Chapter 2 · Question 2

Describe the reaction of zinc granules with dilute sulphuric acid. How can you test that the gas evolved is hydrogen? Write the balanced chemical equation.

Answer Revealed

Direct Answer:

Zinc granules react with dilute sulphuric acid to produce zinc sulphate and hydrogen gas:

\text{Zn(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{ZnSO}_4\text{(aq)} + \text{H}_2\text{(g)}

. Bubbles appear on the surface of zinc granules and the metal gradually dissolves. To test for hydrogen, pass the gas through a soap solution to form soap bubbles filled with the gas; bring a burning candle near a gas-filled bubble — the gas burns with a characteristic pop sound, confirming it is hydrogen.

Simple Explanation

When zinc pieces are dropped into dilute sulphuric acid, you see fizzing — tiny bubbles of hydrogen gas forming on the surface. Collect the gas in soap bubbles and bring a flame near one: it burns with a sharp 'pop' sound, like a tiny explosion. That pop is the classic test for hydrogen. The equation is

\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2

— zinc pushes hydrogen out of the acid and takes its place to form zinc sulphate.

Exam-Ready Structure

The reaction of zinc granules with dilute sulphuric acid is the standard NCERT demonstration of how acids react with metals to produce hydrogen gas. 1. Experimental setup (Figure 2.1 in the textbook): A test tube is clamped on a stand. About 5 mL of dilute sulphuric acid is added, followed by a few pieces of zinc granules. A delivery tube carries the gas produced into a beaker containing soap solution, where the gas forms bubbles. 2. Observations: (a) Fine bubbles appear on the surface of the zinc granules. (b) The zinc granules gradually get consumed, and the reaction mixture may become warm. (c) The gas evolved is colourless and odourless. 3. Balanced chemical equation:

\text{Zn(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{ZnSO}_4\text{(aq)} + \text{H}_2\text{(g)}

. The salt formed is zinc sulphate, which remains dissolved in water. 4. General reaction of an acid with a metal: Acid + Metal

\rightarrow

Salt + Hydrogen gas. The metal displaces hydrogen from the acid. 5. Testing for hydrogen gas: (a) The gas is passed through a soap solution to trap it as bubbles. (b) When a burning candle is brought near a gas-filled soap bubble, the hydrogen burns with a characteristic pop sound. (c) This pop sound is the confirmatory test for hydrogen gas. 6. NCERT asks students to repeat the activity with acids such as HCl, HNO₃, and CH₃COOH and compare whether the observations are the same or different, reinforcing that different acids may show different vigour under the same test conditions.

Key Points

Zinc + dilute sulphuric acid: $\text{Zn(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{ZnSO}_4\text{(aq)} + \text{H}_2\text{(g)}$
Observation: bubbles/fizzing on zinc surface; metal gradually dissolves
Hydrogen gas test: pass through soap solution, bring lit candle near bubble — pop sound confirms H₂
General reaction: Acid + Metal $\rightarrow$ Salt + Hydrogen gas
The reaction is a displacement — zinc displaces hydrogen from the acid

Common Mistakes

Confusing the pop sound test: a glowing splint is for oxygen, not hydrogen
Writing the product as zinc oxide — the product is zinc sulphate, a salt

Describe the reaction of zinc granules with dilute sulphuric acid. How can you test that the gas evolved is hydrogen? Write the balanced chemical equation.