Reaction Kinetics

label Chemistry
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In a reaction of sodium thiosulphate and sulphuric acid, what causes the cross to disappear.?

Mar 8th, 2015

I'm going to take a wild guess and suppose that you are referring to a demonstration--something like what happens in this video:

https://www.youtube.com/watch?v=eCW3uHuBsUE

The video uses HCl (hydrochloric acid) instead of H2SO4 (sulfuric acid), but the products remain the same in general: SO2, S, an aqueous sodium salt, and water.  The balanced equation for sodium thiosulphate when mixed with sulfuric acid is given by...

H2SO4(aq) + Na2S2O3(aq) --> SO2(g) + S(s) + Na2SO4(aq) + H2O(l)

As you can see from the balanced equation that solid sulfur forms a product.  It is this solid sulfur which makes the solution cloudy, thus making the cross disappear. 

Mar 8th, 2015

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