Neutralizing an Acid Spill Before the Bell
During a Grade 10 lab, a 400 mL beaker of hydrochloric acid solution (0.75 mol/L) tips over inside a large plastic tray. The teacher hands one student a 500 g box of sodium bicarbonate and asks her to make the spill safe before the bell. Her written plan says: "Add three spoonfuls of baking soda, stir, and pour the liquid down the sink." Before treatment a pH strip in the tray reads pH 1; after three spoonfuls (about 30 g) the liquid foams strongly and a fresh strip reads pH 5. A thermometer left in the tray shows the temperature fell from 21 °C to 17 °C. The room has no fume hood, the windows are closed, and 25 minutes remain in the period. School rules allow only liquids between pH 6 and pH 9 to go to the drain, and the teacher asks the student to justify every remaining step.
1. Calculate the minimum mass of sodium bicarbonate the spill requires, then explain why 30 g still left the liquid at pH 5.
Read the worked answer
Amount of acid: n = C × V = 0.75 mol/L × 0.400 L = 0.300 mol HCl.
The balanced equation is HCl + NaHCO₃ → NaCl + H₂O + CO₂, a 1:1 ratio, so 0.300 mol of bicarbonate is needed. Mass = 0.300 mol × 84.0 g/mol ≈ 25.2 g.
So 30 g is already a slight excess on paper, which means the pH 5 is not caused by adding too little powder. The most likely causes are:
- the spoonfuls went in as lumps, so only the surface dissolved and reacted;
- the foam layer sits on the liquid and slows mixing;
- pH strips are only reliable to about ±1 unit;
- some acid remains in the film on the tray walls and under the beaker.
The correct move is not to dump in another 25 g. Stir, wait for the foaming to stop, re-test, and add bicarbonate in ~1 g increments until the reading falls between pH 6 and pH 9.
2. Explain the 4 °C temperature drop and identify the real hazards the student must manage.
Read the worked answer
The drop confirms a chemical reaction is running: HCl reacting with NaHCO₃ is endothermic, so it absorbs heat from the solution and the tray, and the thermometer reads lower. That is why the student should not describe this as a "heat" hazard - there is no fire risk here.
The real hazards are the products and the handling:
- Gas: 0.300 mol of CO₂ is roughly 7 L at room conditions. If the tray were covered or the liquid sealed in a bottle, the pressure would build and could burst the container.
- Foam: the bubbles expand fast and can overflow the tray rim onto the floor, creating a slip hazard and spreading the acid.
- Closed room: CO₂ is denser than air and collects low; with windows shut and no hood, ventilation matters.
Practical answer: leave the tray open, add bicarbonate slowly, wear goggles, ventilate the room, and keep the liquid below the tray rim so the foam has room to expand.
3. The teacher also asks why the student may not simply pour the pH 5 liquid down the sink, and why sodium hydroxide pellets would be a worse choice than baking soda.
Read the worked answer
The pH rule exists because whatever goes down the drain reaches the sewer system and eventually a receiving stream. A liquid at pH 5 is still ten times more acidic than the allowed lower limit; diluted or not, it is corrosive to metal pipes and harmful to aquatic organisms. A pH strip is the evidence the rule demands, so the student must neutralize, re-test and record the reading before pouring.
Sodium hydroxide would neutralize the acid, but it is the wrong reagent for a spill:
- it is a strong base and corrosive in its own right, so a splash is as dangerous as the acid;
- pellets dissolve exothermically, heating the spill;
- there is no visual end point, so it is very easy to overshoot from pH 1 to pH 12.
Baking soda is weakly basic and self-limiting - even a saturated solution stays near pH 8.3 - so an excess cannot push the liquid into the danger zone. That safety margin, not speed, is why it is the right choice.