11.04 Properties and Uses of Alkali’s and Bases
Definitions and Key Concepts
- Acids:
- Contain an excess of H⁺ ions in solution.
- Have a pH less than 7.
- Turn blue litmus red.
- Neutralised by bases to form salt and water only.
- Examples of common acids and their formulas:
- Sulfuric acid: H₂SO₄
- Hydrochloric acid: HCl
- Nitric acid: HNO₃
- Bases:
- Oxides and hydroxides of metals.
- Neutralise acids to form salt and water only.
- Mainly insoluble in water (e.g., magnesium oxide, zinc oxide).
- Examples:
- Calcium oxide: CaO
- Magnesium oxide: MgO
- Alkalis (a subset of bases):
- Bases that dissolve in water.
- Contain an excess of OH⁻ ions in solution.
- Have a pH greater than 7.
- Turn red litmus blue.
- Feel soapy to the skin.
- Examples:
- Sodium hydroxide: NaOH
- Potassium hydroxide: KOH
- Ammonium hydroxide: NH₄OH
Properties and Reactions
- Neutralisation:
- Reaction between an acid and a base/alkali to form salt and water.
- General equations:
- Acid + Base → Salt + Water
- Ionic equation: H+(aq)+OH−(aq)→H2O(l)H⁺(aq) + OH⁻(aq) → H₂O(l)H+(aq)+OH−(aq)→H2O(l)
- Reaction with Ammonium Salts:
- Bases react with ammonium salts to release ammonia gas: Ammonium salt+Base→Salt+Water+Ammonia gas
- Example: Ammonium nitrate+Calcium oxide→Calcium nitrate+Water+Ammonia gas
- Ammonia gas turns red litmus blue and can be detected by its characteristic smell.
- Preparation of Ammonia in the Laboratory:
- Ammonia gas is dried by passing it through a tower of quicklime (CaO).
- Reaction example: 2NH₄NO₃(s)+CaO(s)→Ca(NO₃)₂(s)+H₂O(l)+2NH₃(g)
Key Properties Summary
| Property | Acids | Bases | Alkalis |
|---|---|---|---|
| pH | Less than 7 | N/A | Greater than 7 |
| Ions | Excess H⁺ ions | Metal oxides and hydroxides | Excess OH⁻ ions |
| Litmus Test | Turn blue litmus red | N/A | Turn red litmus blue |
| Solubility | Soluble in water | Mostly insoluble | Soluble in water |
| Feel | N/A | N/A | Soapy to the skin |
Common Alkalis and Bases
| Type | Name | Formula | Strength | Uses |
|---|---|---|---|---|
| Alkalis | Sodium hydroxide (caustic soda) | NaOH | Strong | Degreasing agent, soap and paper production, industrial uses. |
| Potassium hydroxide (caustic potash) | KOH | Strong | Making soft soaps and biodiesel. | |
| Calcium hydroxide (limewater) | Ca(OH)₂ | Strong | Neutralising soil acidity, treating acidic gases from power stations. | |
| Ammonium hydroxide | NH₄OH | Weak | Cleaning fluids (degreasing agent), making fertilizers. | |
| Bases | Calcium oxide | CaO | N/A | Neutralising soil acidity, making cement and concrete. |
| Magnesium oxide | MgO | N/A | Used in antacid tablets to treat indigestion. |
Frequently Asked Questions
- What is the formula for:
- a) Sulfuric acid: H₂SO₄
- b) Hydrochloric acid: HCl
- What can we say about the concentrations of ions in water:
- Water contains equal concentrations of H⁺ and OH⁻ ions.
- Which ion is in excess in an alkali solution?
- OH⁻ ions are in excess.
- Which ions are present in the following solutions?
- Nitric acid solution: H⁺ and NO₃⁻.
- Calcium hydroxide solution: Ca²⁺ and OH⁻.
- Ammonia solution: NH₄⁺ and OH⁻.
- Which compounds are alkalis?
- Potassium hydroxide (KOH) and Ammonium hydroxide (NH₄OH) are alkalis.
- Zinc oxide, magnesium oxide, and aluminum hydroxide are not alkalis as they are insoluble in water.
Quizzes
Quiz 1