a cork and place it in a 100 mL beaker. Connect the nails to the two terminals of a 6 volt battery through a bulb and a switch.
  • Pour some dilute HCl in the beaker and switch on the current. Repeat with dilute H2SO4.
  • In both cases, bulb glows. It means there is an electric current through the acidic solution by ions.
  • Repeat the experiment using glucose & alcohol solutions. In these cases, bulb does not glow because glucose & alcohol solutions do not conduct electricity.

Acids contain H+ ion as cation and anion (Cl in HCl, NO3 in HNO3, SO24in H2SO4, CH3COO in CH3COOH).

Acidic properties are due to H+(aq) ions in solution.

Repeat the same Activity using alkalis such as sodium hydroxide, calcium hydroxide, etc. NaOH & KOH conduct electricity as they are broken down into ions when dissolved in water. The movement of the ions, Na+, Kand OH- in solution generate electricity.

What Happens to an Acid or a Base in a Water Solution?

Acids produce ions only in aqueous solution. It can be proved by the following experiment.
  • Take 1g solid NaCl in a clean dry test tube.
  • To this, add some conc. sulphuric acid.
  • HCl gas comes out of the delivery tube. [In very humid climate, pass the HCl gas through a guard tube (drying tube) containing calcium chloride to dry the gas].
  • When HCl gas is tested with wet blue litmus paper, it becomes red colour. But with dry litmus paper, no colour change occurs.
  • It means dry HCl gas (absence of water) cannot produce H+ ions. So it does not behave as an acid.
  • HCl solution (presence of water) can produce H+ ions and behave as an acid.

HCl + H2O H3O+ + Cl

H+ ions cannot exist alone. They exist after combining with water molecules. Thus H+ ions must be shown as H+(aq) or hydronium ion (H3O+).

H+ + H2O H3O+

Action of base with water:

Bases generate hydroxide (OH) ions in water.

All bases do not dissolve in water.An alkali is a base that dissolves in water. They are soapy, bitter and corrosive. Never taste or touch them as they cause harm.

NaOH, KOH, Mg(OH)2, NH4OH etc. are alkalis.

Neutralisation reaction:

Acid + Base Salt + Water

H  X + M  OH MX + HOH

H+(aq) + OH (aq) H2O(l)

Mixing of acid or base with water:

This process is highly exothermic. E.g.

  • Take 10 mL water in a beaker. Add a few drops of concentrated H2SO4 and swirl the beaker slowly.
  • Touch the base of the beaker. It is hot. So the reaction is exothermic.  
  • Repeat this activity with sodium hydroxide pellets. It is also exothermic reaction.

Mixing of concentrated nitric acid or sulphuric acid with water must be done carefully. Add the acid slowly to water with constant stirring. Otherwise, excessive local heating may cause the mixture to splash out and cause burns. The glass container may also be broken.

Mixing an acid or base with water results in a 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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