WHAT DO ALL ACIDS AND ALL BASES HAVE IN COMMON?
All acids have similar chemical properties. For example, all acids generate hydrogen gas on reacting with metals, so hydrogen seems to be common to all acids.
However, not all compounds containing hydrogen are acidic. This can be demonstrated 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, the bulb glows, indicating an electric current through the acidic solution by ions.
- Repeat the experiment using glucose and alcohol solutions. The bulb does not glow because glucose and alcohol solutions do not conduct electricity.
Acids contain H⁺ ions as cations and anions (Cl⁻ in HCl, NO₃⁻ in HNO₃, SO₄²⁻ in 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 and KOH conduct electricity as they dissociate into ions (Na⁺, K⁺, and OH⁻) when dissolved in water, generating electricity through ion movement.
What Happens to an Acid or a Base in a Water Solution?
Acids produce ions only in aqueous solution. This can be demonstrated by the following experiment:
- Take 1g solid NaCl in a clean, dry test tube.
- Add some concentrated sulphuric acid.
- HCl gas comes out of the delivery tube. [In a very humid climate, pass the HCl gas through a guard tube containing calcium chloride to dry the gas.]
- When HCl gas is tested with wet blue litmus paper, it turns red. With dry litmus paper, no colour change occurs.
- This indicates that 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 behaves as an acid.
- H⁺ ions cannot exist alone. They exist after combining with water molecules, forming H⁺(aq) or hydronium ion (H₃O⁺).
Action of Base with Water:
- Bases generate hydroxide (OH⁻) ions in water.
- Not all bases 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:
Mixing of Acid or Base with Water:
This process is highly exothermic. For example:
- 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, indicating an exothermic reaction.
- Repeat this activity with sodium hydroxide pellets. It is also an exothermic reaction.
Mixing concentrated nitric acid or sulphuric acid with water must be done carefully. Add the acid slowly to water with constant stirring to avoid excessive local heating, which may cause the mixture to splash out and cause burns. The glass container may also break.
Mixing an acid or base with water results in a decrease in the concentration of ions (H₃O⁺/OH⁻) per unit volume. This process is called dilution, and the acid or base is said to be diluted.