| 6.1 |
The Role of Water in Showing Acidic and Alkaline Properties |
Acid
- Acid is a chemical compound that produces hydrogen ions, \(H^+\) or hydroxonium ions \(H_3O^+\) when dissolved in water.
- The dissociation of acid in water produces hydrogen ions and an anion.
- An example of dissociation of hydrochloric acid in water is as follows;
\(HCl(aq) \xrightarrow[]{H_2O} H^+ (aq) + Cl^- (aq)\)
- Without the presence of hydrogen ions, that means there are no acidic properties on the chemicals.
Basicity of acids
- Refers to the number of hydrogen ions, \(H^+\) that can be produced by an acid molecule that ionises in water.
|
Monoprotic
acid
|
Diprotic
acid
|
Triprotic
acid
|
| One \(H^+\) per acid molecule |
Two \(H^+\) per acid molecule |
Three \(H^+\) per acid molecule |
|
Hydrochloric acid,
\(HCl\)
|
Sulphuric acid,
\(H_2SO_4\)
|
Phosphoric acid,
\(H_3PO_4\)
|
Alkali
- The base is a chemical compound that can neutralise an acid to produce salt and water.
- Example of formation of salt and water is:
- \(HCl (aq) + NaOH(aq) \rightarrow NaCl (aq)+ H_2O(l)\)
- Most bases are not soluble in water.
- Bases that are soluble in water are known as alkalis.
- Bases that insoluble in water are zinc oxide, zinc hydroxide, copper(II) oxide and copper(II) hydroxide.
- An alkali is a chemical compound that dissolves in water and produces freely moving hydroxide ions, \(OH^- \).
- Without the presence of hydroxide ions, the chemical substances do not show alkaline or basic properties.
- Example of alkali is potassium hydroxide:
- \(KOH(aq) \xrightarrow[] {H_2O}K^+(aq) + OH^-(aq)\)
- Examples of akali;
- Sodium hydroxide, \(NaOH\)
- Potassium hydroxide, \(KOH\)
| Definition of Alkali |
| Chemical compound that dissolves in water and produces freely moving hydroxide ions, \(OH^-\) |
Uses of acids, bases and alkalis in our daily life.
|
Acid
|
Uses
|
|
Sulphuric acid
|
Make paints, polymer, fertilisers and detergent
|
|
Hydrochloric acid
|
To clean metal before electroplating
|
|
Benzoic acids
|
To preserve food
|
|
Bases
|
Uses
|
|
Ammonia
|
Make fertilisers, nitric acid, and to maintain latex in liquid form
|
|
Magnesium hydroxide
|
To make toothpaste, gastric medicine
|
|
Calcium hydroxide
|
To make cement, limewater and to neutralise acidic soil
|
Colour of indicators in acidic and alkaline solutions
| Indicator |
Colour of indicator in acidic solution |
Colour of indicator in alkaline solution |
| Methyl orange |
Red |
Yellow |
| Universal indicator |
Red for strong acid and yellow for weak acid |
Purple for strong alkali and blue for weak alkali |
| Blue litmus paper |
Red |
Blue |
| Red litmus paper |
Red |
Blue |
