Mass Never Disappears

Why it matters

A zinc granule fizzes away in acid inside a sealed flask, yet the balance never budges — the gas is trapped, so no mass is lost.

The idea in plain words

Count the atoms on both sides of Zn + 2HCl → ZnCl₂ + H₂. The balance holds because the atom count is identical.

Predict first

If you weigh the flask before and after the reaction (sealed), what happens to the total mass?

Atoms are neither created nor destroyed; they just rearrange into new molecules.

• Mass increases because gas is created
• Mass stays exactly the same — correct
• Mass decreases because the solid zinc disappears

What you do

Watch the see-saw balance. As zinc dissolves and hydrogen gas forms, does the beam tip?

Check yourself

Why does the see-saw remain balanced during this reaction?

Law of Conservation of Mass: mass of reactants = mass of products.

What observation confirms hydrogen gas is produced?

H₂ is identified by the 'pop' test with a burning splinter.

If the flask were open, the mass would seem to decrease. Why?

In an open system, gaseous products escape, making the remaining mass appear lower.

• The number and type of atoms are conserved — correct
• The gas escapes, balancing the loss of solid
• Zinc is lighter than hydrogen
• The solution turns blue
• Bubbles form and a burning splinter pops — correct
• A white precipitate settles
• Mass is destroyed
• Hydrogen gas escapes into the air — correct
• The acid evaporates completely