In a science demonstration, a teacher mixed zinc (Zn) with hydrogen chloride (HCl) in a flask and quickly attached a balloon over the mouth of the flask. Bubbles formed in the solution and the balloon inflated.

When the teacher mixed zinc (Zn) with hydrogen chloride (HCl), a chemical reaction occurred between the zinc metal and the hydrochloric acid (which contains hydrogen chloride dissolved in water). The equation for this chemical reaction is:

Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)

Here's a step-by-step breakdown of the reaction:

1. Zinc (Zn), a solid metal, reacts with hydrochloric acid (HCl), which is in aqueous form—that means the HCl is dissolved in water.

2. The zinc displaces the hydrogen (H) from the hydrochloric acid. This is known as a single displacement reaction, where an element in its free form displaces another element from a compound.

3. As a result of this reaction, zinc chloride (ZnCl2) is formed and remains in aqueous solution, while hydrogen gas (H2) is produced as a byproduct.

4. The hydrogen gas bubbles you observed are the hydrogen forming as a gas. These bubbles indicate that a chemical reaction is taking place.

5. The balloon inflates because the hydrogen gas produced escapes from the liquid and fills the balloon. Since gas takes up more space than the same amount of liquid or solid, the balloon expands as more and more hydrogen gas is generated from the reaction.

This demonstration is a good example of a metal-acid reaction that produces hydrogen gas. It visually shows the production of a gas during a chemical reaction and is often used in classrooms to illustrate chemical reactivity and stoichiometry.