Disappearing Cross Virtual Lab: Concentration and Rate

Updated 2026-10-07

This disappearing cross virtual lab matches the second method in the AQA GCSE Chemistry required practical on rates of reaction: following a reaction by the change in turbidity. Students mix sodium thiosulfate with hydrochloric acid over a cross drawn on paper and time how long sulfur takes to hide it. They repeat at five concentrations, plot 1/t against concentration, then raise the temperature. A colorimeter mode covers the AP Chemistry crystal violet kinetics lab (see the Extension).

Rate of reaction by turbidity and colour – disappearing cross and crystal violet

Curriculum links

AQA GCSE Chemistry 4.6.1.2 (factors that affect the rates of chemical reactions) and its required practical on how concentration affects rate, using the change in turbidity. In Combined Science: Trilogy the same content is 5.6.1.2. For the gas-volume method of the same practical, see the rates of reaction virtual lab.
AP Chemistry Unit 5, topics 5.1 Reaction Rates, 5.2 Introduction to Rate Law and 5.3 Concentration Changes Over Time (first-order reactions and ln A against time).
NGSS HS-PS1-5: explain the effect of changing concentration or temperature on the rate of a reaction.

Simulic is not affiliated with or endorsed by the College Board or AQA.

Before the lab (5 min)

Ask students to commit to a prediction, on paper or as question 1 of the class link:

"At 20 °C, 50 mL of sodium thiosulfate solution makes the cross disappear in about 25 seconds. You use 25 mL of thiosulfate and 25 mL of water instead. How long will the cross take to disappear?"

Some students say "the same time, because the volume is the same". Don't correct them yet.

Method in the simulation

Every run uses a fresh flask: the chosen volume of 0.15 M Na₂S₂O₃, topped up with water to 50 mL, plus 10 mL of 2 M HCl.

Set Method to "Disappearing cross", Temperature to 20 °C and Time speed to ×2.
Set Na₂S₂O₃ 0.15 M to 50 mL. Press Add HCl, start timer.
Watch the "View from above" circle. Press Cross gone – stop (or tap the picture) as soon as you cannot see the cross.
Repeat twice more and calculate the mean time. Auto measure times a run for you, with a scatter of about 3%, like a careful observer.
Repeat for 40, 30, 20 and 10 mL of thiosulfate. The water is added for you.
Calculate 1/t for each mean. Press 1/t – concentration to see the graph.
Set the volume back to 50 mL and repeat at 30 °C and 40 °C. Press 1/t – temperature.

V Na₂S₂O₃ (mL)	V water (mL)	[S₂O₃²⁻] (M)
50	0	0.125
40	10	0.100
30	20	0.075
20	30	0.050
10	40	0.025

the cloudy flask with the cross just gone, and the 1/t against concentration line below

Expected results

After each run, the sim shows its model time, the moment the cross really disappears. Single runs scatter by about 3% around it. At 20 °C:

V Na₂S₂O₃ (mL)	50	40	30	20	10
Model time t (s)	25.2	31.6	42.2	63.8	130
1/t (s⁻¹)	0.0397	0.0317	0.0237	0.0157	0.0077

1/t is proportional to the thiosulfate concentration: the points lie on a line through the origin. Halving the concentration (50 to 25 mL) doubles the time, from 25.2 s to 50.8 s.
Real runs scatter: our auto runs at 50 mL gave 26.3, 25.4 and 25.5 s.
Temperature, 50 mL: 52.0 s at 10 °C, 25.2 s at 20 °C, 12.8 s at 30 °C and 6.8 s at 40 °C. Each 10 °C rise roughly doubles 1/t.

Questions for students

(Prediction, asked again after the lab) How does the time change when you use 25 mL of thiosulfate and 25 mL of water instead of 50 mL of thiosulfate?
Why is water added to make the volume up to 50 mL in every run?
With 20 mL of thiosulfate and 30 mL of water at 20 °C, what is the mean time for the cross to disappear?
With 50 mL of thiosulfate, how many times bigger is 1/t at 30 °C than at 20 °C?
Why can 1/t be used as the rate, and how could the timing be made more accurate?

Answers for teachers: (1) It about doubles, from 25 s to 51 s. (2) To keep the total volume, and the depth of liquid you look through, the same, so only the thiosulfate concentration changes. (3) About 64 s (accept 59–69 s). (4) About 2.0 (accept 1.75–2.2). (5) Every run stops when the same amount of sulfur has formed, so the rate is a fixed amount divided by t, which is proportional to 1/t. The end point is a judgment, so use the same observer, cross and lighting, repeat and average, or use a light sensor to detect the end point.

Common misconceptions

"The total volume is the same, so the concentration is the same." Adding water instead of thiosulfate lowers the concentration of the reactant.
"A longer time means a faster reaction." Time and rate are inversely related, which is why the graph uses 1/t.
"Heating only makes the particles collide more often." The bigger effect is that a larger share of collisions have enough energy to react.

Extension

AP: crystal violet colorimetry. Switch Method to "Colorimeter", set [OH⁻] to 0.02 M and 25 °C, and press Start measuring. Compare the A – t, ln A – t and 1/A – t graphs: only ln A – t is straight (R² 0.9999), so the reaction is first order in the dye. We got k' = 4.0×10⁻³ s⁻¹ and t½ = 173 s. At 0.04 M OH⁻, k' doubled to 8.0×10⁻³ s⁻¹ (R² 0.9996 for ln A, 0.88 for the other two graphs), so it is also first order in OH⁻, with k = 0.200 M⁻¹ s⁻¹.

FAQ

Why not just run the real practical?

The real reaction gives off sulfur dioxide, which is toxic and can trigger asthma. The simulation gives a full data set safely, with repeats in seconds. Use it before a wet lab or instead of one.

Why do my times differ slightly from the table?

Each run is timed by eye, or by Auto measure with about 3% scatter. That is why students repeat each run and use the mean.

Does this replace the gas-volume method?

No. The AQA practical asks for both methods: one measuring gas volume and one following a change in color or turbidity. The gas-volume method is in the rates of reaction virtual lab, so the two packs together cover the whole practical.