Disappearing cross sim
Make 50 cm³ of mixture from 0.20 mol/dm³ sodium thiosulfate and water, warm it to the temperature in a water bath, and stand the flask on the card.
Disappearing cross · covers the GCSE, IGCSE, O-Level topic
What the sim shows
Make 50 cm³ of mixture from 0.20 mol/dm³ sodium thiosulfate and water, warm it to the temperature in a water bath, and stand the flask on the card. Add 5 cm³ of 2.0 mol/dm³ hydrochloric acid and start the clock at the same moment. Na₂S₂O₃ + 2HCl → 2NaCl + S + SO₂ + H₂O. Fine sulfur clouds the liquid pale yellow; stop the clock when you look down and can no longer see the cross. Less than a milligram of sulfur (0.88 mg) is enough, and only 0.6% of the acid has been used by then, so the thiosulfate alone sets the speed. The total volume is always 50 cm³, so you look through the same depth every time and the cross goes at the same amount of sulfur. 1/t is the rate: against concentration it is a straight line through the origin, and each 10 °C warmer about doubles it. In a real lesson the end is a judgement, so use the same eyes, flask and card every time; the figure calls it gone at 5% contrast. The sulfur dioxide is toxic, so work where the air moves. The clock runs 10 times faster than real time.
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