Acids

Definition of acid:

v  A substance which produces hydrogen ions as the only positive ions when it is dissolved in water.

Two types of acids:

v  Strong acids

Ø  Is one that ionises/dissociates completely in water to produce hydrogen ions

Ø  No molecules left  

Ø  Solution contains a high concentration of ions

Ø  Good electrical conductor.

v  Weak acids

Ø  Is one that ionises/dissociates incompletely/partially in water to produce few hydrogen ions

Ø  Most of the acid molecules remain as molecules

Ø  Solution contains a low concentration of ions

Ø  Poor electrical conductor.

Properties of acids:

v  Sour taste.

v  Change the colour of indicators (e.g. turn blue litmus paper red).

v  Contain hydrogen ions and conduct electricity.

v  React with metals, carbonates and bases.

Reaction with metals:

v  Acid + metal → salt + hydrogen

v  Example: 2HCl (aq) + Zn (s) → ZnCl₂ (aq) + H₂ (g)

v  Observation:

Ø  Effervescence observed. Colourless, odourless gas evolved which extinguished a lighted wooden splint with a ‘pop’ sound.

Ø  Grey solid reduced in size (when in excess).  Grey solid dissolves to form a colourless solution

Ø  Heat liberated

Reaction with carbonates:

v  Acid + carbonate → salt + carbon dioxide + water

v  Example: Na₂CO₃ (s) + H₂SO₄ (aq) → Na₂SO₄ (aq) + CO₂ (g) + H₂O (l)

v  Observation:

Ø  Effervescence occurs. Colourless, odourless gas evolved which reacts with calcium hydroxide to form a white precipitate.

Ø  White solid dissolves to form a colourless solution

Reaction with bases:

v  Acid + base → salt + water

v  Example: MgO (s) + H₂SO₄ (aq) → MgSO₄ (aq) + H₂O (l)

v  Observation:

Ø  Heat is evolved.  No visible change

Colour change of indicators:

v  Litmus paper: Turns blue litmus paper red

v  Methyl orange: Turns methyl orange red

v  Phenolphthalein: Remains colourless

v  Universal indicator: Between red to green