The Balance That Seemed to Lose Mass
At a school science exhibition, three benches are set up. On Bench A, 5.0 g of zinc granules are dropped into 50 mL of dilute sulphuric acid in an open conical flask standing on a digital balance. The reading falls from 213.40 g to 213.18 g in three minutes; the flask feels warm, and the gas escaping burns with a pop when a burning splinter is brought near. On Bench B, 5.0 g of green ferrous sulphate crystals are heated in a dry boiling tube; the crystals first turn white, then a reddish-brown residue remains, and a gas with the smell of burning sulphur comes off. On Bench C, sodium sulphate solution is mixed with barium chloride solution; a white solid appears at once and the thermometer shows no change. A visitor tells the students that Bench A proves the law of conservation of mass is wrong.
1. The visitor says mass has been destroyed on Bench A. Explain what is actually happening, write the balanced equation, identify the gas with its test, and state what the balance would read if the flask were closed.
Read the worked answer
Mass has not been destroyed; it has left the open flask as a gas. A balance reads only what is on the pan, so gas escaping is no longer weighed. The law of conservation of mass applies to a closed system, not an open one.
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
The gas burns with a pop, the standard test for hydrogen. 5.0 g of zinc is about 0.076 mol, which would give about 0.15 g of hydrogen if the acid is in excess; the observed fall of 0.22 g is of that order, the extra coming from water vapour and fine spray carried out with the gas. If the flask were tightly stoppered, the reading would stay almost constant because the hydrogen would remain inside; the flask would still feel warm and the pressure inside would rise. That is why the equation is balanced atom by atom, not by the reading of an open balance.
2. For Bench B and Bench C, name the type of reaction, write the balanced equation, and state one observation that proves a chemical change has occurred.
Read the worked answer
Bench B is thermal decomposition: one reactant is broken down by heat. The water of crystallisation is lost first (green crystals turn white), then
2FeSO₄ → Fe₂O₃ + SO₂ + SO₃
The reddish-brown residue is Fe₂O₃, and the smell of burning sulphur is SO₂, an acidic gas that turns moist blue litmus red. The colour change and the new gas prove a chemical change.
Bench C is a double displacement (precipitation) reaction:
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq)
Barium sulphate is insoluble, so it separates as a white solid. A new substance with different properties has formed, and the thermometer shows no change, so the change is not simply a physical warming. In both benches the atoms are only rearranged; none are created or destroyed.
3. How do the observations on Bench A show that the reaction is exothermic? Predict what would happen if the zinc were replaced by copper turnings, and justify your prediction.
Read the worked answer
The flask feels warm without any external heating, so heat is released to the surroundings: the reaction is exothermic. The energy released when bonds form in ZnSO₄ and H₂ is greater than the energy needed to break the bonds in H₂SO₄ and the zinc lattice, so the surplus appears as heat.
Copper is below hydrogen in the reactivity series, so it cannot displace H⁺ from dilute sulphuric acid. With copper turnings there would be no bubbles, no pop test, no fall in the balance reading and no warming; the copper would simply remain at the bottom. This is exactly why zinc, not copper, is used in the laboratory preparation of hydrogen, and why copper vessels are safe for dilute acids.