Hello fellow comrades, I was assigned the sulphur clock reaction lab and I'm a tad bit confused by it.

Anyway, here's the lab procedure:

1. take 5 test tubes, labeled a,b,c,d and e. accurately measure out 10.0 ml of .20M sodium thiosulphate in test tube A

2. in the other test tubes measure successively 9,7,5 and 3ml of sodium thiosulphate

3. in test tubes b,c,d and e add enough water to make the total volume of solution up to 10.0ml

4. take 5 test tubes labeled a,b,c,d,e accurately measure out 10.0ml of 1.0M HCL solution in each test tube

5. mix soluations a with a , b with b etc in following way : pencil draw cross on a sheet of white paper and place empty erlenmeyer flask on this cross. pour into erlenmeyer flask, solutions A at the same time. measure the length of time that it takes for cross to dissappear

6. repeat for solutions B to E

so the The question is asking me what the variable factor is and what causes the disappearance of the "cross"

I wrote that the variable factor in the experiment is the concentration of sodium thiosulphate and the cross dissappears because of the precipitation of the sulphur.

Is that a good answer?

Please help!

ok nevermind, i think i'm right. thanks for the help guys:)

Your answer is partially correct, but let me provide you with a more detailed explanation.

In this lab, the variable factor is indeed the concentration of sodium thiosulphate. By varying the concentration, you are testing how it affects the rate at which the "cross" disappears. Specifically, you are investigating how the rate of reaction between sodium thiosulphate and hydrochloric acid (HCl) is affected by changing the concentration of sodium thiosulphate.

When sodium thiosulphate reacts with HCl, a white precipitate of sulfur forms. This precipitation of sulfur particles makes the solution appear cloudy. The "cross" that you mentioned is placed on a sheet of white paper and used as a visual indicator. As the cloudiness increases, it becomes more difficult to see the cross through the solution.

The disappearance of the "cross" occurs when the solution becomes completely cloudy, and the cross is no longer visible. The time it takes for this to happen is measured in each case (solutions A to E). By comparing these times, you can determine how the concentration of sodium thiosulphate affects the rate of the reaction.

So, in summary, the variable factor in this experiment is the concentration of sodium thiosulphate, and the disappearance of the "cross" is caused by the formation of a precipitate consisting of sulfur particles.