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The Role of Water in Showing Acidic and Alkaline Properties

 
6.1  The Role of Water in Showing Acidic and Aklaine 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 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. 
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, \(H_3PO_4\)

 
 
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, \(KOH\)
 
Definition of Alkali
Chemical compound that dissolves in water and produces freely moving hydroxide ions, \(OH^-\).
 
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
 
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.
 
The image is a diagram showing different types of indicators. At the center, there is a blue circle labeled ‘Indicator.’ Four arrows point to this central circle from four surrounding circles. The surrounding circles are labeled as follows: ‘Methyl orange’ (top left), ‘Universal indicator’ (top right), ‘Blue litmus paper’ (bottom left), and ‘Red litmus paper’ (bottom right). The background is white, and the text is in blue. The Pandai logo is at the bottom center of the image.
 
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
 

 

 


 

The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of pandai.

The Role of Water in Showing Acidic and Alkaline Properties

 
6.1  The Role of Water in Showing Acidic and Aklaine 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 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. 
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, \(H_3PO_4\)

 
 
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, \(KOH\)
 
Definition of Alkali
Chemical compound that dissolves in water and produces freely moving hydroxide ions, \(OH^-\).
 
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
 
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.
 
The image is a diagram showing different types of indicators. At the center, there is a blue circle labeled ‘Indicator.’ Four arrows point to this central circle from four surrounding circles. The surrounding circles are labeled as follows: ‘Methyl orange’ (top left), ‘Universal indicator’ (top right), ‘Blue litmus paper’ (bottom left), and ‘Red litmus paper’ (bottom right). The background is white, and the text is in blue. The Pandai logo is at the bottom center of the image.
 
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
 

 

 


 

The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of pandai.
Chapter : Acids, Bases and Salts
Topic : Role of Water in Showing Acidic and Alkaline Properties
Form 4 Chemistry
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