Iodine Clock Lab

OBSERVE THE REACTION

Two clear solutions. One sudden change.

Ready to prepare
Iodine clock laboratory equipmentLabeled reagent bottles, a graduated measuring cylinder, a beaker of Solution A, Solution B, a thermometer, and a digital stopwatch on a laboratory bench. IODINE CLOCK / BENCH 01 KIO₃0.10 MSTOCK SOLUTION DI H₂ODEIONIZED 20 mL ICE BATH 40302050 mLSOLUTION A · empty 22.0 °C 10 mLSOLUTION B6.4 mL SOLUTION BStarch +Na₂S₂O₅ 0.00SECONDSZORNSCIENCE
Measure the potassium iodate to begin.

The beaker is empty. Both reagents are colorless.

Solution A0.0 mL
Solution A temperature22.0 °C
Mixture volume-
ClockNot started
What makes the clock change color?

Iodate reacts in the bisulfite-containing mixture. While bisulfite remains, iodine is consumed and the solution stays clear. Once that iodine-consuming reagent runs out, iodine accumulates and forms a dark blue complex with starch. The blue color is an indicator of that endpoint.

Increasing iodate concentration speeds up this model; increasing temperature makes a greater fraction of collisions energetic enough to react. The clock time is an inverse-rate proxy for comparing runs with a fixed amount of Solution B.

Model limits: This is an illustrative kinetics model, not a prediction of measured lab times. At 22 °C, 4.0 mL stock gives a 32 s clock. Time varies inversely with iodate concentration and follows an Arrhenius temperature factor (activation energy 45 kJ/mol). Solution B begins at 22 °C; mixing uses a volume-weighted temperature and assumes no subsequent heat loss. Bath conditioning is accelerated. No empirical calibration is supplied in the handout.

Solution B is the handout’s premix using 2% starch, 0.20 M sodium metabisulfite, and water; its internal proportions are not specified. The displayed concentration is for Solution A before adding B. Mixing dilutes it further by 16.0/22.4.

How to use the iodine clock lab

Prepare Solution A, mix in Solution B, and watch for the sudden dark blue endpoint.

  1. Concentration: choose 2.0, 4.0, or 8.0 mL of 0.10 M KIO₃, or set another volume. DI water makes Solution A up to 16.0 mL.
  2. Prepare: measure KIO₃, then add the measured DI water. In Temperature mode, condition Solution A in an ice or warm water bath, or leave it at room temperature.
  3. Mix & time: add 6.4 mL of prepared Solution B. The stopwatch starts when the solutions first meet and stops at the blue endpoint. Playback speed changes the waiting time, not simulated seconds.
  4. Investigate: start a new trial and change one condition. The temperature investigation fixes Solution A at 0.025 M. In the challenge, aim for a 20.0 s endpoint within ±1.0 s.

This simulation has no data-entry areas. Use your class handout separately if assigned.

Real-lab safety: potassium iodate is harmful if swallowed and irritating to body tissues; sodium metabisulfite is an irritant. Avoid skin and eye contact. Follow your teacher’s handling and disposal directions. Simulated timing is illustrative.