Lower secondary · 10 min

Weighing a chemical reaction: conservation of mass

Let limestone react with acid in a flask on a balance, then repeat with the flask open — and explain the difference.

Goal

Verify that mass is conserved in a closed system and identify where the “missing” mass goes when the system is open.

Apparatus and reagents

Limestone (marble chips, $\ce{CaCO3}$ ), dilute hydrochloric acid $\ce{HCl}$ , conical flask with stopper, electronic balance.

Procedure

Choose “Before reaction”: note the balance reading for the closed flask containing $\ce{CaCO3}$ and $\ce{HCl}$ .
Switch to “After reaction (still closed)”: the mixture has fizzed, but compare the balance reading with the first step.
Now choose “Flask opened”: watch $\ce{CO2}$ leave and read the new mass.
Conclude: write the balanced equation $\ce{CaCO3 + 2HCl -> CaCl2 + CO2 + H2O}$ and account for every gram.

What to observe

In the closed flask the balance reading is identical before and after fizzing: $m_{\text{before}} = m_{\text{after}}$ .
Once the stopper is removed the reading drops by about 3.8 g — the mass of the $\ce{CO2}$ that escaped.

Explanation

Atoms are neither created nor destroyed in a reaction; they are rearranged. In a closed vessel every product stays on the balance, so the total mass is unchanged — the law of conservation of mass (Lavoisier). Opening the flask lets the gas leave and the balance only weighs what remains.

Chemists behind it

Antoine Lavoisier