Bases (OH− furnishing) MCQs Quiz | Class 10
This Class X Science (Code 086) quiz covers Unit I: Chemical Substances – Nature and Behaviour, specifically focusing on Bases. The questions test your understanding of the definition of bases in terms of furnishing hydroxide (OH-) ions in aqueous solutions. Test your knowledge, review the educational content below, and download your result as a PDF.
Understanding Bases: Furnishing OH- Ions
In Chemistry, specifically within the Arrhenius definition used in Class 10 Science, a base is defined as a substance that dissociates in water to produce hydroxide ions (OH-). This property is what gives bases their characteristic slippery feel and bitter taste.
Key Concepts
- Dissociation: When a base dissolves in water, it breaks apart into positive ions (cations) and negative hydroxide ions (anions). For example: NaOH(aq) yields Na+(aq) + OH-(aq).
- Alkalis: Bases that are soluble in water are called alkalis. All alkalis are bases, but not all bases are alkalis. Examples include Sodium Hydroxide (NaOH) and Potassium Hydroxide (KOH).
- Strong vs. Weak Bases:
- Strong Bases: Completely ionize in water, producing a high concentration of OH- ions (e.g., NaOH, KOH).
- Weak Bases: Partially ionize in water, producing a low concentration of OH- ions (e.g., Ammonium Hydroxide, NH4OH).
Common Bases and Their Formulas
| Name | Chemical Formula | Nature |
|---|---|---|
| Sodium Hydroxide | NaOH | Strong Alkali |
| Potassium Hydroxide | KOH | Strong Alkali |
| Calcium Hydroxide | Ca(OH)2 | Strong Base (Sparingly soluble) |
| Magnesium Hydroxide | Mg(OH)2 | Weak Base (Milk of Magnesia) |
| Ammonium Hydroxide | NH4OH | Weak Base |
Quick Revision Points
- The presence of OH- ions is responsible for the basic character of a substance.
- Dry pellets of a base (like NaOH) do not show basic behavior until dissolved in water to release ions.
- Bases turn red litmus paper blue.
- When a base reacts with an acid, the H+ from the acid combines with the OH- from the base to form water (Neutralization).
Extra Practice Questions
- Question: What happens to the concentration of OH- ions when excess base is dissolved in a solution?
Answer: It increases per unit volume. - Question: Why does dry calcium hydroxide not change the color of dry red litmus paper?
Answer: Because no OH- ions are present in the dry state; water is required for ionization. - Question: Name the base found in window cleaner.
Answer: Ammonium Hydroxide. - Question: Which ion is common to all alkalis?
Answer: Hydroxide ion (OH-). - Question: Write the ionization equation for KOH.
Answer: KOH(s) + Water -> K+(aq) + OH-(aq).
