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| 6.1 |
The Role of Water in Showing Acidic and Aklaine Properties |
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| 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.
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| Basicity of Acid |
| Definition of Basicity of Acids |
- Refers to the number of hydrogen ions, đ»+ that can be produced by an acid molecule that ionises in water.
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| Classification of Acid based on Acidity |
| Monoprotic Acid |
Diprotic Acid |
Tripotic 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, \(H_3PO_4\) |
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| 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:
- \(KOH(aq) \xrightarrow[] {H_2O}K^+(aq) + OH^-(aq)\)
- Example of alkali;
- Sodium hydroxide, \(NaOH\)
- Potassium hydroxide, \(KOH\)
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| Definition of Alkali |
| Chemical compound that dissolves in water and produces freely moving hydroxide ions, \(OH^-\). |
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| Uses of Acids, Bases and Akalis in Our Daily Life |
| Acid |
Uses |
| Sulphuric acid |
Make paints, polymer, fertilisers and detergent |
| Hydrochloric acid |
To clean metal before electroplating |
| Benzoic acid |
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 |
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| The Role of Water to Show Acidity and Alkalinity |
- Acids only show acidic properties in the presence of water.
- When an acid is dissolved in water, acid molecules will ionise to produce hydrogen ions, \(H^+\).
- The presence of hydrogen ions \(H^+\) allows the acid to show its acidic properties.
- Therefore, blue litmus paper changes to red.
- Without water, solid oxalic acid, C2 H2 O4 only exist as molecules.
- Hydrogen ions, \(H^+\) are not present.
- Thus, the colour of blue litmus paper remains unchanged.
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| Indicators and their Colours in Acid and Alkaline Solution |
| 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 |
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