Safe concentration of HCl for Year 8 students to use?: A year 8 textbook has an activity which includes an example for a colour change (blue litmus paper with HCl added to it). The teacher thinks this is a bit boring and has asked me to look into an alternative colour change reaction.  As the 4 examples of a chemical reaction all involved HCl I decided to look for a colour change reaction involving HCl.  One of the reactions I am looking at is the reaction of concentrated HCl with CuSO4.6H2O.  This gives a lovely colour change from blue to green [CuCl4]2-.  Excess NH3 solution can then be added to give a lovely deep blue/purple [Cu(NH3)4(OH)2].

So far I have been able to get the CuSO4 solution to change colour to the green [CuCl4]2- using 6M HCl (with conc. HCl obviously been approx 12M).  I am concerned about the students using 6M HCl.  The 6M HCl would be in a Stuhl dropper bottle, it would be in the fumehood and the students would have all their PPE on.  Do you think it is safe for the students to do the reaction if closely supervised by myself and the staff member, or should it be a demonstration only?  Just out of curiosity, what molarity of HCl would you consider to be safe for students to use?

Can you suggest an alternative colour change reaction involving HCl at a lower concentration?