[Bel-p]

We did an experiment using dilute hydrochloric acid & sodium thiosulphate and I asked why we didn't need to wear gloves.  I was told it was because the HCl was very diluted and therefore not harmful if you got it on your hands.  

The concentration of HCl(aq) was 2M (ie, I think, 73g of HCl per 1 litre of water?).  I'm curious to know - what concentration of HCl would be harmful on the skin?  Can anyone tell me please?

Thanks.

[Bel-p]

Also: why do we talk about sodium thiosulphate "solution" and "dilute" hydrochloric acid?  why don't we talk about dilute sodium thiosulphate and/or hydrochloric acid solution?  I'm sorry if this sounds a stupid question, but I'm trying to get my head around concentrations & dilutions & solutions etc.  

[rcflyingboy]

Noramlly with hydrochloric acid (in my experience) you can get the concentrated acid on your hands and it wont burn or even affect your skin for a few minutes. Also the hydrochloric acid is considered dilute because it has to start as a solution for us to use it in liquid form. The sodium thiosulfate is a solution since it starts as a solid dissolved in a liquid.

[Borek]

Term "diluted" has no precise meaning.

In case of HCl it usually means something made by diluting stock acid (about 35%). 2M acid is not that diluted (you are right - it is about 7%) - depending on the context you may refer to it as concentrated or diluted.

In my practical experience 2M is not dangerous for the skin if the contact is not prolonged (ie I have sprinkled 10% acid on my hand several times and washed it in a 30 seconds or so without any signs of skin irritation). No idea how long will it take to get burned with 10% and I am not planning such experiment. Stock acid (35%) IS dangerous.

Also don't bother yourself with solution/dilution - it is not precise and context dependent too. One you will get some experience you will know which name to use, but you may for now refer to all solutions as 'just solutions' and it will be OK.

[mike]

I asked why we didn't need to wear gloves

This year we have started making all of our students wear gloves all of the time.