The Pop Test

Why it matters

A few grey lumps of zinc sit at the bottom of a test tube. We are about to pour in dilute hydrochloric acid. Something invisible is going to escape — and it announces itself with a pop.

The idea in plain words

Now balance it yourself. Use the arrows to set the numbers so every element has the same count of atoms on both sides — atoms are never created or destroyed.

Predict first

What will happen when dilute hydrochloric acid is poured on zinc granules?

zinc is more reactive than hydrogen, so it kicks hydrogen out of the acid — the bubbles are hydrogen gas escaping.

• Nothing — metals don't react with dilute acids
• Bubbles of a gas will rise from the zinc — correct
• The zinc will melt into a liquid
• The acid will just turn cloudy and nothing else

What you do

Pour the acid and watch all three views at once: the bubbles in the tube, the atoms swapping partners, and the equation writing itself.

Check yourself

Which gas bubbles out when zinc reacts with dilute HCl?

Zinc displaces hydrogen from the acid: Zn + 2HCl → ZnCl₂ + H₂↑. A burning splinter pops in hydrogen.

In the balanced equation Zn + 2HCl → ZnCl₂ + H₂, why is the 2 needed before HCl?

The right side has 2 Cl (in ZnCl₂) and 2 H (in H₂). One HCl only brings 1 of each, so we need 2HCl. Coefficients balance atoms — they never change what the substances are.

During the reaction, what happens to the zinc atoms?

Atoms are conserved. The zinc lump shrinks because its atoms leave the solid and pair with chloride ions in solution as ZnCl₂ — count them in the particle view: none are missing.

In this reaction, zinc ends up with the chloride and hydrogen escapes as gas. What really happened?

Zinc is more reactive than hydrogen, so it claims the chloride and hydrogen is set free as gas. A few lessons ahead you'll learn the official name for this move.

• Oxygen
• Hydrogen — correct
• Chlorine
• Carbon dioxide
• Because the acid is twice as strong
• So that hydrogen and chlorine atoms are equal on both sides — correct
• Because zinc has a valency of 2, so everything doubles
• It is just a convention chemists follow
• They disappear as the zinc dissolves
• They melt and become part of the water
• Every zinc atom is still there — now inside zinc chloride — correct
• They turn into hydrogen atoms
• The more reactive zinc pushed hydrogen out of the acid and took its place — correct
• Zinc and hydrogen merged into a single new element
• The acid simply soaked into the zinc like a sponge