← Back to Lab
Chemistry · 6.2 Rate of reaction · Paper 5/6 practical

Disappearing Cross. Cloud it.

Sodium thiosulfate + hydrochloric acid → a fine sulfur precipitate that slowly turns the solution opaque. Time how long a cross drawn under the flask takes to vanish. Vary the concentration (or temperature) and plot 1/time — a measure of rate — against it.

0620 Topic 6.2 — Rate of reaction Na₂S₂O₃ + 2HCl → S↓ + SO₂ Paper 5/6 — Practical
Setup — mix thiosulfate and water, then add acid and start the timer.
00.0 s
cross clear

Shortcuts Space start · Enter cross gone · R reset.

Variables — total volume kept constant

50
0
25
Diluting with water lowers the thiosulfate concentration but keeps the volume — and the acid — the same.

Live readouts

[Thiosulfate]
— g/dm³
Time t
0.0 s
Rate ∝ 1/t
—
Opacity
0%
The cross "disappears" once enough sulfur has formed — a fixed amount of precipitate, so 1/t ∝ rate.

Results table

Run at several concentrations to build the table.

1/t vs concentration — straight line ⇒ rate ∝ conc

📋 Method (Cambridge practical procedure)
  1. Draw a cross on a piece of paper and stand a conical flask on it.
  2. Measure a volume of sodium thiosulfate solution into the flask; add water to keep the total volume constant (this sets the concentration).
  3. Add a fixed volume of dilute hydrochloric acid, start the stopwatch and swirl once.
  4. Look down through the solution and stop the clock when the cross can no longer be seen.
  5. Repeat at different concentrations (or temperatures), keeping everything else the same.
  6. Plot 1/t against concentration. A straight line through the origin shows rate is proportional to concentration.
⚠ Sources of error & precautions
  • Judging "disappeared" is subjective — the same person should judge every run, viewing from directly above with the same lighting.
  • Temperature must be controlled when concentration is the variable (and vice-versa) — change only one variable at a time.
  • SO₂ is toxic — work in a well-ventilated room or fume cupboard; it irritates the lungs.
  • Use the same flask and cross each time so the depth of liquid (and path length) is identical.
  • Wash the flask between runs so old sulfur does not affect the next reading.
🎯 Syllabus reference (0620)
  • 6.2 Rate of reaction — investigate and describe the effect of concentration and temperature on rate; interpret data, including graphs of 1/time.
  • 12.1 — measurement of time and volume.

Ask the lab assistant