Properties of Acids and Bases (HCl & NaOH): Zinc Metal MCQs Quiz | Class 10

Understanding Reactions of Acids and Bases with Zinc Metal

The reaction of acids and bases with metals like zinc is a fundamental concept in chemistry, often explored in practical sessions. This section delves into the observations and inferences drawn from such reactions, particularly focusing on hydrochloric acid (HCl) and sodium hydroxide (NaOH) with zinc metal.

Reaction with Dilute Hydrochloric Acid (HCl)

When dilute hydrochloric acid (HCl) reacts with zinc metal (Zn) granules, a vigorous reaction occurs. The key observations and inferences are:

• Observation: Effervescence (formation of gas bubbles) is observed around the zinc granules. The reaction mixture might also feel warm, indicating an exothermic reaction.
• Inference: The gas evolved is hydrogen gas (H₂). This is confirmed by bringing a burning splinter near the mouth of the test tube; it extinguishes with a ‘pop’ sound.
• Chemical Equation:

  Zn (s) + 2HCl (aq) → ZnCl₂ (aq) + H₂ (g)

  Here, zinc metal reacts with hydrochloric acid to form zinc chloride (a salt) and hydrogen gas.

Reaction with Sodium Hydroxide (NaOH)

Bases also react with certain metals, including zinc, although the reaction is generally less vigorous than with acids, especially with dilute bases. For a noticeable reaction with NaOH, it is often performed with a concentrated solution or by heating.

• Observation: Similar to acids, effervescence (gas bubbles) is observed. The solution might also warm up.
• Inference: The gas evolved is again hydrogen gas (H₂), identifiable by the ‘pop’ sound test.
• Chemical Equation:

  Zn (s) + 2NaOH (aq) → Na₂ZnO₂ (aq) + H₂ (g)

  Here, zinc reacts with sodium hydroxide to form sodium zincate (a complex salt) and hydrogen gas.

Key Takeaways and Revision

• Both acids (like HCl) and certain bases (like NaOH) react with active metals such as zinc.
• A common observation in these reactions is the evolution of gas bubbles (effervescence).
• The gas evolved when metals react with acids or bases is hydrogen gas (H₂).
• Hydrogen gas is identified by the characteristic ‘pop’ sound produced when a burning splinter is brought near it.
• These reactions are typically exothermic, meaning they release heat.
• The products formed are generally a salt and hydrogen gas.

Practice Questions

Test your understanding further with these additional questions:

1. Which of the following metals does not react with dilute acids to produce hydrogen gas?
   a) Zinc b) Magnesium c) Copper d) Iron
2. The salt formed when zinc reacts with dilute sulfuric acid is:
   a) Zinc chloride b) Zinc sulfate c) Zinc nitrate d) Sodium zincate
3. What would you observe if you add a few drops of phenolphthalein indicator to the reaction mixture of NaOH and Zn after the reaction?
   a) It turns red b) It remains colorless c) It turns blue d) It turns yellow
4. Why is hydrogen gas collected by downward displacement of water?
   a) It is heavier than air b) It is highly soluble in water c) It is lighter than air and insoluble in water d) It is reactive with water
5. Which of the following is an amphoteric metal, reacting with both acids and strong bases?
   a) Copper b) Gold c) Zinc d) Silver

Answers: 1) c) Copper, 2) b) Zinc sulfate, 3) b) It remains colorless (if NaOH is completely reacted), 4) c) It is lighter than air and insoluble in water, 5) c) Zinc