When an acid reacts with a metal, hydrogen gas (H₂) is usually liberated. The reaction between an acid and a metal is a type of single-displacement or single-replacement reaction, where the more reactive metal displaces hydrogen from the acid, forming a metal salt and releasing hydrogen gas.

Here’s an example of the reaction between hydrochloric acid (HCl) and zinc (Zn):

Hydrochloric Acid + Zinc → Zinc Chloride + Hydrogen Gas

2HCl + Zn → ZnCl₂ + H₂

Testing for the Presence of Hydrogen Gas:

To test for the presence of hydrogen gas, you can perform the “pop” test. This test is a simple way to detect the presence of hydrogen gas, as hydrogen is highly flammable. When hydrogen gas is ignited, it produces a distinctive “pop” sound due to its rapid combustion in the presence of oxygen from the air.

Here’s how you can perform the “pop” test:

1. Take a test tube or a small glass container and fill it with the reaction mixture (the acid-metal reaction) that you suspect contains hydrogen gas.
2. Carefully bring a lit splint (a thin wooden stick or a twisted paper strip) close to the mouth of the test tube without immersing it in the gas mixture.
3. If hydrogen gas is present, it will immediately ignite with a “pop” sound at the end of the splint, creating a small flame.

Safety Precautions:

When conducting any chemical test, including the “pop” test for hydrogen gas, safety should be a top priority. Take the following precautions:

• Perform the test in a well-ventilated area to avoid the buildup of potentially flammable gases.
• Keep the flame at a safe distance from the mouth of the test tube to prevent accidents or burns.
• Wear appropriate safety gear, such as safety goggles and gloves.
• Use small quantities of chemicals for the test, and dispose of the reaction mixture safely after the experiment.

Note: The “pop” test is a qualitative test and only indicates the presence of hydrogen gas. If you need a quantitative analysis of the amount of hydrogen gas produced, specialized equipment and techniques are required.