6.1 - The Role of Water in Showing Acidic and Alkaline Properties

Acids, Bases and Alkalis

• An acid is a chemical substance that ionises in water to produce hydrogen ions, ${\mathrm{H}}^{+}$.
  • $\mathrm{HCl}(\mathrm{aq})\to {\mathrm{H}}^{+}(\mathrm{aq})+\mathrm{C}{\mathrm{l}}^{-}(\mathrm{aq})$
  • $\mathrm{HN}{\mathrm{O}}_3(\mathrm{aq})\to {\mathrm{H}}^{+}(\mathrm{aq})+\mathrm{N}{\mathrm{O}}_3^{-}(\mathrm{aq})$
• In water, ${\mathrm{H}}^{+}$ combines with ${\mathrm{H}}_2\mathrm{O}$ to form the hydroxonium ion, ${\mathrm{H}}_3{\mathrm{O}}^{+}$. For simplicity, ${\mathrm{H}}^{+}$ is normally used to represent it.
• Basicity of an acid is the number of hydrogen ions that one acid molecule can produce when it ionises in water.

Acid type	Hydrogen ions produced per molecule	Example
Monoprotic	1	$\mathrm{HCl}$
Diprotic	2	${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$
Triprotic	3	${\mathrm{H}}_3\mathrm{P}{\mathrm{O}}_4$

• A base reacts with an acid to produce salt and water only. Metal oxides and metal hydroxides are bases.
• An alkali is a base soluble in water; it ionises in water to produce hydroxide ions, $\mathrm{O}{\mathrm{H}}^{-}$.
  • $\mathrm{NaOH}(\mathrm{aq})\to \mathrm{N}{\mathrm{a}}^{+}(\mathrm{aq})+\mathrm{O}{\mathrm{H}}^{-}(\mathrm{aq})$
  • $\mathrm{N}{\mathrm{H}}_3(\mathrm{aq})+{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\rightleftharpoons \mathrm{N}{\mathrm{H}}_4^{+}(\mathrm{aq})+\mathrm{O}{\mathrm{H}}^{-}(\mathrm{aq})$
• Every alkali is a base, but an insoluble base is not an alkali.

Uses of Acids and Alkalis

• Alkaline toothpaste neutralises acid on teeth; acidic vinegar is used to pickle chillies.

Role of Water in Acidity

Experiment 6.1 aim: Determine whether water is needed for an acid to show acidic properties.

• Hypothesis: Water is needed for an acid to show acidic properties.
• Variables: presence of water (manipulated), blue-litmus colour (responding), type of acid (fixed).
• Materials: solid oxalic acid, ${\mathrm{C}}_2{\mathrm{H}}_2{\mathrm{O}}_4$, distilled water and dry blue litmus paper.
• Apparatus: test tube and test tube rack.
• Procedure:
  1. Place one spatula of solid oxalic acid in a test tube and insert dry blue litmus paper.
  2. Observe its colour, add $2\mathrm{c}{\mathrm{m}}^3$ distilled water and shake.
  3. Observe the litmus colour again.
• Precaution: acid is corrosive; rinse an affected area with running water if contact occurs.
• Dry oxalic acid contains molecules but no ${\mathrm{H}}^{+}$ ions, so blue litmus remains blue.
• In water, acid molecules ionise and the resulting ${\mathrm{H}}^{+}$ ions turn blue litmus red.
• Conclusion: An acid shows acidic properties only in the presence of water.
• Acidic solutions have a sour taste, are corrosive, have pH below 7 and turn moist blue litmus red.

Role of Water in Alkalinity

Experiment 6.2 aim: Determine whether water is needed for an alkali to show alkaline properties.

• Compare a sodium hydroxide pellet and sodium hydroxide solution using red litmus paper.

• Without water, $\mathrm{O}{\mathrm{H}}^{-}$ ions remain fixed in the ionic lattice of solid $\mathrm{NaOH}$; red litmus does not change colour.

• In water, the ions dissociate and mobile $\mathrm{O}{\mathrm{H}}^{-}$ ions turn moist red litmus blue.

• Ammonia gas likewise shows alkaline properties only after water enables ammonia molecules to ionise and form $\mathrm{O}{\mathrm{H}}^{-}$.

• Conclusion: An alkali shows alkaline properties only in the presence of water.

• Alkaline solutions taste bitter, feel slippery, are corrosive, have pH above 7 and turn moist red litmus blue.

• Precaution for Experiment 6.2: use one sodium hydroxide pellet; if alkali contacts skin, rinse continuously until the slippery feeling disappears.