(a) What happens when an acid reacts with a metal? Give the chemical equation of the reaction involved.

(b) Which gas is usually liberated when an acid reacts with a metal? How will you test for the presence of this gas?

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Solution

(a) Those metals which are above hydrogen in standard reduction potential series ( $- v e E^{0}$ values) will liberate hydrogen gas on reaction with dil. acids

Eg. Zin with sulphuric acid.

For dilute acid, zinc sulfate and hydrogen gas at produced.

$Z n + H_{2} S O_{4} ⟶ Z n S O_{4} + H_{2}$

For concentrated sulphuric acid, zinc sulfate, Sulphur dioxide, and water are produced.

$Z n + c o n c H_{2} S O_{4} ⟶ Z n S O_{4} + S O_{2} + 2 H_{2} O$

(b) Hydrogen gas is usually liberated when an acid reacts with a metal.

Take a few pieces of zinc granules and add 5 ml of dilute $H_{2} S O_{4}$ . Shake it and pass the gas produced into a soap solution. The bubbles of the soap solution are formed. These soap bubbles contain hydrogen gas.

$Z n + H_{2} S O_{4} ⟶ Z n S O_{4} + H_{2} ↑$

We can test the evolved hydrogen gas by its burning with a pop sound when a candle is brought near the soap bubbles.

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