======Disappearing X Reaction======


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**Safety: **{{$demo.safety_description}}\\
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**Alternative titles:** Rate of Reaction of Sodium Thiosulfate and Hydrochloric Acid
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====Summary====                   
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====Procedure====  
  -Label five 100-mL beakers 1–5 and draw a large “X” on the bottom of each. Place them on an overhead projector or light box so the marks are visible.
  -Measure the specified volumes of sodium thiosulfate solution (0.15 M) and distilled water into each beaker to prepare different concentrations:
    *Beaker 1: 50 mL Na2S2O3, 0 mL water  
    *Beaker 2: 40 mL Na2S2O3, 10 mL water  
    *Beaker 3: 30 mL Na2S2O3, 20 mL water  
    *Beaker 4: 20 mL Na2S2O3, 30 mL water  
    *Beaker 5: 10 mL Na2S2O3, 40 mL water
  -Measure 5 mL of 2 M HCl into a separate container for each beaker.
  -Add HCl all at once to the sodium thiosulfate solution in beaker 1, stir briefly, and start timing.
  -Stop timing when the black “X” is no longer visible through the cloudy solution. Record the reaction time.
  -Repeat the procedure for beakers 2–5.
  -Calculate 1/time for each trial and plot concentration versus reaction time and concentration versus 1/time to analyze the rate law.



====Links==== 
The Disappearing Cross Experiment - Your Science Teacher: 
{{youtube>_nYmRdDMM8A?}}\\

Reaction Rate - Hydrochloric acid + Sodium Thiosulfate (Concentration) - Angles and Acid: 
{{youtube>J8zyMnMzbLA?}}\\

📄 Rate of Reaction of Sodium Thiosulfate and Hydrochloric Acid - Flinn Scientific: [[https://www.flinnsci.com/api/library/Download/78da6c8204aa48a294bd9a51844543ad?srsltid=AfmBOoqLBsxWmXyZ3iFfLDoScbD_tGWUaU4mgduwhXls9Z9IMRjttrNt]]\\


====Variations====
  *Repeat the experiment varying HCl concentration to confirm the reaction order with respect to acid.  
  *Investigate the effect of temperature by warming or cooling the sodium thiosulfate solution before mixing.  
  *Downscale the experiment using a 6-well reaction plate or medicine cups for individual student trials.



====Safety Precautions====
  *Wear safety goggles, gloves, and a chemical-resistant apron.  
  *Hydrochloric acid is corrosive; avoid skin or eye contact.  
  *Sodium thiosulfate solution is an irritant; handle with care.  
  *The reaction produces sulfur dioxide gas, which is an irritant; perform in a well-ventilated area and avoid inhalation.  
  *Clean beakers promptly after use to prevent sulfur deposits from hardening.




====Questions to Consider====
  *Why does the “X” disappear more quickly in higher concentrations of sodium thiosulfate? (Because reaction rate increases with concentration, producing sulfur faster.)  
  *Why is 1/time used as a measure of reaction rate? (Because rate is inversely proportional to the time taken for the visible change to occur.)  
  *What does the straight-line relationship between concentration and 1/time suggest about the reaction order? (It suggests the reaction is first order with respect to sodium thiosulfate concentration.)  
  *Why is the reaction considered acid-catalyzed even though HCl concentration does not directly affect the measured rate? (The acid produces reactive intermediates like HS2O3⁻ that drive the decomposition.)