6.1The Role of Water in Showing Acidic and Alkaline Properties
Acids, bases, and alkalis are chemical substances widely encountered in daily life, such as in lime used to shine coins, toothpaste used to neutralise acids on teeth, and vinegar used for pickling. According to the Arrhenius theory, acidic and alkaline properties only appear when these substances are dissolved in water. Without water, acids and bases cannot produce free-moving ions and will not exhibit their characteristic properties.
Learning Objectives
- Define acid and alkali.
- State the meaning of basicity of an acid.
- Investigate the role of water in showing acidic and alkaline properties through experiment.
The Dissociation and Ionisation of Acids and Alkalis in the Presence of Water
Water releases free-moving H⁺ and OH⁻ ions that give acidic and alkaline properties.
Red litmus stays red
Red litmus stays red
Red litmus stays red
Red litmus turns blue
Brief Explanation
Understand the key ideas about acids, bases, alkalis and the role of water.
1. Acids and Basicity
An acid ionises in water to produce H⁺ ions. The basicity of an acid is the number of H⁺ ions produced per acid molecule.
- Monoprotic acid: produces 1 H⁺ per molecule, e.g. HCl.
- Diprotic acid: produces 2 H⁺ per molecule, e.g. H₂SO₄.
- Triprotic acid: produces 3 H⁺ per molecule, e.g. H₃PO₄.
2. Bases and Alkalis
A base reacts with acids to produce salt and water only. Metal oxides and metal hydroxides are bases.
An alkali is a water-soluble base that produces freely moving OH⁻ ions in water.
| Type of acid | Number of H⁺ produced per molecule | Example |
|---|---|---|
| Monoprotic acid | 1 | Hydrochloric acid, HCl |
| Diprotic acid | 2 | Sulphuric acid, H₂SO₄ |
| Triprotic acid | 3 | Phosphoric acid, H₃PO₄ |
Try These Questions First
Answer mentally, then check the explanation.
What is the definition of an acid based on the Arrhenius theory?
Why is sulphuric acid, H₂SO₄, classified as a diprotic acid?
Why does a dry pellet of sodium hydroxide, NaOH, fail to change the colour of dry red litmus paper?
Common Mistakes
Avoid these errors when studying acids, bases and alkalis.
Common Misconceptions
Compare each misconception with the correct fact.
Hydrogen ions, H⁺, float completely isolated and unattached in an aqueous acidic solution.
H⁺ ions actually bond with water molecules (H₂O) to form hydroxonium ions (H₃O⁺), although chemists conventionally write H⁺ for simplicity.
Solid sodium hydroxide does not contain hydroxide ions until water is added.
Hydroxide ions are already present in solid NaOH, but they are locked firmly within the ionic lattice. Water dissolves the lattice and allows the ions to move freely.
Ammonia is an acid because its formula is NH₃ and contains hydrogen.
When ammonia gas dissolves in water, it ionises to form ammonium ions (NH₄⁺) and hydroxide ions (OH⁻), making aqueous ammonia an alkali.
Summary
Key facts to remember.
- Acid: Chemical substance that ionises in water to produce hydrogen ions, H⁺ (forming hydroxonium ions, H₃O⁺).
- Basicity of Acids: Monoprotic produces 1 H⁺ per molecule (e.g. HCl); diprotic produces 2 H⁺ per molecule (e.g. H₂SO₄, H₂CO₃); triprotic produces 3 H⁺ per molecule (e.g. H₃PO₄).
- Base: A substance that reacts with acid to produce salt and water only (metal oxides and metal hydroxides).
- Alkali: A base that dissolves in water to produce freely moving hydroxide ions, OH⁻ (e.g. NaOH, KOH, aqueous NH₃).
- Role of Water: Water is mandatory for acids to ionise to produce H⁺ and for alkalis to produce free-moving OH⁻. Without water, acidic and alkaline properties cannot be exhibited.
Quick Activity
Complete all parts before checking your score.
A. Objective Quiz
Which of the following acids is classified as a diprotic acid?
Why does a piece of dry blue litmus paper not change colour when placed in solid oxalic acid?
B. Fill in the Blanks
An alkali is defined as a chemical substance that ionises in water to produce __________ ions.
Hydrogen ions produced when an acid dissolves in water combine with water molecules to form __________ ions, H₃O⁺.
C. Match the Substance with Its Classification
Drag each substance on desktop. On mobile, tap a substance and then tap the matching box.
Answer Choices
Phosphoric acid, H₃PO₄ Magnesium oxide, MgO Aqueous ammonia, NH₃(aq) Hydrochloric acid, HClClassification / Description
Keywords
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